The value of maintaining a minimum streamflow objective on average is lessened when there is considerable dispersion around the average. An integrated economic and hydrology model is presented which provides water policy planners with a way to accurately measure both the economic cost and hydrologic consequences of maintaining a minimum streamflow level in an irrigated river basin at alternative probabilities of maintaining the target flow level. Water markets for streamflow augmentation are shown to be the most cost-effective policy in the study area.