Farmers relying on the South Platte River in northeastern Colorado for irrigation water have been found to be adversely affected by pumping from a groundwater deposit associated with the river. A simulation model of the hydrologic-legal-farmer decision system is developed and employed to analyze several institutional alternatives for managing the system. The "augmentation plan" approach recently developed by the state was shown to yield highest net incomes under both normal water supply and drought scenarios while protecting the prior rights of river water users. Anticipated opportunity costs of ground water use are internalized into pumpers' decisions.