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A Dynamic Watershed Simulation Model (DWSM) is being developed to simulate rainfall-runoff, propagation of flood waves, soil erosion, and transport of sediment and commonly used agricultural chemicals in agricultural and rural watersheds. The model is being tested on the Big Ditch watershed in Illinois using rainfall, flow, sediment, nutrient, and pesticide data collected during 1998 and 1999 rainfall events. This 100- square-kilometer watershed is a subwatershed of the Upper Sangamon River basin draining into the Lake Decatur, a water supply reservoir. The lake periodically violates drinking water standards on nitrate-nitrogen concentration. It has high sedimentation rate reducing the lake capacity. The goal is to use the DWSM to evaluate alternative land use and best management practices in reducing soil erosion, and sediment and agricultural chemical discharges, to help solve the pollution and sedimentation problems in the lake Decatur. The DWSM-hydrology and sediment components and some results on Big Ditch are presented here.