Watershed based plan for the Fort Cobb Watershed

              Page 1 of 38
June 2009
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WATERSHED BASED PLAN
FOR THE
FORT COBB WATERSHED
Prepared By:
Oklahoma Conservation Commission
Water Quality Division
4545 N. Lincoln Blvd., Suite 11A
Oklahoma City, OK 73105
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FORT COBB WATERSHED BASED PLAN
Table of Contents
PAGE
PREFACE 3
INTRODUCTION 6
CAUSES AND SOURCES 7
LOAD REDUCTIONS 14
CRITERIA 15
NPS MANAGEMENT MEASURES 15
TECHNICAL AND FINANCIAL ASSISTANCE NEEDED 18
IMPLEMENTATION SCHEDULE 21
INTERIM MILESTONES 26
PUBLIC OUTREACH 27
MONITORING PLAN 31
REFERENCES 36
APPENDICES 37
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PREFACE
The Fort Cobb Watershed
covers 314 square miles in
southwestern Oklahoma in
Caddo, Washita, and Custer
Counties. Ft. Cobb Reservoir’s
designated beneficial uses
include public and private
water supply, warm water
aquatic community, agricul-ture,
municipal and industrial
uses, primary body contact
recreation, and aesthetics.
The reservoir is the primary
drinking water source for the
Cities of Anadarko and
Chickasha. The watershed is located in the Central Great Plains Ecoregion in
southwestern Oklahoma. Landuse in the watershed includes agricultural fields, cattle
operations, rural communities, and one hog operation. Most soils in the watershed are
highly erodible, sandy clays and loams. The water quality of the reservoir and its tributaries
has been of concern for more than a decade with water quality problems identified
beginning in 1981.
Oklahoma Water Quality
Standards list Fort Cobb
Reservoir as a Nutrient
Limited Watershed (due to
high primary productivity) and
a sensitive public and private
water supply. 1998 Oklahoma
Water Resources Board
(OWRB) data showed the lake
was hypereutrophic and in
1999, eutrophic (OWRB
2002). Studies indicated
biological, chemical, and
habitat degradation within the
Ft Cobb Reservoir Watershed.
DDT was detected in fish flesh tissue in 1981. Ft. Cobb Reservoir and six waterbody
segments in its watershed were listed on the 1998 303(d) list as being impaired by
nutrients, pesticides, siltation, suspended solids, and unknown toxicity (Table 1). The
Reservoir and three streams, Cobb, Willow, and Fivemile Creek, are currently listed on the
2008 303(d) list as being impaired (see Table 1; ODEQ 2008). In addition, concerns have
been expressed by the Master Conservancy District reservoir managers regarding the
nutrient and sediment loads.
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Table 1. 303(d) Listed Causes of Impairment in Fort Cobb Watershed.
303(d)
list year OK Waterbody ID Name Cause of Impairment
1998 OK 310830050020 Fort Cobb
Reservoir pesticides, suspended solids, turbidity
1998 OK 310830060030 Willow Creek nutrients, siltation, suspended solids
1998 OK 310830060040 Lake Creek
unknown toxicity, pesticides, nutrients,
siltation, other habitat alterations, suspended
solids
1998 OK 310830060050 Cobb Creek pesticides, nutrients, siltation, suspended
solids
1998 OK 31080060080 Fivemile Creek nutrients, siltation, suspended solids
1998 OK 31080060130 Crowder Lake nutrients, organic enrichment/D.O.,
suspended solids
2002 OK310830050020 Fort Cobb
Reservoir phosphorus
2002 OK 310830060030 Willow Creek pathogens
2002 OK 310830060040 Lake Creek low dissolved oxygen1, turbidity
2004 OK310830050020 Fort Cobb
Reservoir phosphorus
2004 OK 310830060030 Willow Creek Fecal coliform, Enterococcus, E. coli
2004 OK 310830060040 Lake Creek selenium
2006 OK310830050020 Fort Cobb
Reservoir phosphorus, turbidity
2006 OK 310830060030 Willow Creek Fecal coliform, Enterococcus, E. coli
2006 OK 310830060040 Lake Creek selenium
2006 OK 31080060130 Crowder Lake turbidity, dissolved oxygen
2008 OK310830050020 Fort Cobb
Reservoir turbidity
2008 OK 310830060030 Willow Creek Fecal coliform, Enterococcus, E. coli
2008 OK 310830060050 Cobb Creek ammonia, Enterococcus, E. coli
2008 OK 31080060080 Fivemile Creek Enterococcus, E. coli
2008 OK 31080060130 Crowder Lake turbidity, chlorophyll-a, dissolved oxygen
Considerable efforts have been made to identify the causes, sources, and extent of water
quality threats and impairments in the basin, and extensive remedial efforts have occurred
in the past several years. Previous studies of the reservoir and watershed were conducted
by the U.S. Fish and Wildlife Service (USFWS), the Bureau of Reclamation (BOR), and the
U.S. Geological Survey (USGS). These studies identified the causes, extent, and some of
the sources of water quality impairment in the watershed.
1 Listing for D.O. later determined to be in error during TMDL development.
In 2006, the Oklahoma Department of Environmental Quality (ODEQ) released the final
draft of a TMDL for phosphorus loading to Fort Cobb Reservoir (Appendix A). This TMDL
recommended a 78% phosphorus load reduction to restore beneficial use support to the
reservoir. Because there are no point source dischargers in the watershed, this reduction
must come entirely from nonpoint sources in the watershed.
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The TMDL was based on watershed data collected between 1990 and 2001; therefore,
loading reduction recommendations are based upon loading during that period. Since that
period, many changes have taken place in the watershed which suggests that Oklahoma is
making significant progress towards the TMDL goal. These efforts include, but are not
limited to, a decrease in peanut production in the watershed following the loss of
government subsidies of peanut production, a 2001 §319 Project focused on education and
demonstration of practices to reduce sediment and
nutrient pollution in the watershed, a 2005 §319 Project
focused on no-till, and continued effects of previous NPS
education programs in the watershed which have resulted
in the voluntary implementation of best management
practices such as riparian zones, nutrient management,
and conservation tillage.
Additional work in the watershed includes education
programs developed by the Oklahoma Cooperative
Extension Service (OCES), the Deer Creek, West Caddo,
North Caddo, and Mountain View Conservation Districts,
the Natural Resources Conservation Service (NRCS),
and the Oklahoma Conservation Commission (OCC), and
various programs to reduce nonpoint source loading in
the watershed. As a result of these efforts, Lake Creek
was delisted for pesticides and unknown toxicity in 2002.
A Conservation Reserve Enhancement Program (CREP) is planned for the Fort Cobb
watershed, which will further address sediment and nutrient loading. This watershed based
plan (WBP) discusses the efforts which have already occurred as well as those necessary
to expand the programs ongoing in the watershed to reach the load reduction goals
established by the TMDL and to restore beneficial use support to Fort Cobb Reservoir and
the waterbodies in its watershed.
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INTRODUCTION
In 1997, on the 25th anniversary of the 1972 Federal Clean Water Act, Vice President Al Gore
initiated development of a nationwide strategy to protect water quality. This initiative resulted
in the development of the Clean Water Action Plan (CWAP), which established goals and
implementation schedules for numerous strategies dealing with point and nonpoint sources.
Oklahoma’s Office of Secretary of Environment (OSE) was designated as the state lead
agency to implement the provisions of the CWAP in Oklahoma.
Under OSE’s leadership, Oklahoma has successfully met the CWAP requirement to establish a
Unified Watershed Assessment (UWA) strategy. Oklahoma’s UWA is a written document
whose development and implementation relied upon input from the state’s UWA Work Group.
Through the UWA process, the Work Group identified “Category I” watersheds in Oklahoma
that were recognized as significantly impaired and in need of immediate federal and state
funding to target restoration activities. Fort Cobb Watershed was one of these high priority
watersheds (Figure 1).
EPA’s Nonpoint Source Program and Grants Guidelines for States and Territories for FY 2004
and Beyond requires a Watershed-Based Plan (WBP) to be completed prior to implementation
using incremental funds. The guidance defines the 9 key components to be addressed in a
watershed-based plan, much of which builds from the strategies outlined in a Watershed
Restoration Action Strategy (WRAS). These components are: 1) identification of causes and
sources that will need to be controlled to achieve load reductions, 2) estimate of load
reductions expected from the management measures described, 3) a description of the
management measures that will need to be implemented to achieve load reductions, 4) an
estimate of the amounts of technical and financial assistance needed, associated costs, and/or
the sources or authorities who will bear responsibility, 5) an information/education component
that will be used to enhance public understanding of the project and encourage early
participation in the overall program, 6) a schedule for implementing the Non-Point Source
(NPS) management measures identified in this plan that is reasonably expeditious, 7) a
description of interim, measurable milestones for determining whether control actions are being
implemented, 8) a set of criteria that can be used to determine whether loading reductions are
being achieved over time and substantial progress is being made or whether the Watershed
Plan or Total Maximum Daily Load (TMDL) needs to be revised, and 9) a monitoring
component to evaluate the effectiveness of the implementation efforts over time.
The WBP for the Fort Cobb Watershed has been developed as a dynamic document that will
be revised, when necessary, to incorporate the latest information, address new strategies, and
define new partnerships between watershed shareholders following this initial documentation.
Also, it is understood that the water quality goals set forth in this WBP, as well as the technical
approach to address the goals, may not be comprehensive and it may be necessary to revise
or expand them in the future.
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Figure 1. Fort Cobb Watershed.
West Caddo CD
Mountain View CD
Deer
Creek
CD
North Caddo CD
Fivemile Creek
Lake Creek
Cobb Creek
Willow Creek
Federal and state funding allocations for future water quality projects designed to address the
Fort Cobb Watershed problems should not be based solely upon their inclusion in this WBP,
rather the WBP should be considered a focal point for initial planning and strategy
development. In order for this WBP to become an integral part of the entire watershed
restoration program, it must be amenable to revision and update. It is anticipated that at least
biannual revisions may be necessary, and that the responsibility for such revisions will rest
primarily with the OCC with support from the Office of the Secretary of the Environment (OSE)
and the NPS Working Group.
CAUSES AND SOURCES
Causes
Currently, Fort Cobb Reservoir, Willow Creek, Cobb Creek, and Fivemile Creek are impaired
by turbidity (reservoir), bacteria (all creeks), and ammonia (Cobb Creek) (Table 1). The Fort
Cobb TMDL (ODEQ 2006) focuses on phosphorus as the primary cause of impairment in Fort
Cobb Reservoir and suggests that the dissolved oxygen listing for Lake Creek was in error. In
addition, it confirms that pesticide impairments cited by the 1998 303(d) list are no longer
present, as indicated by current water quality and biological data (Appendix A).
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Sources
Point Sources
The TMDL verified that there were no permitted point source dischargers in the Fort Cobb
Watershed. However, there are two Concentrated Animal Feeding Operation (CAFO) farms in
the watershed, both with total retention NPDES permits. Permits on these farms, one a cattle
farm with 2700 animal units, and the other a swine farm with 800 animal units allow overflows
only under 25 year, 24 hour storm events. According to the TMDL, these provisions are
determined sufficient to protect the waters in the Cobb Creek watershed. The TMDL
recommends no additional measures for these CAFO farms. In order to rule out effects of
these facilities on nearby stream health, the relative load contribution attributable to these
facilities should be considered by the State to verify that these facilities are not significant
contributors to local or watershed-wide water quality problems. Based on these findings, the
TMDL may need to be revised.
Nonpoint Sources
In rural settings, the primary sources of nutrients may include runoff of applied fertilizer and
manure to agricultural land, runoff of animal wastes associated with the erosion of sediments in
grazing fields, runoff from concentrated animal operations, failing septic tanks, and
contributions from wildlife. The TMDL used the Soil and Water Assessment Tool (SWAT)
model to estimate NPS loadings from landuse in the watershed (Appendix B). This is the same
model and model runs that were used to target NPS implementation with an FY 2001 §319
project in the Fort Cobb Watershed. The model subdivided the basin into 90 subbasins, based
on 10-meter USGS Digital Elevation Model data for the basin (Figure 3). Loading estimates for
these 90 subbasins as predicted by SWAT are seen in Table 2. Loading estimates from Fort
Cobb landuses as predicted by SWAT are seen in Table 3. Figure 4 displays the SWAT
predictions related to phosphorus loading from subbasins in the Fort Cobb watershed. The
darkest red basins produce the highest phosphorus in runoff. The SWAT model estimated a
total sediment load to the lake (excluding roads) of 276,000 metric tons per year and a total
phosphorus load of approximately 70,000 kg P/year.
Typical landuse in the Fort Cobb Watershed (photo courtesy of Storm et al. 2003).
Page 9 of 38
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Figure 1. Sub-basin layout used in the Cobb Creek SWAT model (Storm et al. 2003).
Figure 3. Total phosphorus loading by sub-basin as predicted by SWAT (Storm et al. 2003).
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Table 2. SWAT Estimated Sub-basin Loading.
