Sedimentation and Hydrologic Processes in Lake Decatur and Its Watershed

Abstract

One of the end products of erosion is the accumulation of sediment in lakes and reservoirs, which results in the degradation and impairment of use of these water bodies. Lake Decatur, a water supply reservoir in the Upper Sangamon River watershed in east-central Illinois. has lost one-third of its storage capacity to sedimentation since its construction in 1922. The lake has been surveyed six times (in 1931-1932, 1936, 1946, 1956, 1966, and 1983) for the purpose of determining sediment accumulation rates. This report includes information on the history of the lake and on the physical and geological characteristics of the Upper Sangamon watershed. Changes in reservoir storage capacity over time and the temporal, spatial, and geotechnical variations in sediment deposition are analyzed. Also presented is an analysis of the relative contribution of sediment from various areas of the watershed. Over the period 1922-1983 Lake Decatur lost 9100 acre-feet of storage capacity through the accumulation of 9,830,000 tons of sediment. On the average each acre of watershed delivered 21.4 tons of soil to the lake over this 61-year period. Rates of sediment accumulation have generally decreased over time. The 15% of the watershed area nearest the lake contributed approximately one-half of the sediment in the lake. STATE OF ILLINOIS HON. JAMES R. THOMPSON, Governor DEPARTMENT OF ENERGY AND NATURAL RESOURCES DON ETCHISON, Ph.D., Director BOARD OF NATURAL RESOURCES AND CONSERVATION Don Etchison, Ph.D., Chairman Walter E. Hanson, M.S., Engineering Seymour O. Schlanger, Ph.D., Geology H. S. Gutowsky, Ph.D., Chemistry Robert L. Metcalf, Ph.D., Biology Judith Liebman, Ph.D. University of Illinois John C. Guyon, Ph.D. Southern Illinois University STATE WATER SURVEY DIVISION RICHARD G. SEMONIN, B.S., Chief