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Executive Summary Atthe request of the Department of the Interior, the Bureau of Reclamation (Reclamation) has prepared a preliminary analysis of the effects of draft endangered fish flow recommendations (flow recommendations) for the Gunnison and Colorado rivers on the purposes and operations of the Wayne N. Aspinall Unit (Aspinall Unit). The conclusions presented in this report are based on this preliminary analysis and can be refined with additional data and review. The draft flow recommendations include a spring peak on the lower Gunnison River, base flows, and a minimum flow for fish passage. With existing physical and operational constraints and the current state of forecasting and modeling, flow recommendations cannot be fully met. Operating to meet them more often would increase negative impacts on Aspinall Unit water yield and reservoir related recreation and fisheries. The Aspinall Unit, which includes Blue Mesa, Morrow Point, and Crystal Dams, Powerplants, and Reservoirs, is located along a 40-mile reach of the Gunnison River in western Colorado. The Aspinall Unit was authorized by the Colorado River Storage Project Act of 1956. Its authorization calls for meeting a variety of purposes including regulating the flow of the Colorado River; storing water for beneficial consumptive use; providing for the reclamation of arid and semi-arid land; providing for the generation of hydroelectric power; providing for fish and wildlife enhancement and public recreation; providing for the control of floods; and allowing the Upper Basin states to develop Colorado River Compact apportioned waters. The Upper Colorado River Endangered Fish Recovery Program (Recovery Program) has prepared draft flow recommendations I to provide the annual and seasonal patterns of flow in the Gunnison River and in the Colorado River downstream from their confluence to help recover four species of endangered fish-the Colorado pikeminnow, razorback sucker, humpback chub, and bonytail. Impacts associated with these recommendations are evaluated in this report. However, it should be noted that the recommendations are still under review by the Recovery Program and are subject to change. A hydrologic model using RiverWare software has been developed and used to conduct this preliminary analysis of the effects of the draft flow recommendations on the Aspinall Unit and its purposes. Aspinall Unit operating criteria used in the model include recognized constraints/demands such as flood control criteria, trout habitat protection criteria, power generation criteria, minimum stream flow criteria, outlet capacities, and others. Results from the model were used to determine the effects of meeting peak, base, and minimum flow recommendations. A baseline operation run representing existing conditions was developed for comparison purposes. This baseline run did not include releases for endangered fish nor for the National Park Service (NPS) reserved right proposed flows (The NPS has filed an application to IMcAda, C. W. 2001. (March Draft) Flow Recommendations to Benefit Endangered Fishes in the Colorado and Gunnison Rivers. Prepared by U.S. Fish and Wildlife Service for the Upper Colorado River Endangered Fish Recovery Program. Grand Junction CO. 11