R2001-120 AUTHORIZING AGREEMENT - Laserfiche WebLink

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SEDIMENTATION SURVEY OF BIG CREEK AND SANDY CREEK BASINS OF LAKE DECATUR � Introduction Lake Decatur is a multi-purpose lake located in central Illinois. The dam for Lake Decatur I,, is in section 22 of Decatur Township in Macon County, immediately south of downtown Decatur. The lake has a surface area of approximately 3,072 acres. The maximum depth of Lake Decatur was 13.1 feet in 1983, the average depth was 6.1 feet. Including the lake's surface, the Lake Decatur watershed encompasses approximately 925 square miles total surface area in Illinois. The lake was built in 1922 to serve as a source of water supply for the city of Decatur. Portions of the lake have been dredged since 1988. In addition to its use as a public water supply, Lake Decatur also provides a variety of recreational opporlunities. The lake is owned by the city of Decatur. -- The primary land use in the watershed is row crop production. Soil and organic material have had the opportunity to wash into the lake since its construction. These materials have the capability to negatively impact all of the lake's current uses. The severity of the sedimentation problems in the lake have been documented by sedimentation surveys conducted by the Water Survey or the U.S. Soil Conservation Service (predecessor to the Natural Resource Conservation Service) in 1936, 1946, 1956, 1966, and 1983 (Fitzpatrick, et al, 1987). The 1983 survey - indicated a sedimentation rate of 150 acre-feet per year for the period 1922 to 1983. A survey of Basin 6 of Lake Decatur, located upstream of Reay's Bridge Road, was conducted in 2000. The Illinois State Water Survey will undertake a sedimentation re-survey of the Big Creek and Sandy Creek Basins of the lake. The transects on the main body of the lake between the dam and Raey's Bridge Road will be surveyed but not analyzed for sedimentation rates. The study is designed to delineate current sedimentation conditions for the two lake basins and document a baseline condition for the main lake area that would be available for future analyses of sedimentation rates.. Sediment Accumulation The sedimentation survey will consist of a survey of water depth and sediment accumulation along 8 transects for the Big Creek basin of the lake and 4 transects of the Sandy Creek basin of the lake. This cross sectional information will be used in a calculation of the present water volume of these section of the lake as well as the volume of sediment accumulated since the construction of the reservoir. The water volume and the measured sedimentation rate will be discussed and compared to conditions in previous surveys of the lake. -` Transects on the main body of the lake will be re-surveyed and analyzed to document water depth only. Due to past dredging programs, this portion of the lake can not be analyzed for sedimentation rates at this time. The ends of each of the surveyed transects were permanently monumented in 1936. � Surviving monuments will be recovered using original descriptions from the 1956 survey and - recovery notes from the 1983 survey. - 1