Sub-basin
AREA
(km2)
Surface
Runoff
(mm)
Baseflow
(mm)
Total Water
Yield (mm)
Sediment
(mg/ha)
Organic
Nitrogen
(kg/ha)
Organic P
(kg/ha)
Nitrate in surface
runoff (kg/ha)
Soluble
Mineral P
(kg/ha)
Sediment
bound mineral
P (kg/ha)
Total P
(kg/ha)
1 1.92E+01 27.198 21.307 51.494 3.02 4.895 0.603 0.106 0.009 0.452 1.064
2 2.12E+01 44.085 35.825 84.136 6.228 6.636 0.803 0.308 0.005 0.828 1.636
3 1.86E+01 45.708 41.17 91.644 4.087 7.324 0.917 0.139 0.015 0.589 1.521
4 8.41E+00 59.531 54.213 121.906 3.919 6.681 0.814 0.173 0.01 0.589 1.413
5 1.51E+01 59.522 35.941 105.415 1.371 9.323 1.123 0.21 0.119 0.951 2.193
6 1.15E+01 54.575 44.673 104.907 1.299 10.146 1.179 0.153 0.092 0.869 2.14
7 7.76E-01 64.588 93.128 175.716 4.213 3.414 0.427 0.221 0.006 0.538 0.971
8 1.18E+01 83.927 68.263 158.491 5.242 7.35 0.92 0.285 0.03 0.837 1.787
9 1.48E+01 50.333 41.2 94.889 2.573 4.644 0.576 0.149 0.012 0.465 1.053
10 2.92E+01 31.725 28.935 64.179 3.398 6.12 0.763 0.081 0.007 0.482 1.252
11 8.49E+00 49.722 43.356 100.008 4.762 7.552 0.933 0.143 0.014 0.688 1.635
12 3.92E-01 81.615 63.218 150.528 4.692 4.613 0.591 0.288 0.008 0.637 1.236
13 4.08E+00 57.373 45.845 109.125 4.154 5.687 0.689 0.172 0.012 0.647 1.348
14 1.49E+01 51.162 45.908 101.745 3.902 6.746 0.844 0.145 0.016 0.619 1.479
15 6.40E-01 67.495 56.309 133.605 4.144 3.42 0.445 0.244 0.006 0.555 1.006
16 1.04E+01 66.203 48.74 118.653 5.349 7.315 0.898 0.219 0.007 0.786 1.691
17 3.25E+00 65.768 53.966 125.363 3.876 5.338 0.673 0.233 0.007 0.628 1.308
18 8.27E+00 61.75 65.052 135.163 4.894 6.626 0.815 0.238 0.009 0.844 1.668
19 2.34E+00 63.825 41.101 109.131 3.083 4.99 0.623 0.227 0.007 0.557 1.187
20 1.56E+01 52.451 43.091 98.176 4.097 6.665 0.816 0.174 0.006 0.643 1.465
21 1.17E+01 62.091 37.157 119.144 2.322 3.437 0.384 0.103 0.026 0.413 0.823
22 1.58E+01 54.363 47.485 112.845 5.096 7.144 0.845 0.14 0.03 0.761 1.636
23 1.54E-01 57.438 54.933 119.686 5.707 6.978 0.85 0.235 0.006 0.7 1.556
24 2.58E+01 63.747 38.638 116.515 1.651 3.385 0.38 0.135 0.035 0.413 0.828
26 8.30E+00 55.895 41.336 102.702 3.631 6.642 0.812 0.148 0.013 0.595 1.42
27 3.16E-01 70.184 82.381 159.887 3.497 3.602 0.441 0.251 0.006 0.47 0.917
28 1.11E+01 58.754 90.282 162.144 3.27 4.629 0.573 0.157 0.01 0.497 1.08
29 1.33E+00 60.497 53.606 127.77 3.833 4.113 0.503 0.207 0.005 0.525 1.033
30 1.63E+01 56.704 79.111 143.377 4.087 5.737 0.709 0.168 0.006 0.602 1.317
31 1.56E+01 59.96 35.11 96.799 3.669 5.706 0.705 0.214 0.008 0.694 1.407
32 1.60E+01 44.063 49.617 102.287 4.17 3.912 0.497 0.155 0.004 0.559 1.06
33 9.64E+00 45.049 45.392 95.578 4.119 5.626 0.685 0.162 0.004 0.61 1.299
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Sub-basin
AREA
(km2)
Surface
Runoff
(mm)
Baseflow
(mm)
Total Water
Yield (mm)
Sediment
(mg/ha)
Organic
Nitrogen
(kg/ha)
Organic P
(kg/ha)
Nitrate in surface
runoff (kg/ha)
Soluble
Mineral P
(kg/ha)
Sediment
bound mineral
P (kg/ha)
Total P
(kg/ha)
34 1.38E+01 42.272 36.619 82.554 4.841 7.858 0.943 0.14 0.018 0.752 1.713
35 8.03E+00 45.779 47.536 103.311 4.865 5.756 0.713 0.147 0.015 0.648 1.376
36 1.63E+01 41.155 36.611 81.13 5.508 8.637 1.034 0.123 0.016 0.808 1.858
37 7.86E+00 71.821 93.901 178.555 3.963 4.737 0.582 0.301 0.009 0.714 1.305
38 6.23E-01 60.379 52.918 117.398 10.491 8.53 1.027 0.221 0.005 1.078 2.11
39 2.97E+01 51.589 100.085 167.169 3.579 4.675 0.575 0.184 0.018 0.599 1.192
40 1.10E+01 32.863 32.221 66.38 3.782 6.5 0.791 0.094 0.003 0.554 1.348
41 2.39E-01 37.573 89.244 141.895 1.859 2.549 0.322 0.117 0.003 0.256 0.581
42 9.76E+00 35.479 57.285 99.696 2.994 5.537 0.682 0.104 0.014 0.562 1.258
43 5.64E+00 50.394 37.238 90.2 2.031 3.753 0.47 0.159 0.007 0.432 0.909
44 2.39E-01 68.272 51.862 126.06 1.636 2.306 0.328 0.197 0.005 0.257 0.59
45 3.41E-01 54.859 69.637 137.479 0.968 1.207 0.175 0.151 0.003 0.149 0.327
46 1.08E+01 44.676 82.178 133.882 2.73 3.618 0.436 0.141 0.005 0.472 0.913
47 3.17E+01 67.633 71.945 148.966 5.84 6.645 0.821 0.233 0.007 0.844 1.672
48 9.09E+00 72.984 51.113 128.756 4.478 6.09 0.747 0.267 0.008 0.694 1.449
49 1.56E+01 48.316 64.608 122.413 2.924 4.499 0.556 0.148 0.018 0.534 1.108
50 7.69E+00 59.272 119.231 185.652 2.76 3.342 0.397 0.198 0.007 0.509 0.913
51 4.69E-01 52.32 99.866 172.358 0.875 0.793 0.11 0.14 0.003 0.14 0.253
52 4.18E-01 72.596 53.314 139.115 4.258 4.933 0.624 0.248 0.006 0.58 1.21
53 4.18E-01 51.149 59.582 117.475 5.527 3.89 0.455 0.164 0.005 0.634 1.094
54 1.02E+01 51.24 42.769 97.537 4.672 5.997 0.734 0.157 0.005 0.678 1.417
55 3.56E+00 55.822 69.517 133.307 3.071 3.307 0.396 0.186 0.006 0.494 0.896
56 1.80E+00 56.706 56.26 120.711 2.619 3.573 0.456 0.177 0.005 0.413 0.874
57 8.04E+00 50.824 75.133 131.671 2.129 3.164 0.4 0.172 0.006 0.418 0.824
58 2.83E+01 37.324 73.863 116.036 1.448 3.002 0.372 0.105 0.01 0.297 0.679
59 2.56E-04 29.149 122.047 151.789 5.553 8.264 0.982 0.258 0.009 0.686 1.677
60 1.20E+01 43.583 55.189 103.275 2.564 4.431 0.551 0.138 0.006 0.494 1.051
61 5.99E-02 92.043 48.162 145.011 1.77 2.39 0.302 0.375 0.009 0.366 0.677
62 1.11E+01 34.114 31.551 67.489 5.613 7.922 0.949 0.099 0.003 0.739 1.691
63 3.92E+00 61.29 95.521 171.36 3.253 3.11 0.386 0.206 0.007 0.551 0.944
64 9.31E+00 45.097 120.841 184.917 3.077 2.532 0.3 0.148 0.005 0.492 0.797
65 1.03E+01 45.126 41.258 88.964 2.588 5.11 0.63 0.123 0.016 0.505 1.151
66 1.57E+01 53.374 106.726 177.098 2.706 3.754 0.457 0.168 0.014 0.48 0.951
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Sub-basin
AREA
(km2)
Surface
Runoff
(mm)
Baseflow
(mm)
Total Water
Yield (mm)
Sediment
(mg/ha)
Organic
Nitrogen
(kg/ha)
Organic P
(kg/ha)
Nitrate in surface
runoff (kg/ha)
Soluble
Mineral P
(kg/ha)
Sediment
bound mineral
P (kg/ha)
Total P
(kg/ha)
67 3.85E+00 59.375 71.051 137.048 2.903 4.243 0.522 0.191 0.005 0.466 0.993
68 8.70E-03 43.584 63.164 112.436 0.007 0.009 0.001 0.094 0.001 0.001 0.003
69 1.80E+00 44.714 46.247 95.734 2.079 3.344 0.417 0.135 0.004 0.344 0.765
70 1.30E+01 26.598 126.714 163.971 1.14 1.639 0.199 0.077 0.003 0.215 0.417
71 7.55E+00 37.126 32.073 71.802 2.793 4.819 0.579 0.094 0.007 0.482 1.068
72 1.40E+00 53.081 82.98 143.678 3.572 5.059 0.614 0.176 0.005 0.548 1.167
73 3.34E+00 45.478 55.954 107.349 1.931 3.024 0.382 0.129 0.004 0.336 0.722
74 8.29E+00 59.304 81.656 151.916 3.641 4.695 0.59 0.187 0.005 0.512 1.107
75 1.24E+01 55.807 87.467 155.92 4.042 5.144 0.618 0.183 0.014 0.651 1.283
76 2.75E+00 96.8 81.318 192.216 12.309 9.954 1.204 0.411 0.012 1.557 2.773
77 1.13E+00 70.043 55.222 132.27 6.565 7.172 0.872 0.246 0.006 0.836 1.714
78 2.70E+00 68.549 64.252 144.007 4.496 5.311 0.699 0.259 0.008 0.717 1.424
79 1.25E+00 68.765 58.196 139.176 4.478 4.673 0.581 0.235 0.006 0.63 1.217
80 1.36E+01 40.901 125.391 186.441 2.599 3.061 0.369 0.132 0.004 0.408 0.781
81 3.33E-01 47.632 90.536 159.356 4.14 4.9 0.674 0.138 0.003 0.388 1.065
82 1.71E-02 35.8 84.674 120.944 1.205 2.99 0.397 0.09 0.004 0.207 0.608
83 9.91E+00 40.605 55.685 99.948 2.405 4.03 0.499 0.123 0.005 0.432 0.936
84 5.80E-01 49.712 86.352 148.054 1.111 1.33 0.175 0.132 0.003 0.17 0.348
85 9.08E+00 53.278 124.399 194.81 2.157 2.228 0.262 0.185 0.021 0.465 0.748
86 1.68E+00 33.15 34.701 69.55 1.149 2.413 0.299 0.092 0.004 0.242 0.545
87 1.96E-01 53.203 84.646 154.727 0.935 1.425 0.2 0.137 0.002 0.121 0.323
88 7.79E+00 39.372 118.206 168.503 1.84 2.465 0.3 0.125 0.005 0.342 0.647
89 8.69E+01 26.772 81.711 113.273 1.679 2.725 0.336 0.076 0.003 0.292 0.631
90 1.62E+01 53.325 61.977 120.823 2.375 4 0.494 0.197 0.006 0.423 0.923
Page 13 of 38
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The SWAT model predictions are subject to the following limitations:
• Loads are subject to all the same limitations as those presented in the report:
Fort Cobb Basin – Modeling and Land Cover Classification 2003;
• The loads are from upland sources only and do not consider bank or stream bed
erosion, instream nutrient processes, or deposition of sediment in reservoirs or
flood control structures on main channels;
• These data contain significantly more uncertainty than absolute load predicted to
the lake or basin outlet. With limited calibration data, these data would be best
utilized to relatively rank subbasins in terms of their nutrient contributions.
Although these predictions are subject to limitations, the estimates provide valuable
information about areas contributing most significantly to watershed loading and suggest
areas where incentives and other implementation programs should be targeted to have the
greatest impact on water resources. These high priority subwatersheds (highest
contributing watersheds as depicted in Figure 3) account for approximately 66.17 or 20% of
the 329.35 square miles in the watershed and about 30% of the load. Including the next
highest contributing set of subwatersheds increases the area to 210.83 square miles or
47% of the watershed and approximately 61% of the load.
The TMDL estimated phosphorus loading from septic tanks to be 3,608 kg/year, assuming
all watershed residents used septic systems and using a worst case scenario where:
• All septic tanks were failing,
• Every household was assumed to have one septic tank, equaling 1,124 septic tanks
in the watershed,
• Effluent from the tanks (11.6 mg P/L) drained directly to streams and lakes,
• Persons in the watershed produced 75 gallons of wastewater per day.
This loading would be approximately five percent of the total phosphorus loading to the
watershed. Given that this is an over estimate of the loading from the current systems, the
TMDL determined that loading from septic tanks was insignificant.
The primary crops grown in the watershed are wheat (80% of cropland), peanuts, sorghum,
and cotton (Storm et. al 2003). Wheat, peanuts, and sorghum are the landuses that
provided the highest nutrient and sediment loading in the watershed (Table 3); croplands,
which are about 50.4% of the total land in the watershed, account for 90.4% of total P load.
With the loss of peanut
subsidies, peanut pro-duction
has declined in
the watershed, and
many formerly peanut
fields have been con-verted
to cotton fields.
The SWAT model esti-mated
that the conver-sion
of peanuts to
cotton without BMPs to
address cotton could
result in increased
phosphorus and sediment loading to the lake (Table 4).
Cotton is one of the row crops produced in the Fort Cobb
Watershed (photo courtesy of Storm et al. 2003).
Page 14 of 38
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Table 3. SWAT simulated loads by land cover for the Fort Cobb Basin for the period 1/1990 -
10/2001 (from Storm et al. 2003).
Land Cover Fraction of
Basin (%)
Surface
Runoff (mm)
Total Stream
Flow (mm)
Sediment
(Mg/ha)
Total N
(kg/ha)
Total P
(kg/ha)
Forest 6.0% 23.98 178.98 0.01 2.20 0.01
Pasture-Range 41.4% 40.34 105.36 1.61 3.60 0.62
Peanut 7.1% 61.76 147.15 4.06 7.74 1.87
Sorghum 2.8% 96.02 161.33 3.16 6.95 1.20
Urban 0.1% 87.60 100.95 0.05 1.20 0.09
Water 2.1% 0.00 0.00 0.00 0.00 0.00
Wheat for Grain 30.8% 57.58 121.60 5.88 9.90 1.91
Grazeout Wheat 9.7% 56.10 118.77 5.16 8.69 1.81
Basin Average --- 48.47 118.46 3.36 6.26 1.19
Table 4. Load summary for Fort Cobb Basin as predicted by the SWAT model (from
Storm et. al 2003).
Crop Scenario Runoff
(CMS)
Total Water
Yield (CMS)
Sediment
(Mg/yr)
Total P
(kg/yr)
Total N
(kg/yr)
Current 1.37 3.05 301,277 108,031 543,615
Peanuts converted to cotton 1.28 2.95 307,131 110,103 543,461
Further details about the estimation of causes and sources in the Fort Cobb Watershed
can be found in the TMDL (ODEQ 2006) and SWAT model reports (Storm et. al. 2003).
LOAD REDUCTIONS
The draft TMDL estimated that a 78% phosphorus load reduction2 would be necessary to
restore beneficial use support to Fort Cobb reservoir. This sets a goal of reducing
phosphorus loading from 70,000 kg/yr to 15,400 kg/yr. The TMDL addresses both
phosphorus and turbidity impairment to the reservoir because most phosphorus is found
attached to sediment, one of the primary causes of turbidity. The TMDL reasons that if
phosphorus is reduced to meet water quality standards, then turbidity levels in contributing
streams will also be reduced to a level that will meet the turbidity standard. Fortunately,
BMPs recommended by the TMDL will also work to address the other sources of
impairment in watershed streams including pathogens. The TMDL also estimates that
every 1.0% reduction in phosphorus will correspond to a 1.33% reduction in total nitrogen
and a 1.5% reduction in sediment delivery to the lake. Further explanation of the
methodology for arriving at the 78% load reduction can be found in the TMDL and SWAT
model reports (ODEQ 2006; Storm et. al 2003).
2 This includes the load reduction to allow for a margin of safety and potential growth in the watershed.
Page 15 of 38
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CRITERIA
Fort Cobb Reservoir’s designated beneficial uses include public and private water
supply, warm water aquatic community, agriculture, municipal and industrial uses,
primary body contact recreation, and aesthetics. The reservoir is the primary drinking
water source for the Cities of Anadarko and Chickasha.
The goal of the TMDL is to reduce the 1998 – 2001 loading to the lake of approximately
70,000 kg P/year to 15,400 kg P/year. That load reduction is based on the following
endpoints, based on Oklahoma’s Water Quality Standards (OWRB 2004a, b):
• Trophic State Index (chlorophyll-a based) for Fort Cobb Reservoir less than 62
• Dissolved Oxygen (surface water)
o Summer (June 16 – October 15): 4.0 mg/L
o Seasonal (October 16 – June 15): 5.0 mg/L
• Anoxic volume in Fort Cobb Reservoir less than 50% of water column.
