R2020-137 Resolution Authorizing Proposal with Illinois State Water Survey for Hydrologic, Sediment and Nutrient Monitoring Upper Sangamon River Watershed

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Hydrologic, Sediment and Nutrient Monitoring Upper Sangamon River Watershed by Illinois State Water Survey Champaign,Illinois Introduction Lake Decatur is the water supply reservoir for the City of Decatur and major industry in the area. The reservoir was created by constructing a dam to impound the flow of the Sangamon River. The drainage area of the Sangamon River upstream of Decatur is 925 square miles and includes portions of seven counties in east-central Illinois.Nitrate-N concentrations in Lake Decatur have exceeded the Illinois Environmental Protection Agency(IEPA)drinking water standard of 10 milligrams per liter (mg/1)whereby the City of Decatur installed a major ion exchange plant to remove nitrate from the potable water distribution. Also,due to the loss of one-third storage capacity of the original volume of the lake from sedimentation,the city made a major dredging investment to return much of the capacity. To protect and maintain the programs responsible for the reduced impact of nitrate-N and Decatur sedimentation from the upstream watershed,the City ofis moving forward with activities to develop a comprehensive watershed management plan. The purpose of this proposal is to collect data from three locations within the Upper Sangamon River, upstream of Lake Decatur for stream stage, suspended sediment, nitrate-N and total phosphorus to determine current levels in context of previous monitoring data conducted by the Illinois State Water Survey(ISWS)between 1993 and 2008(Demissie and Keefer, 1995; Keefer and Demissie, 1996;Keefer et al., 1997;Keefer and Demissie, 1999,2000, 2002; Keefer and Bauer, 2005,2008;Keefer, Bauer and Markus,2010). This data is for use by city planners and resource managers to develop lake and watershed management plans. Proposed site locations, drainage areas, and sampling plan are presented in Table 1. A map of the Lake Decatur watershed and site locations and subwatersheds are in Figure 1. Table 1. Proposed Locations of ISWS monitoring stations for City of Decatur in the Sangamon River Watershed. Drainage Data monitored Station ID Name area Stage* Discharge Nutrient* Sediment* 101 Lost Creek 66.4 mit X X X X at West Twin Bridge Rd. (172 km ) 102 Friends Creek 26.0 mit X X X X at IL Route 48 (67.4 km ) 113 Sangamon River 627.2 mi22 X X X at Route 32 (1624.4 km ) *Bi-weekly sampling 2