The Rural Storm Water Management Model (RSWMM) project required accurate simulation of hydrology, hydraulics and water quality for 500 sq. km draining to Lake Huron. Assessment of sediment, phosphorus and nitrogen load reduction due to agricultural and urban BMP implementation was also required. Since no existing model could fully meet these goals, a customized model was developed. The RSWMM is a unique tool that can evaluate the impacts on water quantity and quality resulting from field-scale BMPs including conservation tillage, cover crops, and contour farming, among others. The model also simulates the in-stream treatment of nitrogen and phosphorus, and is able to assess the efficacy of designed treatment devices such as sedimentation ponds and denitrifying bioreactors. Field-scale simulations show good agreement with literature-reported sediment- and nutrient-load reduction rates. Five watersheds in Ontario, Canada totaling 500 km2 have been modeled, and results will be presented for a variety of BMP implementation scenarios. Overall, the RSWMM promises to be a robust and flexible tool for use in the practical application of agricultural BMPs.