R99-191 AUTHORIZING AGREEMENT - Laserfiche WebLink

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SEDIMENTATION SURVEY OF LAKE DECATUR Introduction Lake Decatur is a multi-purpose lake located in central Illinois. The dam for Lake , Decatur 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. I, The lake was built in 1922 to serve as a source of water supply for the city of Decatur. In I addition to its use as a public water supply, Lake Decatur also provides a variety of recreational opportunities. The lake is owned by the city of Decatur. T'he primary land use in the watershed is row crop production. Soil and organic material have had the opporlunity 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. The Illinois State Water Survey in collaboration with the city of Decatur will undertalce a sedimentation re-survey of the north (upstream) end of the lake. The study is designed to delineate current sedimentation conditions for the section of the lake north of Rea's Bridge. Sediment Accumulation The sedimentation survey will consist of a survey of water depth and sediment accumulation along 6 transects of the lake. This cross sectional information will be used in a calculation of the present water volume of this 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. The ends of each of the surveyed transects were permanently monumented in 1936. These monuments will be recovered using original descriptions from the 1956 survey and recovery notes from the 1983 survey. The survey will be conducted using an electonic fathometer or physical measurement with a sounding pole and differentially corrected Geodetic Positioning System (GPS). Collection of the electronic hydrographic survey data will be coordinated by a laptop-based survey management program. This survey data will be supplemented by sounding pole measurements of water and sediment depths at a minimum of 5 points on each lake transect. Samples of the accumulated sediments will be collected and analyzed for particle size distribution and unit weight. Twenty-four particle size distribution and twelve unit weight samples will be collected. The results of the laboratory analysis of these samples will be used to discuss distribution patterns of sediment in the lake and calculate the mass of accumulated sediments. Because these sediments typically accumulate in a very loose, unconsolidated mass