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Water: Lakes

Watershed Protection: Clean Lakes Case Study

United States
Environmental Protection
Office of Water
EPA 841-F-95-003
September 1995
Number 2

Watershed and In-lake Practices Improve Green Valley Lake, Iowa

Key Feature: Impoundment heavily impacted by agricultural runoff
Project Name: Green Valley Lake
Location: Union County, Iowa USEPA Region 7
Scope/Size: Watershed Area 1946 hectares (4750 acres)
Lake Area 158 hectares (390 acres)
Land Type: Ecoregion #47, western corn belt plains; planted in soybeans and corn
Pollution Source: Agriculture
Pollutants: Sediment, nutrients
Data Sources: State; project monitoring
Data Mechanisms: Water quality and sediment sampling, bathymetric studies
Monitoring Plan: Yes
Control Measures: Sediment/nutrient retention dikes, grade stabilization structures, tile intake terraces, diversions, grassed waterways, sediment control basins


Green Valley Lake is a 158 ha impoundment located in southern Iowa. It was built in 1952 near the headwaters of the Platte River and by the 1970s it was heavily impacted by erosion from its surrounding agricultural watershed. Soil and nutrients delivered to the lake produced a number of water quality problems including sedimentation, turbidity, excess growths of the blue-green algae Alphanizomenon sp., and low dissolved oxygen. The algal blooms were responsible for early morning oxygen sags, which in turn led to partial kills of the largemouth bass and bluegill populations.

All three phases of EPA's Section 314 Clean Lakes Program were implemented at Green Valley Lake. A Phase I Diagnostic/Feasibility Study identified the causes of the lake's water quality problems, and a Restoration/Implementation Project (Phase II of the Clean Lakes Program) was carried out in Green Valley Lake between 1980 and 1986. The Restoration Project included implementation of best management practices on surrounding agricultural lands and construction of sediment/nutrient retention dikes in the two major arms of the lake. A total of 65 best management practice installation projects were completed, including grade stabilization structures, tile intake structures, a diversion, grassed waterways, terraces, and water sediment control basins. A monitoring program conducted during the Restoration phase indicated that water quality improved in the lake and the dikes seemed to be effective in preventing the movement of sediments into the main lake. The lake was monitored for three years to assess the benefits of the Restoration Project. Criteria for judging the effectiveness of the Restoration Project were established and included lowered suspended solids, lowered nutrients in lake inflows, and fewer algal blooms. The Clean Lakes Program Phase II Restoration Project largely succeeded with respect to these criteria. Additional monitoring conducted from 1991 to 1994 under a Phase III Clean Lakes Post-Restoration Monitoring Study indicates that Green Valley Lake has improved and probably will continue to improve due to both in-lake and watershed restoration efforts.


Regional map showing location of Green Valley Lake location Green Valley Lake was formed as an impoundment of the Platte River in 1952. It lies just north of Creston, Iowa, 88.5 kilometers southwest of Des Moines, Iowa, and 137 kilometers east of Omaha, Nebraska (Figure 1). It has a watershed area of 2105 hectares and a surface area of 158 hectares. The maximum depth of the lake is 7.9 meters and its hydraulic residence time is 1.82 years. The area receives 84 cm of rain per year, 50 percent of which falls between April and July, and 75 percent of which falls during the April through October growing season. Thunderstorms and other violent weather strike the area on average 45 times per year. The average annual runoff for the watershed is 14 cm--6 cm in surface runoff and 8 cm in subsurface runoff.

The entire lake is surrounded by the Green Valley Lake State Park, which is owned by the State of Iowa and provides recreational opportunities for all citizens of the state. The land in the watershed is occupied by 39 landowners. Seventy-two percent of the watershed is farmed, primarily to raise corn and soybeans, and most of the rest is parkland.


Water quality problems in Green Valley Lake first became evident in the 1970s. Aquatic life and recreation suffered from excessive inputs of sediment and nutrients. Between 1968 and 1978 sedimentation reduced the lake area by 10 percent and reduced the lake volume at a rate of 8635 cubic meters per year. Volume reduction was most noticeable in the parts of the lake that received runoff.

In the northwest arm of the lake (inflow from the Platte River), for instance, boating and fishing were eliminated from over 10 hectares (6.3 percent of the lake surface) because of the loss of lake volume and depth. High turbidity was evident after rainfall, and wind alone was enough to resuspend sediment in the shallows.