Additional criteria that apply to causes of impairment in the watershed are (OWRB
2004):
• Turbidity (only applicable during baseflow)
25 NTU for lakes
50 NTU for streams
• Coliform bacteria
Monthly geometric mean <5000 colonies/100 ml at point of intake
• <5% of total samples in any 30 day period will total coliform exceed 20,000
colonies/100 ml
• Enterococci bacteria
Geometric mean of 33 colonies/100 ml
• Escherichia coli (E. coli)
Geometric mean of 126 colonies/100 ml
• Warm Water Aquatic Community
IBI = 22
These criteria stem from Oklahoma’s Water Quality Standards (OWRB 2004a). The
procedures by which the data must be collected and analyzed to verify whether or not
these criteria have been met are identified in Oklahoma’s Use Support Assessment
Protocols (OWRB 2004b). Both of these documents fall under the jurisdiction of the
Oklahoma Water Resources Board.
NPS MANAGEMENT MEASURES
According to the TMDL, croplands account for about 90% of the phosphorus loading in the
watershed; therefore, load reduction efforts should focus on cropland (Table 3). The TMDL
SWAT modeling applied various scenarios relative to landuse and BMPs used in the
watershed to estimate the possible solutions to achieve the recommended 78%
phosphorus load reduction. As shown in Table 5, below, the TMDL evaluated the
Page 16 of 38
June 2009
Update
effectiveness of various BMPs to achieve a phosphorus load reduction. No single BMP
type will fully address the required load reduction; a combination of BMPs will be
necessary.
Table 5. Load reductions for different BMPs (from ODEQ 2006).
Practice
% Reduction In Total Basin Load
Sediment Total N Total P
No-till wheat and row crops -51.10% -42.80% -34.40%
No winter cover on row crops 9.20% 11.10% 6.80%
Worst 1% of cultivated land to pasture -6.00% -3.20% -4.40%
Worst 2.5% of cultivated land to pasture -11.50% -8.10% -8.00%
Worst 5% of cultivated land to pasture -18.00% -13.90% -12.30%
Worst 7.5% of cultivated land to pasture -23.00% -18.30% -15.50%
Worst 10% of cultivated land to pasture -26.50% -21.40% -17.90%
Worst 15% of cultivated land to pasture -33.00% -27.10% -22.10%
Worst 20% of cultivated land to pasture -37.50% -31.10% -25.10%
Worst 25% of cultivated land to pasture -41.50% -34.70% -27.70%
Worst 35% of cultivated land to pasture -48.00% -40.40% -32.00%
Riparian Buffer -75% to -90% -35% to -55% -40% to -60%
Nutrient Management -15% -35%
In addition to the BMPs mentioned above, grade stabilization structures are necessary in
this watershed due to the highly erodible soils; damage is already evident in the watershed
with extensive gullying and rill erosion being relatively common. The SWAT model could
not predict areas where grade stabilization structures would be necessary, nor could it
predict the loading reduction that would result from installation of these structures. Such a
prediction would require extensive reconnaissance in the watershed and ultimately, a
conservation plan for every producer. However, an estimate of the need can be roughly
extrapolated from the need demonstrated with the FY 2001 §319 project, where
approximately 25% of the cooperators required grade stabilization structures to reduce
erosion.
The FY 2001 §319 project funded a targeting exercise based on the SWAT model that was
later expanded into the TMDL. Results of that exercise were used to focus implementation
into areas of origin for the bulk of the sediment and phosphorus loading. Subsequently, the
OCC used these results in conjunction with the recommendations of the TMDL as part of a
FY 2005 §319 project. Figure 6 displays results of the 2003 targeting effort.
Implementation of BMPs in the red areas was expected to reduce nutrient loading to the
watershed by approximately 50%. Implementation of BMPs in the yellow areas could
reduce nutrient loading by an additional 30%.
Page 17 of 38
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Figure 6. Location of areas in Fort Cobb Watershed most likely contributing the
greatest portions of total sediment, and therefore phosphorus loading.
Page 18 of 38
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TECHNICAL AND FINANCIAL ASSISTANCE NEEDED
The amounts of technical and financial assistance needed are closely tied to one another.
All programs to implement NPS BMPs outlined in the above section require technical
assistance in the form of a plan writer, certified by the NRCS. Such a position typically
costs a total of $42,000 - $61,000 per year, including benefits. NRCS funds this technical
support for their own programs (mainly EQIP in this watershed), but programs like a
Conservation Reserve Enhancement Program or §319 must fund technical support through
some other means. In addition, part-time help may be required to address the needs of the
tri-county area. Any staff that provides technical support would be best served to work
through the local conservation district and NRCS offices, as these are the places local
landowners are most comfortable in going to for technical support. Therefore, it is
beneficial to provide assistance to these districts to help support the program.
Funding necessary to implement the BMPs recommended by the TMDL is estimated using
a combination of best professional judgment, based on experience in the watershed, and
use of the PRedICT model. These values are seen in Table 6. An initial value of
approximately $16 million has been estimated as necessary to implement the TMDL
recommended practices. However, this value will likely change as the programs evolve
and the Watershed Based Plan is updated. The actual amount of funding for BMP
implementation in each of the OCC’s projects is given below:
2001 Fort Cobb project (2001-2005):
128 cooperators
$1,386,611 of practices installed,
total:
$365,650 from State funds
$498,054 from Federal 319 funds
$522,907 from landowners (38%)
2005 Fort Cobb project (2005-2008):
60 cooperators
$865,403 of practices implemented,
total:
$502,556 from State funds
$290,250 from Federal 319 funds
$72,597 from landowners (8%)
Table 7 provides some estimates of funding planned or already implemented for technical
support in the watershed. Some of these are multi-year efforts, and some are single-year
efforts. At a minimum, around $160,000 is required for technical support each year to
provide support to the conservation districts and personnel to meet with landowners and
draft conservation plans.
Table 8 estimates funding necessary to support monitoring needs in the watershed. Not all
information is available at this time regarding monitoring costs for USGS or Bureau of
Reclamation; however, available information suggests that at least $230,000 is needed
every five years.
Page 19 of 38
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Update
Table 6. Funding Needs for Technical Support for Implementation of BMPs.
Project/Funding Source Task Federal
State Cost
Share
Funds
Total
FY 2001 §319 Fort Cobb
Project- five year period
On-Site Coordinator $225,000 $225,000
Plan Writer $80,000 $80,000
District Support $75,000
FY 2005 §319 Fort Cobb
TMDL Implementation
Project- salaries and
support for 2 years
beyond 2001 project
On-Site Coordinator $121,000 $121,000
District Support $15,000 $15,000
Conservation Reserve
Enhancement Program
(CREP)- funding for 2-3
years of technical support
Plan Writer $94,000 -
$312,000
$94,000 -
$312,000
NRCS District
Conservationists (3) $52,000 -
$85,0003 $52,000 -
$85,000
Total $609,800 -
$642,800
$94,000 -
$312,000
$703,000 -
$954,800
3 Estimated from GS 9/11 salary range + benefits.
Page 20 of 38
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Update
Table 7. Funding Necessary to Implement TMDL Recommended Practices to Restore
Beneficial Use Support to Fort Cobb Reservoir.
Load Reduction TMDL
Recommended
BMP
Project/Funding
Source
TMDL Federal State/Local Total
target
Anticipated
from this
project
17%
7% No-till in 50% of
wheat and other
row crop
FY 2005 §319
Fort Cobb TMDL
Implementation
$672,380 $586,754 $1,259,1344
10% CSP, EQIP $930,000
25%
Convert 20% of
worst cultivated
land to pasture
FY 2001 §319
Fort Cobb
Project
EQIP, CSP $2,050,0005
30%
1% Riparian Areas
in 60% of
watershed
FY 2001 §319
Fort Cobb
Project
$38,802 $25,867 $64,669
15% 2010 CREP $4,726,790 $945,358 $5,672,148
14% EQIP, CRP, CSP $4,235,204 $1,058,801 $5,294,005
31.5% 31.5%
Nutrient
Management
Plans for 90% of
producers
FY 2001 and
2005 §319
Programs, EQIP,
CRP, CSP
$375,0006
???
??? Grade
Stabilization
Structures
FY 2001 §319
Fort Cobb
Project
$92,804 $61,870 $154,674
??? EQIP,???
Total $15,799,630
4 Represents an estimated start-up costs for no-till on 39% of cropland based on purchase of no-till drills
for the 4 conservation districts, 30% cost-share on purchase of 10 drills for landowners, and $10/acre
incentive payment (rate recommended by Fort Cobb WAG) for a three year period. Does not include
technical support costs seen in Table 3.
5 Assumes a cost of $51 per acre (based on pasture costs in 20% of cultivated land (40,192 acres)
6 $5.00/acre/year for 90% of all crop and pastureland in the watershed, based on annual incentives
offered through other State 319 programs, plus annual cost of soil testing. Most likely would only need to
apply to all cropland, as few producers fertilize pasture, which would reduce costs to $250,000 annually.
Page 21 of 38
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Update
Table 8. Monitoring Funding Needs Associated with Fort Cobb Watershed.
Monitoring
Program Parameters assessed State Federal Total
OCC Rotating
Basin
Stream water quality, biological
community, habitat, hydraulic
budget, riparian condition,
landuse / landcover,
$10,000 -
$30,000 every
5 years
$10,000 -
$30,000
every 5
years
OWRB BUMP
Program Lake Water Quality $10,000
annually $10,000
annually
Watershed
modeling (OSU,
ODEQ, ARS)
Landuse / Land Cover, BMP
implementation, Load reduction
$150,000
every 5
years
USGS Groundwater/Surface Water
Quality, Load reduction ??? ???
Bureau of
Reclamation ??? ??? ???
IMPLEMENTATION SCHEDULE
The TMDL recommends a 78% load reduction from loading seen between 1998 and 2001.
Implementation towards this load reduction has progressed with formal programs such as
the FY 2001 and 2005 §319 Projects and passive changes resulting from the loss of peanut
subsidies. Measures of water quality changes as a result of those efforts are not fully
available at this time; however, information is available on the implementation completed
through the FY 2001 and 2005 programs such that an estimate of potential load reductions
attributed to the project activities thus far has been estimated. These reductions are seen
in Table 7 under the “Load Reduction” column under “Anticipated from this project”.
These efforts are initial steps towards full implementation of the TMDL recommendations.
Table 9 presents a schedule towards implementation of the remaining TMDL
recommendations. Included in table 9 is a column that schedules the evaluation of each
program. Failure of the programs to meet planned implementation level or load reduction
goals will result in adaptations, as possible during the program period or, as necessary,
with follow-up, supplemental programs until the load reduction goals have been met.
The ARS CEAP program provides an excellent opportunity to evaluate the progress of
these programs towards the TMDL-established goals. The Watershed Based Plan will be
updated following the completion of the ARS effort in 2010 to summarize its findings and to
make necessary adaptations to reach the TMDL load reduction goals.
Page 22 of 38
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Update
Table 5. Schedule for Implementation of TMDL-Recommended Practices.
TMDL-recommended
practice
Program proposed to
implement Begin Date Completion
Date Date to evaluate Agency(ies) /
Group(s) involved
No-till 50% of row crops
and wheat pasture
FY 2005 §319 Project October
2005
January
2009 Annually during project,
and following completion
of the CEAP program.
OCC, conservation
EQIP, CSP, ??? Immediate Ongoing districts, USDA
Convert 20% worst
cultivated land to
pasture
FY 2001 §319 Project7 October
2001
September
2006
Annually during the
project, and following
completion of the CEAP
program.
OCC, conservation
districts, USDA
USDA Programs such as
EQIP, CRP, etc. ongoing ongoing following completion of
the CEAP program
NRCS, FSA, ARS,
Conservation Districts
Riparian Buffers in 60%
of Watershed
FY 2001 §319 Fort Cobb
Project
October
2001
September
2006
Annually during the
project, and following
completion of the CEAP
program.
OCC, conservation
districts, USDA
2010 CREP 2010 2025 Annually during the
project period
FSA, NRCS, OCC,
Conservation Districts
EQIP, CRP, CSP, and
??? ongoing ongoing following completion of
the CEAP program
NRCS, FSA, ARS,
Conservation Districts
Nutrient Management
Plans for 90% of
Producers
FY 2001 and 2005 §319
Programs, EQIP, CRP,
CSP, and ???
ongoing ongoing
Annually during the
projects, & following
completion of the CEAP
program
NRCS, FSA, ARS,
Conservation Districts,
OCC
Grade Stabilization
Structures
FY 2001 §319, EQIP,
CSP, and ??? ongoing ongoing
Annually during the
project & following
completion of the CEAP
program
NRCS, FSA, ARS,
Conservation Districts,
OCC
7 The project did not implement much of this conversion; however, based on recommendations of the TMDL, the Project Coordinator attempted to contact
landowners of the worst-cultivated lands to encourage them towards pasture conversion using either the 319 program or USDA programs.
Page 23 of 38
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The following is a summary of the implementation achieved through the OCC’s 2001 and
2005 §319 projects (2001-2008):
21,086 acres of no-till farming
32 grade stabilization structures
8 diversions, 7 grassed waterways,
and 2 terraces
230 acres of riparian area exclusion
fencing
1 stream crossing
10,767 acres of cropland converted to
pasture
957 acres of grass planting for pasture
improvement
35,030 linear ft of cross-fencing
4 wells
4 septic systems
Visible improvements from no-till implemented through the §319 program are obvious throughout
the watershed. Often, large piles of sandy soil accumulate along fence lines and in fields when dry
and windy conditions occur in this area. No-till helped to hold moisture in the soil and reduce the
amount of soil lost by wind and rain erosion, as seen in the photos below (Figure 7). The first two
photos are of a no-till field, while the next two photos are of an adjacent, conventional till field.
Much of the wheat in the conventional till field has been covered by soil which blew or washed over
the plants.
No-till wheat field Fence along no-till wheat
field
Figure 7. Two adjacent wheat fields, the top in no-till and the bottom in conventional till.
Conventional till wheat field Fence along conventional till wheat
field
Eroded soil
mounded along
fence line
Eroded soil
covering wheat in
field
Fence along conventional till
Conventional till wheat field wheat field
Page 24 of 38
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Update
The OCC’s no-till program has resulted in implementation of almost 30% of the TMDL goal
for no-till. An additional 30% of row crops have been converted to conservation tillage, so
at least 60% of the row crop acreage in the watershed is now in some form of conservation
tillage (Table 10). In addition, approximately 63% of the TMDL goal for converting row
crops to pasture has been achieved through the §319 program. NRCS EQIP has provided
funding for both no-till and conservation tillage as well, so additional progress toward the
overall TMDL goal has been made.
Table 10. OCC §319 progress toward TMDL goals, 2001-2008.
Total conventional row crop in basin at start of project: 98,289 acres
BMP
Total Amount
Implemented
(acres)
Goal for
TMDL
(acres)
% Towards
TMDL Goal
Row Crop Converted to No-Till 16,401 58,973 27.8
Row Crop Converted to Conservation Tillage 17,286 58,973 29.3
Convert Worst Row Crop to Pasture 12,462 19,658 63.4
Establish Riparian Buffers 169 8,547 2.0
A phosphorus load reduction of approximately 20% has already been accomplished since
2001 due to a dramatic change in crop production in the watershed (ODEQ 2006).
