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r" Executive Summary At the request of the Department of the Interior, the Bureau of Reclamation (Reclamation) has prepared a preliminary analysis of the effects of Black Canyon of the Gunnison National Park (Park) desired hydrograph scenarios (National Park Service [NPS] desired flows) on the purposes and operations of the Wayne N. Aspinall Unit (Aspinall Unit). The NPS desired flows for the Park include a year-round minimum flow of 300 cubic feet per second (cfs) and a spring peak of 3,500 cfs to 12,000+ cfs. The Aspinall Unit, which includes Blue Mesa, Morrow Point, and Crystal Dams and Reservoirs, is located along a 40-mile reach of the Gunnison River in western Colorado. The Park is located along the Gunnison River downstream from the Aspinall Unit. 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 and and semiarid 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 Park has been granted an unquantified federal reserved water right for Gunnison River flows. Reclamation, the NPS, and other federal agencies are working to provide information for quantifying that right in Colorado State water court. A hydrologic model using RiverWare software has been developed and used to conduct a preliminary analysis of the effects of meeting NPS desired flows 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, and others. Results from the model were used to determine the effects of meeting peak, inundation, and minimum flow goals provided by the NPS. A base operation run representing existing conditions was developed for comparison purposes. This base run did not include special releases for NPS desired flows or endangered species. Alternative operation modes, ranging from placing limitations on Morrow Point bypasses to fully using all of the Aspinall Unit bypasses and spillways, were analyzed to see if the NPS desired flows could be met. Reclamation modeled a combination of natural flows and storage releases to assist in meeting the NPS desired flows. The model is not a water rights model; therefore, it does not evaluate the legal rights of the NPS to flows of the Gunnison River. The model results show that NPS desired flows can partially be met; however, none of the model runs completely met the desired flows under current Aspinall Unit operating criteria. Because of existing physical outlet capacities at all three dams, the NPS desired flows cannot be met in all years under existing operating criteria. This criteria includes, but is not limited to, the winter and spring drawdown criteria for flood control purposes. iii