The lake also received an overabundance of nutrients. Algal blooms occurred in summers and caused dissolve oxygen (DO) sags in the morning hours. These in turn caused partial fish kills and led to poor survival for young-of-the-year fish. The populations of largemouth bass and bluegills were reduced, and a bad taste and odor were noticed in fish caught in the lake, a typical phenomenon in lakes with algal blooms. The algal blooms also made the lake unsuitable and unattractive for swimming.


To address these problems, the Iowa Department of Natural Resources (IDNR) enlisted and coordinated the efforts of the following agencies:

US Environmental Protection Agency, Region 7, Kansas City, Kansas
USDA Natural Resources Conservation Service, Des Moines, Iowa
Iowa Department of Agriculture and Land Stewardship, Des Moines, Iowa
Union County Soil Conservation District, Creston, Iowa
Union County Consolidated Farm Service Agency, Creston, Iowa
Area Extension Office, Creston, Iowa
University Hygienic Laboratory, Iowa City
Participating landowners in Green Valley Lake watershed

The Phase II Restoration Project officially lasted from July 1980 to June 1986.


Based on the Phase I Diagnostic Study, the IDNR established the following goals for the Green Valley Lake Restoration Project:

  1. Reduce sediment/nutrient delivery to the lake to acceptable levels by installing best management practices (BMPs) on croplands in the watershed.
  2. Reduce resuspension of nutrients within the lake bed by deepening shallow water areas and/or containing such areas.
  3. Monitor chemical, physical, and biological parameters to detect changes in water quality.


IDNR selected the following criteria to be used as evidence of an effective restoration of Green Valley Lake:

  1. Improvements in water clarity or transparency measured with a Secchi disk.
  2. Low suspended solids concentrations in the lake relative to other similar lakes.
  3. Low levels of suspended solids and plant nutrients in the inflows relative to similar streams in the region.
  4. Reductions in algal chlorophylls.
  5. Reductions in blue-green algal populations.
  6. Increases in public use of the State Park.


IDNR administered the project. In 1980, $569,100 in EPA Clean Lakes Program funds were received to implement Phase II restoration measures at Green Valley Lake. The Union County Soil Conservation District, in conjunction with landowners, prepared water quality management plans for individual farms surrounding the lake. Costs to implement the BMPs were shared by the Federal government (50 percent), the landowners (25 percent), and the State (25 percent). In-lake BMPs (sediment/nutrient retention dikes) were built by IDNR. Union County Soil Conservation District staff were funded by the Iowa Department of Agriculture and Land Stewardship and the USDA Natural Resources Conservation Service.


Figure 2 indicating the locations of BMPs in the surrounding watershed

Two approaches were used to restore the lake. First, BMPs were installed on agricultural lands to reduce soil erosion. Figure 2 indicates the locations of BMPs in the surrounding watershed. When the project ended, the following BMPs had been installed: 3 grade stabilization structures, 39 tile intake terraces (totaling 32,961 m), 1 diversion (380 m long), 6 grassed waterways (totaling 2946 m), and 16 water sediment control basins. Second, two sediment/nutrient retention dikes were installed, one in each arm of the lake, to retard water flow and cause sediment to settle out. It was hoped that this would reduce sediment and nutrient loads throughout the lake.

Since 1987 additional BMPs, including 4575 m of terraces and 15 water sediment control structures, have been added. A separate crop set aside program had also reduced row cropland from 77 percent (of total cropland farmed in the region) in 1980 to 60 percent by 1989.


Green Valley Lake is classified to be used for primary and secondary contact recreation and as a drinking water supply. The goal of the monitoring program for the Clean Lakes Project was to document short-and long-term trends in the lake resulting from the implementation of BMPs. Monitoring data were collected for 6 years by the Iowa Department of Natural Resources and Iowa State University.

A bottom profile of the lake has been produced every 5 years to determine the sedimentation rate. Water quality samples were taken during runoff events and dry weather. Two in-lake monitoring sites were used. The water was monitored for pH, chlorophyll a, nitrogen, phosphorus, temperature, dissolved oxygen, total suspended solids, and Secchi disk transparency. Pesticides, metals, and fecal coliform levels were monitored in sediments and the water column. Water level, weed growth, fish kills, turbidity, and erosion were monitored as well, and fish populations were sampled twice each year to note any trends. Lake use (recreation) data were also collected.


