Technical Abstract: Lake Eucha is a nexus of water quality conflicts between agribusiness and environmentalists, urban and rural stakeholders, municipalities and state governments. Lake Eucha is a drinking water supply reservoir for the City of Tulsa, declining water quality has been attributed to both municipal wastewater treatment plant effluent and the application of poultry litter as a fertilizer within the basin. Litigation has forced policy changes in the basin with little knowledge of their eventual impact on water quality. The Soil and Water Assessment Tool (SWAT) model was used to evaluate the effect of various nutrient management scenarios on nutrient loads to Lake Eucha. The model was calibrated for streamflow, total phosphorus, and nitrates using a complex shuffled evolution approach on a 75 GHz computer cluster. SWAT was used to predict phosphorus loads to Lake Eucha under differing levels of poultry litter export from the area, municipal waste discharge, and soil test phosphorus. Reducing municipal waste discharge was the most effective scenario. Litter application, cultivated fields, and overgrazed pastures were the most significant nonpoint source contributors. These predictions were coupled with in lake water quality standards to evaluate management changes required to return Lake Eucha to mesotrophic conditions. Data generated by the SWAT model was used to develop a publically accessible decision support system to allow policymakers and stakeholders to evaluate the impact of various scenarios. The use of watershed models for the assessment of potential management scenarios allows policy makers to make more informed water quality management decisions.