Specifically, many acres that were used for peanut production have now been converted to
wheat production or pasture. According to the SWAT watershed model (Storm et al. 2006),
if there was 100% conversion of row crops and wheat to no-till, total phosphorus loading
would be expected to decrease by 34%. Based on the conversion of 16,000 acres to no-till,
total phosphorus loading should be reduced by approximately 6%. The maturation of other
BMPs, installed as part of the 2001 and 2005 projects, will further reduce the phosphorus
loading in the watershed.
Approximately one-third of the implementation from 2001-2008 occurred in areas that were
expected to be contributing high levels of phosphorus, according to the SWAT model:
• Of the 9,188.6 acres that were in the top 10% of phosphorus load supplying
areas, 32% now have BMPs on them;
• Of the 10,033.2 acres in the next 10% of high phosphorus areas, 27% have
BMP implementation.
Figure 8, below, shows the overlay of implementation and targeting. Further details about
the OCC implementation projects can be found in the final reports associated with the 2001
and 2005 projects.
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Update
Figure 8. Overlay of regions of high phosphorus loading (targeted regions)
onto areas of BMP implementation through the §319 program, 2001-2008.
A Conservation Reserve Enhancement Program (CREP) is planned for the Fort Cobb
watershed beginning in 2010. This project aims to restore stable riparian vegetation and
riparian buffers and to reduce livestock access to floodplains. This will result in reduced
overland flow of pathogens and phosphorus to the streams and will lessen streambank
erosion by stabilizing stream banks. Overall, this will lead to better water quality, lower
maintenance requirements to the road and highway system, and will help to preserve
existing floodplain cropland, pasture, and rangeland. The WBP will be updated at the
conclusion of the CREP signup to estimate the load reductions expected from this
implementation.
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INTERIM MILESTONES
Interim milestones towards addressing the recommendations of the TMDL will continue to
be developed as activities are implemented under the Watershed Based Plan. Some of
these have already been completed through various project workplans, others are ongoing
or planned.
Project Description Responsible
Party
Target
Date Complete
TMDL
Compile watershed loading model and link
to lake model ODEQ, OSU 2003 X
Calibrate model to water quality
monitoring data ODEQ 2003 X
Develop draft TMDL ODEQ 2004 X
Solicit public input to draft TMDL ODEQ 2005 X
Submit to EPA ODEQ 2005 X
2001
§319
Project
Hire Local staff- project and education
coordinators and plan writer
OCC,
Conservation
Districts (CDs)
2002 X
Establish agreements with CDs OCC, CDs 2001 X
Establish a WAG and EdWAG CDs 2001 X
Complete GIS-Based Targeting OCC, WAG 2001 X
WAG selection of BMPs and cost-share
rates WAG, OCC 2001 X
Watershed Implementation Plan OCC 2002 X
BMP Demonstration OCC, CDs 2002 –
2006 X
Develop education program to educate
producers and other watershed citizens
about problems and solutions
EdWAG 2002 X
Identify oil and gas related sources in the
watershed Corp. Comm 2001 -
2002 X
Hire companies to plug abandoned wells Corp. Comm. As
needed Ongoing
Educate current operators and when
necessary take enforcement actions Corp. Comm. As
needed Ongoing
Sample creeks, streams, and agricultural
lands in watershed for pesticides and
fertilizer-related parameters
ODAFF8 2002 X
Conduct pesticide education programs ODAFF 2001 -
2003 Ongoing
Summary of Project Activities including
estimation of load reduction due to
practices implemented and comparison of
implementation to TMDL
recommendations
OCC, ODAFF,
Corp. Comm. 2006 X
8 Oklahoma Department of Agriculture, Food, and Forestry
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Project Description Responsible
Party
Target
Date Complete
2005
§319
Project
Further delineate targeted areas based on
TMDL recommendations OCC 2006 X
Implement no-till practices OCC, CDs 2006 -
2008 X
Update WBP OCC 2008 X
Follow-up GIS evaluation of
implementation OCC 2008 -
2009 X
Instream Habitat Monitoring to Support
ARS CEAP Project and evaluate success
of BMPs
OCC 2006 -
2008 X
CEAP
Water Quality monitoring, watershed
modeling, and compilation of BMPs
implemented in watershed to evaluate
impacts of BMPs
ARS, NRCS,
OCC
2005 -
2010 Ongoing
CREP
Develop program plan with FSA and
NRCS
OCC, FSA,
NRCS
2003 –
2005 X
Secure State match and Governor’s
approval OCC, OSE 2007 X
Submit plan to USDA OCC, FSA,
NRCS 2009 Planned
Begin implementation OCC, FSA,
NRCS
2010 -
2013 Planned
EQIP
Explore possibility of declaring watershed
a special emphasis area to secure higher
funding level FSA, NRCS,
CDs Annually Ongoing
Continue to implement EQIP practices
annually in watershed
CSP Designate watershed as a CSP priority
watershed
FSA, NRCS,
CDs ??? ???
WBP
Update Watershed Based Plan and
evaluation of progress towards TMDL
goals with watershed modeling at least
every five years or more frequently upon
completion of major tasks/projects
OCC, WAG 2012 Ongoing
Continue water quality monitoring to identify sources,
causes, and progress towards TMDL goals
OWRB, Bureau
of Recl., USGS,
OCC, ARS
Annually Ongoing
PUBLIC OUTREACH
Many local efforts, as well as efforts by state and federal agencies and other organizations,
are collectively contributing to the Public Outreach efforts in the Fort Cobb Watershed.
Public outreach will need to be continued in order to reach the water quality goals of
restoring beneficial use support and attaining water quality standards in the watershed.
This section identifies those agencies, organizations, and services that are active in the
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watershed (in no particular order). To varying degrees, these groups have been, and will
continue to be, active in development and expansion of the Watershed Based Plan and
other planning efforts in the watershed. The roles of these groups and programs are
summarized below:
1. Deer Creek, West Caddo, North Caddo, and Mountain View Conservation Districts
These agencies are critical to ensuring participation of local landowners in water quality
improvement programs. Local Conservation Districts are generally the most effective
means to bring a large federal or state program to private citizens because the local
agencies know the local people. Local agencies often have the most accurate knowledge
concerning current land management practices and local needs. In addition, these
agencies have existing programs and mechanisms directed towards the goals of the WBP.
The Conservation Districts, partnered with the OCC, NRCS, and Cooperative Extension,
have been among the primary agencies responsible for public outreach in the watershed.
The districts and NRCS work one-on-one with citizens of the watershed to reduce pollution
and educate about the importance of protecting water resources. These groups also
organize or participate in seminars, training sessions, and meetings to interact with local
people and provide technical assistance and information. The Deer Creek Conservation
District has a very active education program through its outdoor classroom. This program
targets mainly elementary school children and teaches them about environmental issues.
In addition, Deer Creek has housed the Education Coordinator for the FY 2001 and 2005
§319 Fort Cobb Projects and served as the hub for education activities of that project.
2. Watershed Advisory Group (WAG) and Education Watershed Advisory Group
(EdWAG)
The success of water quality protection programs in the Fort Cobb Watershed depends on
the approval and cooperation of the local landowners and various government agencies.
The WAGs were made up of local shareholders in the watershed (including private citizens,
representatives of local industries, and local government) who provided guidance in
delivering the §319 programs based on information supplied to them by technical agencies
in conjunction with their knowledge of the needs of the watershed residents. The WAGs
were developed to help insure that the programs most effectively worked towards reducing
water quality impacts, but, at the same time, met the needs of and were acceptable to the
local producers and other landowners. The WAG recommended the practices and cost-share
rates to reduce the NPS pollution problems in the watershed. The EdWAG
considered the issues in the watershed and recommended an education program to help
inform watershed citizens about those issues using a “show and tell” approach.
3. The Oklahoma Conservation Commission (OCC)
With the 2001 project, the OCC devoted almost $2.3 million towards a program to educate
citizens and implement best management practices to reduce nonpoint source pollution in
the watershed. A portion of these funds support the WAG, a portion is devoted to
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Update
identifying the major sources in the watershed and monitoring the success of the program,
another portion is devoted towards education, but the majority of the funds provides cost-share
assistance to farmers to implement WAG-recommended and OCC-approved BMPs
to protect the water resources of the watershed. This effort was extended through the FY
2005 program, which focused on recommendations of the TMDL, primarily no-till.
The OCC’s main function is to provide oversight for successful completion of the program.
To do this, they provide technical guidance and final approval to the WAG and local
conservation districts for implementation of the BMPs. The OCC implemented an
education program targeted towards citizens of the watershed whose change in behavior
could have the most substantial impacts on water quality. The OCC is also responsible for
monitoring the success and providing administrative support for the §319 projects, and
working with NRCS and FSA to implement a CREP Program in the watershed.
In addition, Blue Thumb, OCC’s education program, is active in the Fort Cobb watershed.
Streams are monitored by volunteers and school groups are taught about water quality
through this program.
4. Oklahoma Cooperative Extension Service (OCES)
The Oklahoma Cooperative Extension Service (OCES) is another leader in promoting water
quality education efforts in the State, working closely with the conservation districts and the
NRCS to promote water quality awareness. The OCES provides one-on-one meetings and
education with landowners along with group presentations and other forms of technical
assistance to improve awareness in the watershed. The OCES also develops and utilizes
test plots and demonstration sites to educate producers about the effectiveness of certain
best management practices. One such set of test plots, developed by the Oklahoma State
University Cooperative Extension Service, was utilized to demonstrate methods of
integrated pest management and effectiveness of more managed fertilizer application in
wheat production. The OCES also holds public meetings and workshops to educate
landowners on topics such as pesticide and fertilizer management, animal waste issues,
and general BMPs.
5. NRCS Local Offices and FSA (USDA)
The United States Department of Agriculture Natural Resource Conservation Service
(USDA/NRCS) and Farm Services Agency (FSA) in Oklahoma have several programs
active in or that could be expanded in the Fort Cobb Watershed. These programs include
the Environmental Quality Incentives Program (EQIP), Conservation Reserve Program
(CRP) and Conservation Reserve Enhancement Program (CREP), Wildlife Habitat
Incentives Program (WHIP), and the Wetlands Reserve Program (WRP). These programs
are employed by the USDA to help landowners protect natural resources.
6. Oklahoma Corporation Commission (Corp. Comm.)
Corp. Comm., as the state agency with jurisdiction over oil and gas mining activities, has
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ongoing efforts in the watershed to identify and reduce impacts from oil and gas activities.
These include efforts to identify location and severity of erosion related to well sites and
pipelines, followed by cleanup by the operators and pipeline companies. Corp. Comm. will
begin additional work in the watershed to further identify problem areas in the watershed
and initiate educational and other actions for site operators. These efforts range in extent
from informing landowners about who to contact in the case of pollution occurring at well
sites or exploration sites to what best management practices can be utilized during
exploration and operation of oil and gas sites. Another focus of additional planned Corp.
Comm. activities includes efforts to reduce impacts from abandoned oil and gas activities.
7. Oklahoma Department of Agriculture, Food, and Forestry (ODAFF)
The ODAFF has an ongoing project aimed at reducing impacts of fertilizers and pesticides
to surface and groundwater in the watershed. The program has attempted to locate
sources or likely sources of contamination from these fertilizers or pesticides and conduct
educational programs to reduce the impact of those sources.
8. Bureau of Reclamation
Fort Cobb Reservoir is owned by the Bureau of Reclamation, which has played an active
role in the watershed with cooperative efforts towards water quality monitoring, land
management, and education.
9. Agricultural Research Service (ARS)
The ARS is currently pursuing a project to evaluate the success of BMPs implemented in
the watershed through the Conservation Effects Assessment Project (CEAP). This
program will involve water quality monitoring, watershed modeling, and cooperation with
local conservation districts, NRCS, OCC and similar agencies to obtain current information
on management practices in the watershed. Information will be shared regarding the
success of programs and can be used to improve efficiency with cost-share and other
implementation programs, as well as to evaluate progress towards meeting the goals of the
TMDL.
Youth education is a significant effort pursued by OCES, NRCS, and the conservation
districts. Most youth education activities focus on general water quality maintenance and
improvement and include activities such as 4-H group water quality monitoring and
education, “Earth-Day-Every-Day” activities fair where hundreds elementary school children
and some of their parents are exposed to environmental education, and various other
training sessions.
Newspaper articles and other media are a method that can be used to inform citizens of
the watershed about programs focused on water quality. The OCES, Conservation
Districts, and NRCS often contribute articles that were released to local papers, covering a
wide range of topics related to water quality, and more specifically, advertising education
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Update
events and programs. Many articles serve as promotions for various upcoming trainings or
other events. Other media related activities such as radio spots and logo contests can be
used to further the efforts of the program. However, in using media and advertising in
education programs, efforts must focus on measurable results. An information article about
water quality is not enough; the article must be associated with some additional effort that
is likely to change behaviors. Information alone doesn’t often change people’s behaviors;
people must be persuaded to change their behavior. Persuasion is more likely to occur as
part of a program of repeated contact and interaction than as the result of a well-written
article in a newspaper.
Current outreach programs in the watershed will need to expand and perhaps partially
redirect their public outreach efforts to work towards more measurable results. Although
current education efforts are valuable programs, efforts may need to be expanded to insure
that the target audience is being reached. The target audience is the people whose change
of behaviors could have the most substantial benefits to water quality. In other words, the
target audience in the Fort Cobb Watershed should include people such as county
commissioners and road maintenance crews, agricultural producers, and people in the oil
and gas industry, among others. Existing and planned outreach programs will need to
coordinate among themselves and with other ongoing efforts in the watershed in order to
educate more watershed citizens and more importantly, change behaviors of land users in
the watershed.
Public Outreach to assure support of this and future evolutions the Watershed Based Plan
will come from:
• Conservation District Newsletter and/or website
• Continued support the WAG or a similar group
• Public meetings and listening sessions held throughout the local communities (and
eventually, throughout the watershed)
• Regular media coverage of activities/issues (both at local and State levels)
• Education programs such as the ones developed in the 2001 and 2005 §319
projects that involve segments of the community ranging from school children to
agricultural producers to homeowners and lakeside residents
• Programs that encourage local citizens to experience “ownership and
understanding” of environmental issues such as volunteer monitoring, clean-up
events, and other educational grassroots efforts to address the problem
MONITORING PLAN
Every Watershed Based Plan requires a monitoring plan to gage overall success of
restoration and remediation efforts. The goal of the monitoring plan for this WBP will be to
expand current monitoring efforts into a long-range monitoring program with clearly defined
milestones that will oversee the progress towards the TMDL recommended load reductions,
restoration of the beneficial use support in the watershed, and preservation of natural
resources for future generations.
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The monitoring plan for this WBP provides for development of individual monitoring plans
and associated quality assurance plans and Standard Operating Procedures for each
underlying project or effort working toward the ultimate goal of restoration of beneficial use
support. These monitoring efforts must be based on Oklahoma’s Water Quality Standards
and Use Support Assessment Protocols, which define the process by which beneficial use
support can be determined. Technical assistance in developing these plans can come from
various sources including the Oklahoma State Agency peer review process, and the
Oklahoma Water Quality Monitoring Council. In addition, local stakeholders need to be
involved in developing these plans to ensure that the plans address monitoring needs
identified by stakeholders and that stakeholders remain informed about watershed
monitoring activities.
