ILLINOIS LAKES STUDIES

The USGS performs many of these activities under the FEDERAL-STATE COOPERATIVE PROGRAM, a partnership between the USGS and the State and local tax-supported agencies. Under this program, funds are appropriated by Congress to match funds furnished by State and other local agencies up to a 50-50 cost-sharing basis. The Cooperative Program generally calls for the State and local agencies to provide one-half the funds for a study and the USGS to provide the other half and perform the study. The USGS applies consistent techniques of data collection and archiving throughout the country, with the information being stored in a data base readily available to all users.

Illinois lakes are used by people for water supply and recreation; and by animals and plants for habitat.The continued viability of many of these lakes is threatened by sediment infilling and eutrophication. Eutrophication is the process of nutrient (typically phosphorus and nitrogen compounds) addition to lakes and the chemical and biological processes that accompany it. Sedimentation and eutrophication occurs in all lakes and has the potential, if left unchecked, to degrade the quality of the lake to the point where it is no longer viable for water supply, recreational use, or as habitat for plants and fish.

Water is added to a lake by precipitation directly onto the lake surface, streamflow into the lake, surface runoff of precipitation into the lake from the surrounding watershed, ground-water inflow to the lake, and wastewater discharge to the lake. Water is removed from the lake by evaporation from the lake surface, streamflow out of the lake, flow of lake water out to ground water, and removal for drinking or industrial use. The difference in the amount of water added to, and removed from, the lake from each of these sources provides the water balance of the lake and affects the stage (water level) of the lake. Addition of large amounts of water to the lake can result in flooding, enhanced erosion of the shoreline, and the resultant property damage. Removal of large amounts of water can result in a lowering of the lake stage, decreasing the value of the lake for recreational use and habitat.

Most of the sediment entering a lake is suspended in water from the streams entering the lake, overland runoff, and wastewater discharge; eroded from the shoreline by wave action.. Most of the sediment leaving a lake is suspended in water from the streams leaving the lake or removed for water supply. The amount of sediment entering a lake typically exceeds the amount of sediment leaving the lake, resulting in a net addition of sediment to the lake. Over time, the continual addition of sediment to the lake makes the lake shallower, and promotes the eventual conversion of the lake to wetland.

The amount of nutrients added to the lake is partially determined by multiplying the amount of water and sediment added to the lake from each of the sources by the nutrient concentration of each source. In addition, nutrient inputs from waterfowl and decaying organic matter in the lake also contribute to the lake's nutrient load. The amount of nutrients removed from the lake is determined by multiplying the amount of water and sediment leaving the lake is determined by multiplying the amount of water and sediment removed from the lake by each source by the nutrient concentration in each source. The net load of nutrients to the lake is obtained by subtracting the amount nutrient added from the amount removed.

The addition of large amounts of nutrients to a lake can promote excessive growth of algae and aquatic weeds, which can impact the chemical conditions in the lake and affect the quality of the fishery.