ABSTRACT: Warm summer stream temperatures due to low flows and high air temperatures are a critical water quality problem in many western United States river basins because they impact threatened fish species’ habitat. One way to allevi- ate this problem is for local and federal organizations to pur- chase water rights to be used to increase flows, hence decrease temperatures. Presented is a Decision Support System (DSS) that can be used in an operations mode to effectively use water acquired to mitigate warm stream temperatures. The DSS uses a statistical model for predicting daily stream temperatures and a rule-based module to compute reservoir releases. Water releases are calculated to meet fish habitat temperature targets based on the predicted stream temperature and a user specified confidence of the temperature predictions. Strategies that enable effective use of a limited amount of water throughout the season have also been incorporated in the DSS. The utility of the DSS is demonstrated by an example application to the Truckee River near Reno, Nevada, using hypothetical operating policy and 1988 through 1994 inflows. Results indicate that the DSS could substantially reduce the number of target tempera- ture violations (i.e., stream temperatures exceeding the target temperature levels detrimental to fish habitat). (KEY TERMS: decision support systems; stream temperature; simulation; water quality; planning; water allocation; rivers/streams.) Neumann, David W., Edith A. Zagona, and Balaji Rajagopalan, 2006. A Decision Support System to Manage Summer Stream Temperatures. Journal of the American Water Resources Asso- ciation (JAWRA) 42(5):1275-1284. INTRODUCTION An increasingly common river management prob- lem is that water storage and use for municipal, industrial, agricultural, and power production pur- poses leave insufficient flow to maintain fish popula- tions. Low flows threaten fish by deteriorating habitat and/or water quality. One of the most common sum- mer water quality problems associated with low flows is high stream temperatures – low flows warm up due to warm air temperatures more rapidly than higher flows. High stream temperatures reduce cold water fish populations by inhibiting growth and extremely high temperatures can result in fish kills. Excessive or prolonged low flows can threaten or endanger fish species, necessitating modified management prac- tices. Hence, many National Environmental Policy Act (NEPA) studies of reservoir operations have as an objective to provide additional flows to increase habi- tat and/or improve water quality for fish. In some western basins this problem is addressed by transfer- ring water rights from other uses to supplies reserved for fish flows. To effectively use these water rights to protect fish, water managers must modify operational strategies by incorporating water quality objectives into daily operations and long term planning. The operational objectives involve management of water quantity (i.e., streamflows), to control water quality characteristics such as temperature. Meeting the water quality objective is more challenging than meeting other 1 Paper No. 04010 of the Journal of the American Water Resources Association (JAWRA) (Copyright © 2006). Discussions are open until April 1, 2007. 2 Respectively, Professional Research Assistant and Director, Center for Advanced Decision Support for Water and Environmental Systems, University of Colorado, UCB 421, Boulder, Colorado 80309-0421; and Assistant Professor, University of Colorado, Department of Civil, Envi- ronmental, and Architectural Engineering, UCB 426, Boulder, Colorado 80309-0426 (E-Mail/Neumann: David.Neumann@Colorado.edu). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1275 JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION OCTOBER AMERICAN WATER RESOURCES ASSOCIATION 2006 A DECISION SUPPORT SYSTEM TO MANAGE SUMMER STREAM TEMPERATURES 1 David W. Neumann, Edith A. Zagona, and Balaji Rajagopalan 2