Monitoring methodologies specified in this WBP have been selected to provide: 1) a
quantifiable measure of changes in parameters of concern, 2) success measures that can
be easily understood by cooperators and stakeholders with a variety of technical
backgrounds, and 3) consistent, compatible information throughout the watershed. As the
WBP evolves, it is anticipated that this list will expand and contract.
Monitoring will focus on the primary causes of impairment, as listed in the 303(d) list, but
will also consider related causes that may exacerbate the impacts of the primary causes or
may ultimately reach impairment levels without improved management. The primary types
of monitoring to be conducted in the Fort Cobb Watershed include:
• Surface water quality: nutrients, sediments, suspended solids, fecal bacteria,
dissolved oxygen, temperature, pH, conductivity, alkalinity, hardness, turbidity,
chlorophyll-a, pesticides, BOD
• Hydraulic budget: in-stream flows, infiltration rates, aquifer recovery, groundwater
levels
• Groundwater quality: nutrients, metals, pesticides, pH
• Landuse/Land cover: acreage in different landuses, quality and type of land cover,
timing and other variables of associated management practices
• Riparian Condition: extent and quality of riparian zones in the watershed, to include
quality and type of vegetation, degree of impact or stability, condition of
streambanks, and primary source of threat or impact
• Aquatic Biological Communities: assessment of the condition of fish and benthic
macroinvertebrate communities related to reference streams and biocriteria
• BMP and other implementation effort coverages: type, extent, and when possible,
specific location of practices to include an estimate of the potential load reduction
effected by implementation
• Behavioral change: participation in Watershed Based Plan-related activities and
behavioral changes of affected communities
• Sediment quality: nutrients, pesticides, other organics of concern
With each WBP-related program, as well as for the WBP as a whole, baseline conditions
will be established and monitored prior to implementation. A monitoring schedule and
Quality Assurance Project Plan (QAPP) will be developed based on the type of project and
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Update
timing of its implementation. Monitoring results will be reported to appropriate local, state,
and federal entities as defined in the QAPPs.
Baseline Data
The baseline data to evaluate progress in the Fort Cobb Watershed has been established
by the draft TMDL. This includes watershed data from primarily the period between 1998 –
2001. Specifically, this data is listed below:
• 2000 census data to estimate watershed population and septic tank loading in the
watershed
• SWAT model used:
o Land use was determined using data retrieved from June 10, 2001 30 m
resolution Landsat TM imagery, a crop type breakdown based on 1999-2001
Oklahoma Agricultural Statistics Service data, and center pivot irrigation
locations tagged from aerial photos.
o 1 meter resolution Digital Orthophoto Quarter Quads (DOQQ) from 1995 for
the entire Fort Cobb Basin were used in ground-truthing the Landsat data.
o Soil test phosphorus for common agricultural land covers was derived from
OSU county level averages for the period 1995-1999.
o The model was calibrated for flow for the period January 1990 through
October 2001 and validated for flow in Cobb Creek for the period 1975 –
1989.
o 10 m USGS DEM
o 200 m NRCS MIADS Soils Data
o EPA Reach3 Streams
o National Inventory of Dams
o County level National Agricultural Statistics Service (NASS) cattle estimates
for the period 1996-2000 were combined with land cover data to estimate the
number of cattle within the basin.
o Approximate CAFO locations and animal numbers were taken from an
Oklahoma Department of Agriculture coverage available at the ODEQ
website. The metadata are listed at the following address:
http://www.deq.state.ok.us/deqmap/help/CAFO.htm.
o Few stream gage data were available to calibrate the SWAT Model for the
period Jan 1990 - Oct 2001. The only suitable gage was Cobb Creek near
Eakley (USGS 07325800). The hydrologic calibration was performed almost
entirely with data from this gage. Another gage downstream of the Fort Cobb
Reservoir was also utilized as a check of the calibration.
• OWRB and USFWS lake data collected in 1998-1999 was used to calibrate the
model, and USGS and USFWS data collected in 2000–2001 was used to validate
the model.
• Atmospheric deposition of nutrients was based on annual data for Oklahoma
downloaded from National Atmospheric Deposition Program’s web site. The average
of the data from 1998 to 2001 was used in the model.
• Hourly weather data, daily flow data, and daily loadings (from the SWAT model) to
the lake were also used in the model. Weather data was obtained from Oklahoma
Mesonet for the Fort Cobb station. The data includes hourly atmosphere pressure,
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Update
air temperature, wind speed and direction, relative humidity, rainfall, and solar
radiation. The hydraulic data was downloaded from Army Corps Of Engineer's web
site (http://www.swtwc.usace.army.mil/FCOBcharts.html). The data includes daily
inflow, release, pool elevation, and evaporation. Once again, 1998 – 1999 data was
used in calibration, and 2000 – 2001 data was used in validation.
Data Collection Responsibilities for Current and Future Monitoring
Responsibility for the collection of additional data of the types described above will reside
with project managers of the individual projects as spelled out their individual work plans.
These project managers will be responsible for ensuring that the data is submitted to the
ODEQ for inclusion in the Oklahoma State Water Quality Database, which will ultimately be
uploaded to the National STORET database. Data reporting under individual workplans will
also be the responsibility of the project managers. Monitoring results will be made public
through the ODEQ’s website, at a minimum. In addition, project and monitoring results
should be presented locally with a public meeting or to the WAG or similar group.
In addition to those monitors to be identified in the workplans of the individual projects
under this WBP, the following groups, at a minimum, will be involved in monitoring
activities:
• Oklahoma Water Resources Board: Beneficial Use Monitoring Program and Oklahoma
Water Watch Monitoring Program
• Oklahoma Conservation Commission: Rotating Basin Monitoring Program, Priority
Watershed Project Monitoring, and Blue Thumb Project Monitoring
• U.S. Geological Survey: Surface and Groundwater quality and quantity monitoring and
special studies
• Oklahoma Department of Agriculture, Food, and Forestry: soil sampling associated with
CAFO regulations
• ARS: CEAP associated monitoring
• US Bureau of Reclamation
Currently, the OCC has two sites in the Fort Cobb watershed which are part of the Rotating
Basin monitoring program. These sites were sampled every five weeks from 2004-2006
and will be sampled again from 2009-2011. The parameters measured include water
temperature, dissolved oxygen, pH, specific conductance, alkalinity, turbidity, instantaneous
discharge, nitrate, nitrite, orthophosphate, total phosphorous, total Kjeldahl nitrogen (TKN),
ammonia, chloride, sulfate, total suspended solids, total dissolved solids, 5-day biochemical
oxygen demand (BOD5), and total hardness, as well as biological (fish and
macroinvertebrates) and habitat data.
The OWRB has 6 sites in the reservoir from which physico-chemical data are collected
quarterly. The parameters measured include turbidity, true color, dissolved oxygen, metals,
chloride, sulfates, total dissolved solids, pH, nutrients, temperature, and chlorophyll-a.
The USGS has 5 “real time” gauging stations in streams in the Fort Cobb watershed, as
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Update
well as one reservoir station and a meteorological station from which data may be
accessed. The parameters collected include temperature, instantaneous discharge,
conductivity, dissolved oxygen, pH, nutrients, suspended sediments, and alkalinity.
The ARS has been monitoring 15 sites in the Fort Cobb watershed since 2004 as part of a
national CEAP Watershed Assessment Study. Fortunately, Fort Cobb is included within
one of the 12 benchmark watersheds in the US, and as a result, ARS, working
collaboratively with the Great Plains RC&D, will complete an extensive bi-weekly water
quality monitoring program. This program includes monitoring of the following paramters:
pH, dissolved oxygen, conductivity, salinity, total dissolved solids, temperature, turbidity,
oxygen reduction potential, nitrate concentration, ammonia concentration, suspended
sediment, and phosphorus. The Great Plains RC&D will work collaboratively with ARS to
contact farmers to obtain conservation and production management information relevant to
the assessments.
Benefits of the Monitoring Plan
Implementation of this monitoring plan will enable Fort Cobb partners to meet the goals of
the WBP, which is ultimately to restore beneficial use support to waters of the Fort Cobb
Watershed. Implementation of the monitoring plan will help further define areas of the
watershed where restoration activities should be focused to realize the optimum benefit for
the investment as well as evaluating the impacts (realized and potential) of implementation
efforts. Collection of the data described under this monitoring plan will help define the
relative contributions from various sources in the watershed and the processes contributing
to water quality degradation in the watershed. And finally, continued collection of this data
and evolution of the monitoring plan for the watershed will allow the program to adapt to
meet the changing needs of watershed protection in the Fort Cobb Watershed.
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REFERENCES
ODEQ. 2008. The State of Oklahoma 2008 Water Quality Assessment Integrated Report.
Oklahoma Department of Environmental Quality, Oklahoma City, OK.
ODEQ. 2006. TMDL Development for Fort Cobb Creek Watershed and Fort Cobb
Lake: FY 99 Section 319(h) Grant #C9996100-07 Final Report. Oklahoma
Department of Environmental Quality, Oklahoma City, OK.
OWRB. 2002. 2002 Report of Oklahoma Beneficial Use Monitoring Program.
Oklahoma Water Resources Board, Oklahoma City, OK.
OWRB. 2004a. Oklahoma Water Quality Standards, Oklahoma Administrative Code,
Chapter 45. Oklahoma Water Resources Board, Oklahoma City, OK.
OWRB. 2004b. Implementation of Oklahoma’s Water Quality Standards, Oklahoma
Administrative Code, Chapter 46. Oklahoma Water Resources Board, Oklahoma
City, OK.
Storm, D. E., M.J. White, and S. Stoodley. 2003. Fort Cobb Modeling and Land Cover
Classification Final Report. Oklahoma State University Biosystems and
Agricultural Engineering. Stillwater, OK.
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Update
APPENDIX A:
TMDL Development For Cobb Creek Watershed
And Fort Cobb Lake
FY99 Section §319(h) Grant #C9996100-07
FINAL REPORT
2006
TMDL Development For Cobb Creek Watershed
And Fort Cobb Lake
FY99 Section 319(h) Grant #C9996100-07
FINAL REPORT
Prepared by Paul Yue
Oklahoma Department of Environmental Quality
June 26, 2006
June 26, 2006
Table of Contents
EXECUTIVE SUMMARY …………………………………………………………………….vi
1. INTRODUCTION................................................................................................................... 1
1.1 LATEST REVISION.................................................................................................................. 1
1.2 INTRODUCTION..................................................................................................................... 1
2. PROBLEM DEFINITION...................................................................................................... 6
3. APPLICABLE WATER QUALITY STANDARDS............................................................. 8
3.1 STANDARDS FOR STREAMS.................................................................................................... 8
3.1.a. Standards for nutrients ................................................................................................ 8
3.1.b. Standards for Dissolved Oxygen................................................................................ 10
3.2 STANDARDS FOR FORT COBB LAKE ..................................................................................... 10
3.3 PESTICIDE STANDARDS ....................................................................................................... 11
3.4 ANTIDEGRADATION POLICY ................................................................................................ 13
4. IMPAIRMENT ASSESSMENT & TMDL TARGETS...................................................... 14
4.1. STATUS OF NUTRIENT IMPAIRMENT IN STREAMS ................................................................ 14
4.1.a. Data from OCC.......................................................................................................... 14
4.1.b. Data from USGS ........................................................................................................ 16
4.2. STATUS OF NUTRIENT IMPAIRMENT IN FORT COBB LAKE................................................... 21
4.3. STATUS OF PESTICIDE IMPAIRMENT .................................................................................... 24
4.3.a. Lake Creek ................................................................................................................. 25
4.3.b. Cobb Creek ................................................................................................................ 25
4.3.c. Fort Cobb Lake .......................................................................................................... 27
4.4. STATUS OF DISSOLVED OXYGEN IMPAIRMENT FOR LAKE CREEK ....................................... 27
4.5. ENDPOINT AND TARGETS FOR FORT COBB TMDL.............................................................. 28
5. SOURCE ASSESSMENT ..................................................................................................... 29
5.1. ASSESSMENT OF POINT SOURCES ....................................................................................... 29
5.2. ASSESSMENT OF NONPOINT SOURCES ................................................................................ 31
5.2.a. Septic Systems ............................................................................................................ 31
5.2.b. Migratory Birds ......................................................................................................... 33
5.2.c. SWAT model for Nonpoint Source Loadings ............................................................. 36
6. MODEL DEVELOPMENT.................................................................................................. 41
6.1. MODEL SELECTION ............................................................................................................ 41
6.2. MODEL SETUP.................................................................................................................... 42
6.2.a. Watershed Representation ......................................................................................... 42
6.2.b. Lake Representation................................................................................................... 42
6.2.c. Selection of Model Simulation Period........................................................................ 43
6.2.d. Model Inputs .............................................................................................................. 45
6.3. MODEL CALIBRATION ........................................................................................................ 46
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June 26, 2006
6.3.a. Hydrodynamics .......................................................................................................... 46
6.3.b. Water Quality............................................................................................................. 47
6.4. MODEL VERIFICATION ....................................................................................................... 57
6.4.a. Hydrodynamics .......................................................................................................... 57
6.4.b. Water Quality............................................................................................................. 57
7. NUTRIENT REDUCTION................................................................................................... 64
7.1. LOAD REDUCTION .............................................................................................................. 64
7.2. MARGIN OF SAFETY AND LOAD ALLOCATION.................................................................... 71
7.3. BEST MANAGEMENT PRACTICES........................................................................................ 73
7.3.a. Effectiveness of Best Management Practices............................................................. 73
7.3.b. Options for Implementing BMPs ............................................................................... 80
8. PUBLIC PARTICIPATION................................................................................................. 82
9. REFERENCES...................................................................................................................... 84
10. APPENDIX A....................................................................................................................... 88
ii