  1981 1986
Turbidity (JTU) 12/70a 19/43
Secchi Disk transparency (cm) 71/51 48/30
s (mg/l)
Total P .6/.4 .2/.25
NO2 + NO3 .5/9.8 1/1.3

Algal chlorophyll
60/60 24/20

Blue-green algae

=> Beach-use indicator
(user days x 1000)

complaints of fish taste & odor no fish taste or odor complaints

1.8 4.1

Public use - 4.7-fold increase in swimming
1.5-fold increase in fishing
a deep sampling station (in-lake)/shallow sampling station (near inflow)

Monitoring data collected during the 6-year project indicated improvements were made in most of the criteria for success that were established before the project began (see Table 1). Somewhat surprisingly, water clarity declined. This problem was attributed to an increase in populations of bullhead and carp, bottom-dwelling fish species that stir up sediments in the course of their daily activities. This problem has been well-documented in other Iowa lakes. Sediment delivery was reduced enough to lower lake volume loss from 8635 cubic meters/year in 1981 to 4318 cubic meters/year in 1986, a result attributed to the sediment/nutrient retention basins, grassed swales, and other BMPs. Nitrogen levels decreased in shallows, and phosphorus levels decreased in shallows and depths.

The decline in chlorophyll a was thought to result from reductions in phosphorus loadings. Fewer algal blooms occurred, and fewer complaints of bad odor and taste in fish were received. By 1986 the decrease in algae had also led to a 50 percent decrease in the daytime annual mean level of DO to 5.7 mg/l in depths and 6.6 mg/l in shallows and eliminated DO sags in the morning hours. As a result there were no summer fish kills reported after 1981.


Figure 3. Annual swimming beach use at Green Valley Lake (IDNR)

The success of the Green Valley Lake Clean Lakes Project can be measured in numerous ways, including changes in water quality parameters. Improvements in human and wildlife use of the lake were clearly noticeable. Swimming beach use at the lake more than doubled between 1981-1986 as a result of the decrease in blue-green algae populations (Figure 3). Fishing hours also increased 1.5-fold by 1986.


Costs and funding for the BMPs installed by landowners and the state during the project are summarized in Table 2.


Efforts to keep Green Valley Lake clean and healthy continue since the completion of the Phase II Restoration Project in 1986. As indicated above, some additional BMPs have been installed. Also, IDNR implemented an EPA Clean Lakes Program Phase III Post-Restoration Monitoring Project from 1991 to 1994. The primary objective of this monitoring was to determine if the restoration techniques employed at Green Valley Lake and within its watershed are working to improve the lake's water quality.

Following are the types of monitoring activities that were conducted:

  • From May 1991 to April 1994 water quality was monitored twice per month from May to September and once per month from October to April.
  • Sediments were sampled above and below the dikes to determine the dikes' sediment and nutrient removal efficiency.
  • Light penetration and wind speed and direction measurements were taken in the arms of the lake to evaluate resuspension of sediments due to wind induced-mixing.
  • Soil conservation practice activities were documented for the time period of the project.
  • Sediment basin maps were created for 1991 and 1994 to estimate the lake sedimentation rate.
  • Recreational use information was collected.

The final report on the Phase III Monitoring Program, issued in June 1995, indicates that overall water quality at Green Valley Lake has improved and probably will continue to improve due to both in-lake and watershed restoration efforts. The Phase III study compared current data (1991-1994) with historical data (1979, 1981-1986) and found that water quality at Green Valley Lake, with natural fluctuations, has improved. Also, comparisons of water quality data from opposite sides of the sediment/nutrient retention dikes support their role in the improved quality of the lake. As a result of the improved water quality and fisheries at the lake, usership has increased steadily over the past years.


(Anonymous). 1990. Phase III post-restoration monitoring program on Green Valley Lake, Union County, Iowa.

Iowa Department of Natural Resources. (undated). Green Valley Lake Clean Lakes Project. Union County, Iowa. Six-year summary of activities. July 1980 -June 1986.

Iowa Department of Natural Resources. June 1995. Phase III Project Final Report for Green Valley Lake, Union County, Iowa. 35pp.

For more information contact:
Marion Conover,
Chief, Fisheries Bureau,
Iowa Department of Natural Resources,
Wallace State Office Building,
Des Moines, IA 50319-0034,
phone 515-281-5208.

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