The Clear Lake watershed, including the lake, amounts to 12,079 acres. Clear Lake is Iowa’s third largest natural lake with a surface area of 3,628 acres at full crest. The lake measures 5 miles long and has a maximum width of 2 miles in the eastern portion. Clear Lake is rather shallow with a maximum depth of 19 feet and a mean depth of 9.5 feet. The watershed to lake ratio of 2.3:1 is very small compared to most Iowa lakes. The watershed consists primarily of cropland (59%), urban and roads (14%), and marsh (9%). The lake is fed primarily by groundwater. Precipitation and runoff from both urban and rural land constitutes the remaining water supply of the lake. Runoff from urban land mainly enters the lake via street storm drains. Runoff from rural land enters the lake primarily from tile lines, which feed small intermittent streams. Ventura Marsh is located directly west of the lake.

The primary cause of Clear Lake’s declining water quality is nutrient loading. Of secondary concern is sediment loading. Clear Lake has endured an influx of excess nutrients for many years, which over time has caused the lake to become eutrophic (nutrient rich). The high nutrient levels, primarily phosphorus, cause annual algal blooms. These blooms, which persist throughout the spring, summer and fall, reduce water clarity to less than a foot and could impair lake recreation, fish and wildlife habitat, and the local economy. The communities of Clear Lake and Ventura depend on Clear Lake to sustain their local economies. Tourism generated by Clear Lake for 1999 alone totaled over 37 million dollars (Clear Lake Chamber of Commerce). Being Iowa’s third largest natural lake, Clear Lake is a popular recreation and tourist attraction. According to the DNR, during 2000 over 600,000 campers visited Clear Lake‘s two state parks alone. Clear Lake was placed on Iowa’s 1998 U.S. EPA Section 303(d) List of Impaired Waters due to its excessive levels of nutrients and phytoplankton.

Because much of the watershed lies in the agricultural region to the west of the lake, much of the phosphorus entering the lake comes from agricultural lands. The average phosphorus budget for the lake indicates that 43% derives from the agricultural watershed, 9% from Ventura Marsh, 7% from groundwater, 6% from the City of Clear Lake, 2% from the City of Ventura, and 2% from unconsolidated county urban lands. An average of 31% of the phosphorus budget derives from direct rainfall on the lake, since Iowa’s rainfall phosphorus has been enriched 10-fold with airborne phosphorus over the past 30 years. Although the predominance of agricultural lands in the watershed makes them a major overall nutrient source, nutrient losses per unit land area indicate areas where nutrient losses are most severe. In general, phosphorus losses were somewhat higher (per unit area) from urban lands than agricultural lands. This indicates that substantial reductions in phosphorus input could be achieved by both urban and agricultural communities.