June 26, 2006
List of Figures
FIGURE 1-1. COBB CREEK WATERSHED STUDY AREA................................................................. 5
FIGURE 3-1. STREAM ORDER (COBB CREEK WATERSHED).......................................................... 9
FIGURE 4-1. TOTAL-P, NO3/NO2 CONCENTRATION IN LAKE CREEK ......................................... 15
FIGURE 4-2. TOTAL-P, NO3/NO2 CONCENTRATION IN TRIBUTARY OF LAKE CREEK ................ 16
FIGURE 4-3. USGS MONITORING STATIONS (PROVIDED BY USGS) .......................................... 17
FIGURE 4-4. TOTAL-P, NO3/NO2 CONCENTRATION IN WILLOW CREEK .................................... 18
FIGURE 4-5. TOTAL-P, NO3/NO2 CONCENTRATION IN LAKE CREEK ......................................... 19
FIGURE 4-6. TOTAL-P, NO3/NO2 CONCENTRATION IN COBB CREEK......................................... 20
FIGURE 4-7. OWRB MONITORING STATIONS IN FORT COBB LAKE........................................... 22
FIGURE 4-8. USFWS MONITORING STATIONS........................................................................... 23
FIGURE 5-1 STORM WATER DISCHARGES ................................................................................. 30
FIGURE 5-2. 2000 U.S. CENSUS BLOCKS IN COBB CREEK WATERSHED .................................... 32
FIGURE 5-3. SUBBASIN LAYOUT USED IN THE COBB CREEK SWAT MODEL ............................. 39
FIGURE 5-4. LAND USE COVERAGE ........................................................................................... 40
FIGURE 6-0. ANNUAL PARTICIPATION FOR FORT COBB WATERSHED........................................ 44
FIGURE 6-1. BATHYMETRIC AND COMPUTATIONAL GRID OVERLAY - FORT COBB LAKE.......... 44
FIGURE 6-2. COMPARISON OF MODELED AND OBSERVED LAKE ELEVATION............................. 48
FIGURE 6-3. TEMPERATURE PROFILE NEAR THE DAM................................................................ 49
FIGURE 6-3A. TEMPERATURE PROFILE NEAR THE DAM................................................................ 50
FIGURE 6-4. CHLOROPHYLL-A CONCENTRATION NEAR THE DAM............................................. 51
FIGURE 6-5. CHLOROPHYLL-A IN THE MIDDLE PART OF THE LAKE ........................................... 51
FIGURE 6-6. CHLOROPHYLL-A IN THE UPPER PART OF THE LAKE.............................................. 51
FIGURE 6-7. TROPHIC STATE INDEX NEAR THE DAM ................................................................. 52
FIGURE 6-8. TROPHIC STATE INDEX IN THE MIDDLE PART OF THE LAKE................................... 52
FIGURE 6-9. TROPHIC STATE INDEX NEAR IN THE UPPER PART OF THE LAKE............................ 53
FIGURE 6-10. DISSOLVED OXYGEN NEAR THE DAM .................................................................... 53
FIGURE 6-11. DISSOLVED OXYGEN IN THE MIDDLE PART OF THE LAKE...................................... 54
FIGURE 6-12. DISSOLVED OXYGEN IN THE UPPER PART OF THE LAKE ........................................ 54
FIGURE 6-13. TOTAL-P NEAR THE DAM....................................................................................... 54
FIGURE 6-14. TOTAL-P IN THE MIDDLE PART OF THE LAKE ........................................................ 55
FIGURE 6-15. TOTAL-P IN THE UPPER PART OF THE LAKE........................................................... 55
FIGURE 6-16. LAKE ELEVATION (2000)....................................................................................... 59
FIGURE 6-17. WATER TEMPERATURE NEAR THE DAM (2000)..................................................... 59
FIGURE 6-18. TROPHIC STATE INDEX NEAR THE DAM (2000)...................................................... 60
FIGURE 6-19. TROPHIC STATE INDEX IN THE MIDDLE PART OF THE LAKE (2000) ....................... 60
FIGURE 6-20. TROPHIC STATE INDEX IN THE UPPER PART OF THE LAKE (2000).......................... 60
FIGURE 6-21. CHLOROPHYLL-A NEAR THE DAM (2000)............................................................... 61
FIGURE 6-22. CHLOROPHYLL-A IN THE MIDDLE PART OF THE LAKE (2000) ................................ 61
FIGURE 6-23. CHLOROPHYLL-A IN THE UPPER PART OF THE LAKE (2000)................................... 61
FIGURE 6-24. DISSOLVED OXYGEN NEAR THE DAM (2000) ......................................................... 62
FIGURE 6-25. DISSOLVED OXYGEN IN THE MIDDLE PART OF THE LAKE (2000) .......................... 62
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June 26, 2006
FIGURE 6-26. DISSOLVED OXYGEN IN THE UPPER PART OF THE LAKE (2000)............................. 62
FIGURE 6-27. TOTAL-P NEAR THE DAM (2000) ........................................................................... 63
FIGURE 6-28. TOTAL-P IN THE MIDDLE PART OF THE LAKE (2000)............................................. 63
FIGURE 6-29. TOTAL-P IN THE MIDDLE PART OF THE LAKE (2000)............................................. 63
FIGURE 7-1. CARLSON’S TSI NEAR THE DAM (REDUCTION)...................................................... 67
FIGURE 7-2. CARLSON’S TSI IN THE MIDDLE PART OF THE LAKE (REDUCTION) ....................... 67
FIGURE 7-3. CARLSON’S TSI IN THE UPPER PART OF THE LAKE (REDUCTION).......................... 67
FIGURE 7-4. CHLOROPHYLL-A NEAR THE DAM (REDUCTION).................................................... 68
FIGURE 7-5. CHLOROPHYLL-A IN THE MIDDLE PART OF THE LAKE (REDUCTION) ..................... 68
FIGURE 7-6. CHLOROPHYLL-A IN THE UPPER PART OF THE LAKE (REDUCTION)........................ 68
FIGURE 7-7. DISSOLVED OXYGEN NEAR THE DAM (REDUCTION) .............................................. 69
FIGURE 7-8. DISSOLVED OXYGEN IN THE MIDDLE PART OF THE LAKE (REDUCTION) ............... 69
FIGURE 7-9. DISSOLVED OXYGEN IN THE UPPER PART OF THE LAKE (REDUCTION) .................. 69
FIGURE 7-10. TOTAL-P NEAR THE DAM (REDUCTION)................................................................. 70
FIGURE 7-11. TOTAL-P IN THE MIDDLE PART OF THE LAKE (REDUCTION) .................................. 70
FIGURE 7-12. TOTAL-P IN THE UPPER PART OF THE LAKE (REDUCTION)..................................... 70
FIGURE 7-13. SEDIMENT REDUCTION VS. TOTAL-P REDUCTION.................................................. 75
FIGURE 7-14. TOTAL N REDUCTION VS. TOTAL-P REDUCTION.................................................... 76
FIGURE 7-15. SEDIMENT LOAD REDUCTION DUE TO CONVERSION OF CULTIVATED LAND TO
PASTURE ................................................................................................................ 77
iv
June 26, 2006
List of Tables
TABLE 2-1: 1998 303(D) LIST FOR THE COBB CREEK WATERSHED ........................................... 6
TABLE 2-2. 2002 303(D) LIST FOR COBB CREEK WATERSHED ................................................... 7
TABLE 3-1: THRESHOLD VALUES FOR STREAM TOTAL-P AND NO2+NO3 ................................ 10
TABLE 3-2. PUBLISHED LC50 VALUES FROM DIFFERENT SOURCES ......................................... 13
TABLE 4-1. OCC WATER QUALITY MONITORING STATIONS.................................................... 15
TABLE 4-2. SUMMARY OF TSI DATA........................................................................................ 23
TABLE 4-3. LC-50 VALUES AND TARGET CRITERIA FOR PESTICIDES ......................................... 24
TABLE 4-4. LAND USE COMPARISON FOR COBB CREEK AND LAKE CREEK SUB-BASINS........... 26
TABLE 4-5. PESTICIDE LOADINGS (APRIL 1999 – AUGUST 1999)............................................. 26
TABLE 5-1. ESTIMATED POPULATION IN COBB CREEK WATERSHED........................................ 32
TABLE 5-2. PHOSPHORUS LOADING TO LAKES FROM WATERFOWLS........................................ 34
TABLE 5-3. LAND USE COVERAGE IN THE FORT COBB LAKE WATERSHED.............................. 37
TABLE 6-1. SURFACE AREA AND VOLUME OF FORT COBB LAKE ............................................. 43
TABLE 7-1. NUTRIENT REDUCTION RATE ................................................................................. 65
TABLE 7-2. LOAD ALLOCATIONS.............................................................................................. 72
TABLE 7-3. SIMULATED ANNUAL LOADS BY LAND USE FOR THE FORT COBB BASIN FOR THE
PERIOD 1990-2000 ................................................................................................ 74
TABLE 7-4. LOAD REDUCTIONS FOR DIFFERENT BMPS ........................................................... 75
TABLE 7-5. RELATIVE EFFECTIVENESS OF NUTRIENT MANAGEMENT ...................................... 78
TABLE 7-6. REDUCTION RATE FOR SEDIMENT AND NUTRIENTS FOR VARIOUS BMPS.............. 79
v
June 26, 2006
Executive Summary
Cobb Creek watershed includes two HUC 11 watersheds, 11130302120 & 11130302130 and
crosses three counties in west-central of Oklahoma. Fort Cobb is located at the lower end of the
watershed and there are four tributaries (Cobb Creek, Lake Creek, Willow Creek, and Fivemile
Creek) contributing to the lake. The watershed is primarily rural. There is no point source
discharge in the watershed.
Fort Cobb Lake and four tributaries were listed in the Oklahoma 1998 303(d) list for nutrients,
suspended solids, siltation, and pesticides. Fort Cobb Lake, Lake Creek and Willow Creek are
listed in the 2002 303(d) list. This TMDL report addresses both the 1998 and 2002 303(d) lists.
There are several federal and state agencies collecting water quality data in the watershed. Data
used in this project are gathered from U.S. Geological Survey, U.S. Bureau of Reclamation, U.S.
Fish and Wildlife Service, Oklahoma Water Resources Board and Oklahoma Conservation
Commission. The data were first used to check the status of impairments for all tributaries and
Fort Cobb Lake. It was concluded that Cobb Creek, Lake Creek, Willow Creek and Fivemile
Creek were not impaired with regard to nutrients and pesticides. It was also concluded that the
Fort Cobb Lake was not impaired for pesticides.
The Fort Cobb Lake was used as the endpoint in the TMDL project. The TMDL targets were
dissolved oxygen, anoxic volume and Trophic State Index (TSI) in the lake. Two water quality
models were employed to link pollutant sources to water quality targets. A SWAT (Soil and
Water Assessment Tool) model was calibrated to simulate nutrient loads to the lake. A three
dimensional EFDC (Environmental Fluid Dynamic Code) model was calibrated and verified to
model water quality in Fort Cobb Lake. The calibrated EFDC model was then used to predict
how much reduction would be needed to restore the Fort Cobb Lake to meet all Oklahoma water
quality standards. As a result, the model called for 78% reduction in nutrient load from the
watershed. Due to the BMPs implemented in the recent years, it was estimated by the SWAT
model that about 20% nutrient reduction had been achieved as of 2005. In order to achieve the
recommended nutrient reduction, sediment load to streams and the lake will also be reduced.
Therefore, the suspended solids and siltation impairments in Cobb Creek, Lake Creek, Willow
Creek and Fivemile Creek are also addressed by this TMDL.
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FINAL June 26, 2006
1. Introduction
1.1 Latest Revision
This TMDL report for Cobb Creek Watershed and Fort Cobb Lake was first drafted in 2004 and
went through peer reviews among state agencies. Then, the report was sent to the EPA for
technical review. After receiving the EPA’s technical approval, the report was open for public
review on November 24, 2004. A public meeting was held in the Town of Fort Cobb on January
13, 2005. The public review period ended on February 25, 2005. Five written comments were
received during the public review period. Not all comments are addressed through the response
to the comments process because the SWAT model for the watershed was recalibrated which
leads to recalibration of the EFDC model for the lake. As a result, the following significant
changes have been made to the TMDL reduction goal and this TMDL report:
1). Update on the SWAT Model
Since there were many questions on land use, tillage, fertilizer application rate, hydraulic
calibration and so on, Oklahoma State University conducted a new survey in the Cobb Creek
watershed to collect additional data. A detailed survey was given in 2005 to Oklahoma State
University (OSU) Cooperative Extension Service Agents and Specialists to gain an
understanding of agricultural practices and land cover that occurred from 1996 to 2001. This
survey went into great detail about the different types of crops in the basin along with different
tillage practices, common double crops, fertilization rates, cattle stocking rates, and harvest
dates. With the newly collected data, OSU recalibrated the SWAT model. A pond option was
also added to the SWAT model during the recalibration process. As a result, the SWAT model
calibration was greatly improved. The newly calibrated model was used to generate nutrient
inputs to the Fort Cobb Lake.
It should be emphasized that the SWAT model was calibrated to the conditions when water
quality data were collected. Since then, the land cover in the watershed has been changed and
certain BMPs have been implemented. In order to evaluate the improvement in nutrient
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FINAL June 26, 2006
reduction that has occurred in the past few years, OSU also updated the SWAT model with 2005
land cover. The updated SWAT model predicted that on average 20% phosphorus reduction has
been achieved since 2001.
2). Update on the EFDC Model
Although there is little difference in the average annual total phosphorus loadings (1995-2000)
between the current and previous SWAT model, the difference in loadings from year to year
ranges from -37% to 43%, especially for the calibration and verification periods of the EFDC
model (as shown in red in the following table). The difference is significant enough to require a
new calibration of the EFDC model for the Fort Cobb Lake.
Year Previous MoTdoetla l P (kg/Cyur)r rent Model Difference
1995 257794 197000 30.9%
1996 34543 50000 -30.9%
1997 93353 104000 -10.2%
1998 75933 53000 43.3%
1999 47922 76000 -36.9%
2000 53741 81000 -33.7%
Average 93881 93500 0.4%
Trophic State Index (TSI) is the only TMDL target which is not met currently in the Fort Cobb
Lake. Thus, TSI is the control factor in determining the reduction goal for this TMDL. A point-to-
point comparison between predicted and observed TSI data and R2 which measures the
goodness-of-fit were added to the TMDL report in the model calibration. In addition, the same
comparison was made for lake elevation and temperature calibration. Vertical temperature
profiles were also added to the report to enhance the hydrodynamic calibration.
The recalibrated EFDC model was then used to predict the nutrient reduction rate needed to meet
all TMDL targets. Due to the significant change in nutrient inputs to the lake, the TMDL
reduction goal increased from 65% to 78%.
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FINAL June 26, 2006
3). Nutrient Input from Migratory Birds
One comment suggested that direct defecation by migratory birds or waterfowl might be an
important nutrient source. One section was added to this report to address the potential nutrient
additions from waterfowl to Fort Cobb Lake.
Annual mid-winter waterfowl surveys were obtained from U.S. Fish and Wildlife Service for this
assessment. Waterfowl in the lake are primarily ducks and small Canadian geese. The
waterfowl phosphorus addition to the lake is estimated less than 2% of non-point source loading
and primarily occurs in the winter. Therefore, we believe that waterfowl will have little impact
on algae growth in the summer.
4). Other Revisions
In addition to the above major updates, many other changes were also made to this report. These
changes include annual precipitation plot and EFDC control files etc. The annual rainfall data
from 1975 to 2001 were plotted so that one would be able to see the representativeness and
appropriateness of the calibration and verification period. The EFDC’s master control files were
attached at the end of this report so that those interested in the model parameters could check the
final parameters used in the EFDC model.
1.2 Introduction
Under Section 303(d) of the Clean Water Act (CWA) as amended by the Water Quality Act of
1987 and the United States Environmental Protection Agency’s (EPA) Water Quality Planning
and Management Regulations [Title 40 of the Code of Federal Regulation (40 CFR), Part 130],
states, territories, and authorized tribes are required to develop lists for those waters within their
boundaries not meeting water quality standards applicable to their designated uses. States are
also required to establish priority rankings for waters on the list and develop Total Maximum
Daily Loads (TMDLs) for all pollutants violating or causing violation of applicable water quality
standards for each identified waterbody in the list.
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FINAL June 26, 2006
A TMDL specifies the maximum amount of a pollutant that a waterbody can receive while still
meeting water quality standards, and allocates pollutant load among all point and nonpoint
pollution sources. Such loads are established at levels necessary to meet the applicable water
quality standards with consideration given to seasonal variations and margins of safety. The
TMDL process establishes the allowable loadings of pollutants or other quantifiable parameters
for a waterbody based on the relationship between pollution sources and in-stream water quality
conditions. States then establish water quality-based controls and programs to reduce pollution
from both point and nonpoint sources and restore and maintain the quality of their water
resources [2].
