Currently economic growth in a New York City watershed is limited due to phosphorus concentrations exceeding standards. The phosphorus concentration in the Cannonsville reservoir in New York State must be reduced to safeguard New York City's drinking water supply, and to enable economic growth to continue in the reservoir's watershed. In Delaware County, where the reservoir is located, the problem of finding and selecting promising measures to reduce phosphorus loads is extremely complex and involves numerous stakeholders. A two-level modeling approach based on Visual-Interactive Decision Modeling (VIDEMO) was used to analyse the problem, using game theory and optimization. The analysis resulted in practical guidelines for land use management, taking into consideration the possible strategies by other parties and stakeholders.