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WATERSHED MOIVITORING FOR THE LAKE DECATUR WATERSHED by I Illinois State Water Survey I', 1NTRODUCTION AND BACKGROUND I Lake Decatur is the water supply reservoir for the City of Decatur. The reservoir was created in 1922 by impounding the flow of the Sangamon River and later modified in 1956 to increase the maximum capacity of the lake from 20,000 to 28,000 acre-feet. The drainage area of the Sangamon River upstream of Decatur is 925 square miles. The watershed includes portions of seven counties in east-central Illinois (Champaign, DeWitt, Ford, McClean, Macon, Piatt, and Shelby). The predominant land use in the watershed is row crop agriculture comprising nearly 90 percent of the land area. The major urban areas within the watershed are Decatur, Monticello, and Gibson City. The lake has seasonally high concentrations of nitrate-N. This has created a serious situation for the drinking water supply of the City of Decatur. The City has been issued non- compliance warnings from Illinois Envitonmental Protection Agency (IEPA) for exceeding the drinking water quality standard for nitrate-N levels above 10 mg/L. The Illinois State Water Survey conducted a two-year study, Watershed Monitoring and Land Use Evaluation.for Lake Decatur Watershed, from 1993 to 1995. This study was initiated as a result of a Letter of Commitment (LOC) between the City of Decatur and the Illinois Environmental Protection Agency, which identified Lake Decatur as exceeding nitrate-N drinking water standards. A technical report for that two-yeaz study was published in January 1996 (Demissie, et al., 1996 - ISWS Misc. Pub. 169). Subsequent one-year reports have been published in ISWS Contract Reports 602 (Keefer and Demissie, 1996), 620 (Keefer, Demissie, Shaw, and Howard, 1997), 637 (Keefer and Demissie, 1999), and 2000-06 (Keefer and Demissie, 2000). The latest publication (ISWS Contract Report 2000-06) reports annual watershed monitoring data from May 1993 through April 1999 and monthly data is presented for May�1998 through April 1999. • In order to best evaluate any on going and future land management changes, continued monitoring in the watershed is highly recommended. The additional data will enhance the baseline data established in the watershed and detect any changes or trends in nitrate-N levels in the watershed. The continuation of the watershed monitoring has been supported by several t