Oklahoma’s 1998 303(d) list identified all major streams (Cobb Creek, Lake Creek, Willow
Creek, Fivemile Creek) and Fort Cobb Lake in the Cobb Creek watershed as not supporting their
designated beneficial uses due to nutrients, suspended solids, siltation, pesticides, exotic species,
unknown toxicity, and/or other habitat alterations. By definition, TMDLs can only be developed
for specific pollutants. Exotic species, unknown toxicity and other habitat alterations are not
pollutants that cause impairments of water being studied and are not within the scope of this
report. This report addresses the remaining pollutants in the Cobb Creek watershed.
Cobb Creek watershed includes two HUC 11 watersheds, 11130302120 & 11130302130, which
include portions of Caddo, Washita, and Custer counties in Oklahoma (Figure 1-1). At the lower
end of the Cobb Creek watershed is Fort Cobb Lake.
Land use in the Cobb Creek watershed consists of forest (6%), pasture (41.4%), agricultural land
(50.4%), water (2.1%) and urban area (0.1) [17]. The watershed is in one of the most intensive
agricultural farming areas of the state. Over half of the state’s peanuts are grown in or near the
watershed, along with wheat, alfalfa and many other row crops [6]. The soils are very coarse and
fragile, allowing for high infiltration rates and excessive erosion.
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FINAL June 26, 2006
FIGURE 1-1. COBB CREEK WATERSHED STUDY AREA
This study consists of two modeling efforts: a watershed model to estimate non-point source
loadings to the Fort Cobb Lake and a lake model to simulate hydrodynamics and water quality
conditions in the lake and make comparisons to the applicable water quality criteria.
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FINAL June 26, 2006
2. Problem Definition
Fort Cobb Lake and four streams were included in the Oklahoma 1998 303(d) list due to
nutrients, suspended solids, siltation, pesticides, exotic species, unknown toxicity, and/or other
habitat alternations. Since exotic species, unknown toxicity and other habitat alterations are not
pollutants, they will not be included in this TMDL study and are not included in the following
table.
TABLE 2-1: 1998 303(d) LIST FOR THE COBB CREEK WATERSHED
Waterbody ID Name Area (acres)/
Length
(miles)
Nutrients Siltation Suspended
Solids
Pesticide
OK310830060020 Fort Cobb Lake 3806 X X
OK310830060010 Cobb Creek 17.3 X X X X
OK310830060080 Fivemile Creek 12.2 X X X
OK310830060040 Lake Creek 16.3 X X X X
OK310830060030 Willow Creek 11.0 X X X
All stream segments in Table 2-1 were assigned priority 3 in the 1998 Oklahoma 303(d) list.
Since there are no permitted point source discharges in the entire watershed, the potential
impairments are caused by the non-point sources in the watershed such as agricultural activities,
cattle and limited small concentrated animal feeding operations (CAFO) in the watershed. There
are two CAFOs in the watershed that are considered to be insignificant in the Soil and Water
Assessment Tool (SWAT) model conducted by Oklahoma State University.
Because of the way the 303 (d) list was compiled and new information obtained through
continuing data collection efforts, the 1998 303(d) list was revisited and reevaluated to determine
whether the beneficial uses of waterbodies were still impaired by the listed pollutants. The
Oklahoma 2002 Water Quality Assessment Integrated Report indicated that siltation impairments
for Cobb Creek, Lake Creek, Willow Creek and Fivemile Creek and suspended solids
impairments for Fort Cobb Lake, Cobb Creek, Lake Creek Willow Creek and Fivemile Creek
were listed in error based on samples collected under high flow conditions. The siltation and
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FINAL June 26, 2006
suspended solids impairments for Lake Creek were corrected to turbidity impairment in the
Oklahoma’s 2002 303(d) list.
The Oklahoma 2002 303(d) list (Table 2-2) shows the latest status of impairments and
impairment source codes for streams and lakes in the watershed. The source code of 9000 in
Table 2-2 stands for unknown source. The impairments for Cause Unknown and Pathogens are
beyond the scope of this study and therefore will not be addressed in this report. The remaining
pollutants, together with those in Table 2-1, are re-evaluated in this TMDL report.
TABLE 2-2. 2002 303(D) LIST FOR COBB CREEK WATERSHED
Waterbody ID Name Cause
Unknown
Turbidity Phosphorus Low
DO
Pathogens
OK310830060020_00 Fort Cobb Lake 9000
OK310830060040_00 Lake Creek 9000 9000 9000
OK310830060030_00 Willow Creek 9000
Fort Cobb Lake and all the streams in the watershed are designated in Oklahoma Water Quality
Standards for the following beneficial uses:
• Public and Private Water Supply
• Warm Water Aquatic Community
• Agriculture
• Industrial & Municipal Process and Cooling Water
• Primary Body Contact Recreation
• Aesthetics
• Sensitive Public and Private Water Supply
In addition, the Fort Cobb watershed is also classified as a Nutrient Limited Watershed (NLW).
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FINAL June 26, 2006
3. Applicable Water Quality Standards
3.1 Standards for Streams
3.1.a. Standards for nutrients
The Oklahoma Water Quality Standards (OWQS) do not have numerical criteria for nutrients
that apply to the streams in the Cobb Creek Watershed. However, they contain the following
narrative standard that applies to all streams and lakes in the state:
“785:45-5-19 (c) (2) Nutrients. Nutrients from point source discharges or other
sources shall not cause excessive growth of periphyton, phytoplankton, or aquatic
macrophyte communities which impairs any existing or designated beneficial use”.
The rules for implementation of Oklahoma’s Water Quality Standards (OAC 785-46-15) [4]
provide a framework that is used in assessing threats to waterbodies or impairments to beneficial
uses by nutrients. The implementation rules describe a dichotomous process to be used in
determining whether or not a stream is nutrient-threatened. If the dichotomous process indicates
a stream is not threatened by nutrients, the stream will be considered not impaired by nutrients.
The dichotomous process uses the follow factors to determine if a stream is threatened by
nutrients:
• Stream order
• Stream slope
• Total-Phosphorus (P) concentration
• Nitrate plus nitrite concentration
• Canopy shading
• Turbidity
The application of this dichotomous process to streams in Cobb Creek watershed was utilized to
derive the threshold concentrations for Total-P and nitrate plus nitrite. If the mean value of
Total-P and nitrate plus nitrite samples in a stream is below their corresponding threshold value,
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FINAL June 26, 2006
the stream is considered not threatened by nutrients. Table 3-1 shows stream order, slope and the
threshold values for Total-P and nitrate plus nitrite for streams in the Cobb Creek watershed.
As shown in Figure 3-1, the stream order is determined using the BASINS rf3 reach file [9]. The
stream orders given in Table 3-1 are for those segments where samples were taken.
FIGURE 3-1. STREAM ORDER (COBB CREEK WATERSHED)
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FINAL June 26, 2006
TABLE 3-1: THRESHOLD VALUES FOR STREAM TOTAL-P AND NO2+NO3
Stream
Stream
Order
Slope
(ft/mile)
Total-P
(mg/L)
NO2 + NO3
(mg/L)
Willow Creek 2 <17 0.15 2.40
Lake Creek 2 <17 0.15 2.40
Trib to Lake Creek 1 ≥ 17 0.24 4.95
Cobb Creek 4 <17 0.36 5.00
3.1.b. Standards for Dissolved Oxygen
The Oklahoma Water Quality Standards (OWQS) has the following criteria for dissolved
oxygen:
Summer (Jun 16 – Oct 15): 4 mg/L
Seasonal (Oct 16 – Jun 15): 5 mg/L
The dissolved oxygen criteria must be maintained at all time.
3.2 Standards for Fort Cobb Lake
The Oklahoma Water Quality Standards do not contain numerical standards for nutrients and
suspended solids; only narrative standards for nutrients and suspended solids can be found in the
OWQS. However, it is very difficult to use narrative standards as the targets of this TMDL. The
targets of a TMDL need to be numerical or quantified in some way.
Fort Cobb Lake and its watershed are classified in the OWQS as Nutrient-Limited Watershed
(NLW). Nutrient-Limited Watershed, by definition, means a watershed of a waterbody with a
designated beneficial use that is adversely affected by excess nutrients as determined by
Carlson’s Trophic State Index (using chlorophyll-a) of 62 or greater. According to the
Implementation of Oklahoma’s Water Quality Standards [4], the beneficial uses designated for
Fort Cobb Lake are presumed to be fully supported but threatened. Since the lake is considered
threatened when Carlson’s Trophic State Index (TSI) is 62 or greater, a TSI value less than 62
was chosen as one endpoint of this TMDL.
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FINAL June 26, 2006
In addition to TSI, dissolved oxygen criteria in the Oklahoma Water Quality Standards and the
Implementation of Oklahoma’s Water Quality Standards also apply to Fort Cobb Lake. The
following endpoints are identified for this TMDL:
• Dissolved Oxygen (DO) for the surface water must meet the following requirements:
o Summer (Jun 16 – Oct 15): 4.0 mg/L
o Seasonal (Oct 16 – Jun 15): 5.0 mg/L
• Anoxic volume of water column in the lake must be less than 50%. The anoxic volume is
defined as the vertical water column where the dissolved oxygen concentration is less
than 2 mg/L.
• Carlson’s Trophic State Index (TSI) must be less than 62. TSI can be calculated as
follows:
TSI = 9.81 × Ln (chlorophyll-a) + 30.6
The unit of chlorophyll-a is μg/L.
Dissolved oxygen criteria must be maintained at all times. Anoxic volume and TSI criteria could
not be exceeded more than 10% of the time in order to achieve compliance.
3.3 Pesticide Standards
Because Alachlor and Aldicarb were detected in both surface water and streamside seepage
samples, pesticides were identified in the1998 303(d) list as a cause of impairment.
To determine whether the surface water is actually impaired, water quality criteria for the
surface water need to be checked. Review of the pesticide monitoring data for Lake Creek
indicates that none of the pesticides tested exceeds any water quality standards. .
Oklahoma Water Quality Standards do not have any numerical criteria specifically for Alachlor
or Aldicarb. The following requirements for toxic substances in general apply:
“For toxicants not specified in Table 2 of Appendix G of this Chapter, concentrations
of toxic substances with bio-concentration factors of 5 or less shall not exceed 0.1 of
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FINAL June 26, 2006
published LC50 value(s) for sensitive representative species using standard testing
methods …”.
“For toxicants not specified in Table 2 of Appendix G of this Chapter, concentrations
of toxic substances with bio-concentration factors greater than 5 shall not exceed 0.01
of published LC50 value(s) for sensitive representative species using standard testing
methods …”.
Both Alachlor and Aldicarb are not specified in Table 2 of Appendix G of the OWQS.
The technical fact sheets of EPA’s National Primary Drinking Water Regulations [12][13]
indicate that the bio-concentration factors (BCF) for Alachlor and Aldicarb are 6 and 42,
respectively. Since both BCF values are greater than 5, the target values for Alachlor and
Aldicarb will be 0.01 of their published LC50 values.
Published LC50 values for Alachlor and Aldicarb were found from the following public
resources:
• EXTOXNET, Extension Toxicology Network[15], which is a pesticide information
project of Cooperative Extension Offices of Cornell University, Oregon State
University, the University of Idaho, and the University of California at Davis and the
Institute for Environmental Toxicology, Michigan State University. The
USDA/Extension Service/National Agricultural Pesticide Impact Assessment
Program provided major support and funding.
• Virginia Corporative Extension [14], Virginia Tech and Virginia State University.
• PAN Pesticides Database [8], derived from the U.S. EPA AQUIRE (AQUatic toxicity
Information REtrieval) Database.
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FINAL June 26, 2006
TABLE 3-2. PUBLISHED LC50 VALUES FROM DIFFERENT SOURCES
Reference Chemical LC50 (μg/L)
Name Fathead
Minnow
Catfish
Common,
mirror,
colored, carp
EXTOXNET Extension Alachlor - 6500 -
Toxicology Network Aldicarb - - -
Virginia corporative Alachlor - - -
Extension Aldicarb - - -
U.S. EPA AQUIRE Alachlor 5700 15700 5600
Database Aldicarb 2700 23300 1000
Using the general methodology in the Oklahoma Water Quality Standards and the most stringent
LC50 values in Table 3.2 for sensitive representative species, the target values for Alachlor and
Aldicarb are calculated as 56.0 μg/L and 10.0μg/L, respectively.
3.4 Antidegradation Policy
Oklahoma antidegradation policy (OAC 785:45-3) requires protecting all waters of the state from
degradation of water quality. The targets of this TMDL, resulting load reduction, and load
allocations in this report were set with regard for all elements of the Oklahoma Water Quality
Standards which includes the antidegradation policy. With the implementation of this TMDL,
the water quality in Fort Cobb Lake and the streams in the watershed will be improving rather
than degrading.
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FINAL June 26, 2006
4. Impairment Assessment & TMDL Targets
Oklahoma’s 2002 Water Quality Assessment Integrated Report has concluded that siltation and
suspended solids impairments were listed in error for Cobb Creek, Lake Creek, Willow Creek
and Fivemile Creek in the Oklahoma’s 1998 303(d) list based on high flow high flow suspended
solids and turbidity sampling. The siltation and suspended solids impairments for Lake Creek
were corrected to turbidity impairment in the Oklahoma’s 2002 303(d) list. Therefore, siltation
and suspended solids will not be addressed in this report.
4.1. Status of Nutrient Impairment in Streams
Lake Creek, Willow Creek, Cobb Creek and Fivemile Creek are listed for nutrient impairment in
the 1998 303(d) list. The Oklahoma Conservation Commission (OCC) conducted quarterly
sampling on Lake Creek and its tributary during 1998 and 1999. The U.S. Geological Survey
(USGS) sampled Fort Cobb Lake and its contributing streams during 2000 and 2001. These data
are used to determine the status of nutrient impairment for Lake Creek, Willow Creek, and Cobb
Creek
4.1.a. Data from OCC
The Oklahoma Conservation Commission sampled five (5) sites in Lake Creek from August
1998 to October 1999. Table 2 shows the legal descriptions of the five monitoring sites.
Samples were collected monthly at Sites 1 & 4 for nutrients and salt analysis which included
nitrate/nitrite, total Kjeldahl nitrogen, total P, sulfate, total suspended solids, chloride, and
hardness. Monthly field data were collected concurrently at all five sites. Field monitoring
included flow rate, dissolved oxygen, temperature, pH, specific conductivity, turbidity, and
alkalinity. In addition to regular monthly monitoring, two high flow events were sampled for
water quality and field data at Site 1 on April 25, 1999 and June 21, 1999.
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FINAL June 26, 2006
TABLE 4-1. OCC WATER QUALITY MONITORING STATIONS
Monitoring Sites Latitude Longitude Legal County
Lake Creek #1 35° 15’ 30.4” N 98° 31’ 54” W S12, T9N, R13W Caddo
Lake Creek #2 35° 18’ 16.6” N 98° 31’ 36.2” W S36, T10N, R13W Caddo
Lake Creek #3 35° 20’ 01.2” N 98° 31’ 36.2” W S24, T10N, R13W Caddo
Lake Creek #4 35° 21’ 45.7” N 98° 30’ 56.8” W S7, T10N, R12W Caddo
Lake Creek #5 35° 24’ 21.9” N 98° 31’ 14.5” W S 25, T11N, R13W Caddo
Sampling Site #1 was located on Lake Creek and Site #4 on a tributary to Lake Creek. Figures
4-1 and 4-2 show the total phosphorus (TP) and nitrogen (NO2 + NO3) data and the
corresponding threshold values for Lake Creek and its tributary.
