Water resources management in Arizona is critical to the development and sustainability of local communities. Several regions in the state are facing immediate challenges to meet water demands. However, tools to assist the wide range of water interests understand the impact of decisions are lacking. This paper describes the development of a dynamic simulation based decision support system for the Upper San Pedro Partnership (USPP). This paper briefly presents the planning that they USPP has undertaken to meet the challenges. The emphasis of the paper is the DSS description. The Upper San Pedro is the one of the last uncontrolled rivers in Arizona and has a thriving riparian zone that also serves as a bird passageway. However, groundwater demands of the growing population and industry exceed natural recharge. The USPP was formed by interested parties to develop a strategy to maintain the riparian zone health while allowing for continued water use. The DSS will support the USPP in identifying water management options that eliminate groundwater overdraft, allow for growth and development within the Upper San Pedro basin, and maintain/raise groundwater levels at desired locations along the San Pedro River. In addition to a mass balance, a groundwater model has been represented through response functions to provide information on the impacts of decisions on river levels. Plans for including measures of riparian zone health are underway. Model development was completed over two years with near monthly interactions with the USPP to verify assumptions and determine structure of decisions to incorporate and output to present to users. These meetings were critical to ensure the DSS properly represented the system and was acceptable to all USPP members. Copyright ASCE 2006.