TP Concentration On Lake Creek (OCC)
0
0.5
1
1.5
2
2.5
4/15/98
7/24/98
11/1/98
2/9/99
5/20/99
8/28/99
12/6/99
3/15/00
Date
TP (mg/L)
Sit e # 1
Threshold
NOx Concentration On Lake Creek (OCC)
0
0.5
1
1.5
2
2.5
3
3.5
4/15/98
7/24/98
11/1/98
2/9/99
5/20/99
8/28/99
12/6/99
3/15/00
Date
NOx (mg/L)
Site # 1
Threshold
FIGURE 4-1. TOTAL-P, NO3/NO2 CONCENTRATION IN LAKE CREEK
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FINAL June 26, 2006
TP Concentration On Tributary Of Lake Creek (OCC)
0
0.1
0.2
0.3
0.4
4/15/98
7/24/98
11/1/98
2/9/99
5/20/99
8/28/99
12/6/99
3/15/00
Date
TP (mg/L)
Site # 1
Threshold
NOx Concentration On Tributary Of Lake Creek (OCC)
0 1 2 3 4 5 6 7
4/15/98
7/24/98
11/1/98
2/9/99
5/20/99
8/28/99
12/6/99
3/15/00
Date
NOx (mg/L)
Site # 1
Threshold
FIGURE 4-2. TOTAL-P, NO3/NO2 CONCENTRATION IN TRIBUTARY OF LAKE CREEK
If the mean of the samples does not exceed the threshold, according to the dichotomous process,
the stream is not threatened by nutrients.
As shown in Figure 4-1 & 4-2, the mean values of TP or NO2 + NO3 of all samples are well
below their corresponding threshold values. Both Lake Creek and its tributary are not nutrient-threatened
so they are not nutrient-impaired.
4.1.b. Data from USGS
Bi-monthly monitoring was conducted from June 2000 to June 2002 at 26 sites (Figure 4-3).
Sixteen sites are located in Fort Cobb Lake and ten sites in three major tributaries, namely Lake
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FINAL June 26, 2006
Creek, Cobb Creek and Willow Creek. The sites in the lake were designed to characterize the
spatial trend of the lake water quality. The sites in the tributaries were intended to determine the
source and load of nutrients to the lake.
Parameters monitored included temperature, pH, DO, specific conductivity and Oxidation
Reduction Potential (ORP), hardness, nitrate/nitrite, ammonia, total nitrogen, total phosphorus,
Soluble reactive phosphorus (SRP), particulate organic carbon.
FIGURE 4-3. USGS MONITORING STATIONS (PROVIDED BY USGS)
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FINAL June 26, 2006
TP Concentration On Willow Creek (USGS)
0
100
200
300
400
500
3/15/00
6/23/00
10/1/00
1/9/01
4/19/01
7/28/01
11/5/01
2/13/02
5/24/02
9/1/02
Date
TP (ug/L)
Threshold
Site # 13
Site # 15
Nitrogen Concentration On Willow Creek (USGS)
0 1 2 3 4 5 6
3/15/00
6/23/00
10/1/00
1/9/01
4/19/01
7/28/01
11/5/01
2/13/02
5/24/02
9/1/02
Date
NO2+NO3 (mg/L)
Threshold
Site #13
Site #15
FIGURE 4-4. TOTAL-P, NO3/NO2 CONCENTRATION IN WILLOW CREEK
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FINAL June 26, 2006
TP Concentration On Lake Creek
0
100
200
300
400
500
3/15/00
6/23/00
10/1/00
1/9/01
4/19/01
7/28/01
11/5/01
2/13/02
5/24/02
9/1/02
Date
TP (ug/L)
Threshold
Site # 18
Site # 20
Nitrogen Concentration On Lake Creek (USGS)
0
0.5 1
1.5 2
2.5 3
3/15/00
6/23/00
10/1/00
1/9/01
4/19/01
7/28/01
11/5/01
2/13/02
5/24/02
9/1/02
Date
NO2+NO3 (mg/L)
Threshold
Site #18
Site #20
FIGURE 4-5. TOTAL-P, NO3/NO2 CONCENTRATION IN LAKE CREEK
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FINAL June 26, 2006
TP Concentration On Cobb Creek (USGS)
0
100
200
300
400
500
3/15/00
6/23/00
10/1/00
1/9/01
4/19/01
7/28/01
11/5/01
2/13/02
5/24/02
9/1/02
Date
TP (ug/L)
Threshold
Site #25
Site #21
Nitrogen Concentration On Cobb Creek (USGS)
0 1 2 3 4 5 6
3/15/00
6/23/00
10/1/00
1/9/01
4/19/01
7/28/01
11/5/01
2/13/02
5/24/02
9/1/02
Date
NO2+NO3 (mg/L)
Threshold
Site #25
Site #21
FIGURE 4-6. TOTAL-P, NO3/NO2 CONCENTRATION IN COBB CREEK
As shown in Figure 4-4, 4-5 & 4-6, the mean values of TP or NO2 + NO3 are well below their
corresponding threshold values (Table 3-1). Cobb Creek, Lake Creek and Willow Creek are not
nutrient-threatened and therefore are not nutrient-impaired.
There is not enough data on Fivemile Creek to assess the status of nutrient impairment. USGS
collected only three samples on site 29 & 30. No samples exceeded TP or TN threshold values.
In addition, the 2002 Water Quality Assessment Integrated Report [16] indicated that the nutrient
impairment for Fivemile Creek was listed in error in the 1998 303(d) list.
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FINAL June 26, 2006
4.2. Status of Nutrient Impairment in Fort Cobb Lake
In addition to the data collected by USGS in Fort Cobb Lake, Oklahoma Water Resources Board
(OWRB) and U.S. Fish & Wildlife Service (USFWS) also conducted quarterly sampling in the
lake. These data are used to determine the status of nutrient impairment for Fort Cobb Lake.
Fort Cobb Lake was not listed in the 1998 303(d) list for nutrient impairment but was included
on the 2002 list. The available data support the listing.
Oklahoma Water Resources Board has conducted quarterly water quality monitoring at six sites
in Fort Cobb Lake from July 1998 to July 1999. Figure 4-7 shows the six sampling sites. The
monitored water quality parameters include NH3, NO2, NO3, Total N, Organic N, TKN, Ortho-P,
Total P, Settleable and Suspended Solids, Chloride, Chlorophyll-a and Turbidity. Field data
include temperature, dissolved oxygen, pH, Conductivity, Total Dissolved Solid (TDS) and other
parameters at different depths in the water column. USGS conducted bi-monthly water quality
sampling on sixteen sites in Fort Cobb Lake, (Figure 4-3). The sampling started in June of 2000
and ended in June of 2002. Depth profiles of temperature, pH, DO, specific conductivity and
Oxidation Reduction Potential (ORP) were conducted for sites in the lake. Water samples for
laboratory analysis were collected as a surface composite and analyzed for nutrients (TN, TP,
NO2/NO3, NH3, SRP), Chlorophyll-a, particulate organic carbon (POC) and physical chemistry
(pH, alkalinity, hardness, turbidity, conductivity, and total dissolved solids). In addition,
samples were collected for algae taxonomy.
U.S. Fish & Wildlife Service, sponsored by U.S. Bureau of Reclamation, conducted quarterly
water quality sampling on sixteen sites on Fort Cobb Lake, its tributaries and outflows (Figure 4-
8). The sampling started in November of 1997 and ended in June of 2000 [11].
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FIGURE 4-7. OWRB MONITORING STATIONS IN FORT COBB LAKE
The constituents analyzed include conductivity, turbidity, chlorophyll-a, COD, total phosphorus,
soluble reactive phosphorus, total alkalinity, chloride, sulfate, total nitrogen, nitrate, nitrite and
ammonia. In addition, other constituents such as metals etc. were also analyzed in water
samples. However, these parameters are not in the scope of this TMDL. A review of the data
for these parameters does not show any violations of water quality standards.
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FIGURE 4-8. USFWS MONITORING STATIONS
The TSI data from USGS, OWRB and USFWS is summarized in Table 4-2. The aesthetics
beneficial use for Fort Cobb Lake is considered not threatened with respect to nutrients if
planktonic chlorophyll-a samples in the water column indicate a Carlson's Trophic State Index of
less than 62.
TABLE 4-2. SUMMARY OF TSI DATA
Agencies Median
TSI
Min
TSI
Max
TSI
# Of
TSI >= 62
Total # Of
TSI
% Of
TSI >= 62
OWRB 63.7 34.0 77.6 21 34 62%
USGS 61.2 38.8 85.2 67 158 42%
USFWS 61.8 41.7 78.8 34 72 47%
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Data in Table 4-2 support the 303(d) status that Fort Cobb Lake does not support the Aesthetics
beneficial use with respect to nutrients.
4.3. Status of Pesticide Impairment
Samples for organics and herbicides were taken by the OCC from August 1998 to June 1999.
Immunoassays for pesticides (2,4-D, Alachlor, Aldicarb, Atrazine, Captan, Carbofuran,
Chlorothalonil, Chlorpyrifos, Cyanazine, Metolachlor, Metribuzin, Paraquat, Picloram, and
Triclopyr) were performed twice monthly during the spring & summer (March – October) and
once monthly during fall and winter (November – February).
TABLE 4-3. LC-50 VALUES AND TARGET CRITERIA FOR PESTICIDES
LC-50 (μg/L)
Pesticides Fathead Minnow Channel Catfish Common, mirror,
colored, carp
Target Criteria
(μg/L)
2,4-D 191500 7000 58271 70.0
Alachlor 5700 15700 5600 56.0
Aldicarb 2700 23300 1000 10.0
Atrzine 15000 4982 28467 49.8
Captan 155 78.3 250 0.78
Carbofuran 1264 629 1405 6.29
Chlorothalonil - 81.5 110 0.82
Chlorpyrifos 178.5 457 76.9 0.77
Cyanazine 18630 12862 - 128.6
Metolachlor 8200 4900 - 490.0
Metribuzin - 32540 - 325.4
Paraquat - 100000 78500 785.0
Picloram 64033 13571 135.7
Triclopyr NA for above species, but >1000 for all other tested species 10.0
Table 4-3 shows the target criteria for each pesticide. The target criteria are determined by
multiplying the minimum LC50 by 0.01 for each pesticide. The LC50 values are derived from the
U.S. EPA AQUIRE database.
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FINAL June 26, 2006
The pesticide data collected by the OCC were compared with the criteria in Table 4-3 for each
pesticide to determine the status of pesticide impairment for Lake Creek. Since no pesticide data
exists for Cobb Creek and Fort Cobb Lake, the evaluation of the status of pesticide impairment
relies on the comparison of the data for Lake Creek and the prediction of the Soil and Water
Assessment Tool (SWAT) model performed by Oklahoma State University.
4.3.a. Lake Creek
OCC collected pesticide data on different sites of Lake Creek from August 1998 through October
1999. Alachlor and Aldicarb are the only two pesticides that were detected in both surface water
and streamside seepage samples. We believe this is the reason that Alachlor and Aldicarb were
listed in the 1998 303(d) list. Alachlor was detected in 13 of the 76 total samples and Aldicarb
was detected in 19 of the 62 total samples. The highest concentration measured was 0.26 μg/L
for Alachlor and 1.58 μg/L for Aldicarb. Both values are well below the corresponding target
values.
Other pesticides were screened against the target criteria (Table 4-3). None of the measured data
exceeds the corresponding target criteria. Therefore, it can be concluded that pesticides do not
impair Lake Creek.
4.3.b. Cobb Creek
In addition to Lake Creek, Cobb Creek and Fort Cobb Lake are listed in the 1998 303(d) list for
pesticide impairment. No monitoring data are available for either of the water bodies.
Oklahoma State University has performed a SWAT model to simulate nutrient and pesticide
loadings from the Fort Cobb Watershed [17]. The model is calibrated for flow and nutrients, but
it is not calibrated for pesticides because of limited pesticide data. The model is not suitable for
predicting the actual pesticide mass loadings from the watershed but is adequate for comparison
of the relative pesticide loadings from different sub-watersheds.
A comparison of land uses in Lake Creek sub-basin and Cobb Creek sub-basin are made in Table
4-4. Both sub-basins have a majority of land used for agricultural practices where pesticides are
normally applied. The percentage of agricultural land in the Lake Creek sub-basin is slightly
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FINAL June 26, 2006
higher than that in the Cobb Creek sub-basin. The SWAT model was calibrated for pesticides
based on data collected in Lake Creek. When the same calibrated parameters are applied to the
Cobb Creek sub-basin, the model should give a conservative prediction of pesticides on a
relative basis.
TABLE 4-4. LAND USE COMPARISON FOR COBB CREEK AND LAKE CREEK SUB-BASINS
Land Use Name Cobb Creek Sub-bLaasnidn UseL a(%ke) Creek Sub-basin
Urban or Built-up Land 0.3% 0.5%
Agricultural Land 85.7% 92.2%
Forest Land 0.1% 1.8%
Range Land 13.6% 5.5%
Barren Land 0.0% 0.0%
Water 0.3% 0.0%
Pesticide loadings and concentrations from the Cobb Creek sub-basin and the Lake Creek sub-basin
as predicted by the SWAT model are shown in Table 4-5.
TABLE 4-5. PESTICIDE LOADINGS (APRIL 1999 – AUGUST 1999)
Pesticide Loading
(kg)
Total Accumulative Flow
(m3)
Average Pesticide
Concentration
(μg/L)
Cobb Creek 394.9 1.42E+07 0.28
Lake Creek 300.4 9.78E+06 0.31
The pesticide loading in Table 4-5 is the loading from April 1999 to August 1999. The loading
for other months of the year is negligible because little or no pesticides are applied in these
months.
As shown in Table 4-5, the predicted pesticide concentration in Cobb Creek is even lower than
that in Lake Creek. Because the observed pesticide concentrations in Lake Creek are well below
the standards and the pesticide concentrations in Cobb Creek are relatively lower than those in
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FINAL June 26, 2006
Lake Creek, we can conclude that the pesticide concentration in Cobb Creek is well below the
standards. In other words, pesticides do not impair Cobb Creek.
4.3.c. Fort Cobb Lake
It is safe to assume that the only source of pesticides to Fort Cobb Lake is pesticides in stream
flows of the tributaries to Fort Cobb Lake. Since none of Fort Cobb Lake’s tributaries are
impaired by pesticides, a simple mixing model can show that Fort Cobb Lake is not impaired by
pesticides.
Assume :
Vi = volume from stream i, (i = 1,2,…n)
V = volume after mixing, V = V1 + V2 + … + Vn
Ci = concentration in stream i, (i = 1,2,…n)
C0 = critical concentration, C0 > Ci for i = 1,2,…n
C = concentration after mixing
Based on mass balance, we get:
V · C = V1 · C1 + V2 · C2 + … + Vn · Cn
Substitute Ci wit