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Gunnison River /Aspinall Unit Temperature Analysis -Phase I Executive Summary, Page-A EXECUTIVE SUMMARY Introduction Hydrology of the Gunnison basin has" been significantly altered by the construction and operation of the Aspinall Unit (Blue Mesa, Morrow Point and Crystal Reservoirs), numerous other smaller reservoirs, and diversion and return flow features related to irrigation in the basin, particularly in the areas surrounding Montrose and Delta. Cold- water releases from the Aspinall Unit reservoirs have been identified as a significant impediment to re-establishment of pikeminnow habitat in the Gunnison River near Delta (Osmundson, 1999). Results of Osmundson's work indicate that increasing mean water temperatures at Delta by 1 °C in June, September and October, and by 2 °C in July and August, would increase the mean annual thermal units (ATU) from 32 to 46 units. Such an increase would put stream temperatures at Delta at a level similar to sites on the Yampa and Colorado Rivers which have abundant populations of pikeminnow. The objective of this phase I study was to determine the feasibility of increasing stream temperatures in the Gunnison River at and below Delta, Colorado through structural and / or operational modifications to the Aspinall Unit reservoirs. The project is being approached in a two-step process. The first phase of the work, which this report summarizes, includes: data collection and assessment; an overview of factors that may constrain the Program's ability to meet temperature objectives; a cursory analysis of the data with the intent of gaining insight into the primary physical processes governing water temperature in the basin; and modeling recommendations for the second phase of the work. Phase II of the project, if approved, would involve development of numerical models of both the Aspinall reservoirs and the Gunnison River downstream. The objective of phase II would be to use these models to simulate temperatures in the river /reservoir system under a variety of Temperature Control Device options and flow regimes. Results and Recommendations We must stress that the results presented here are based on a preliminary analysis of the available data. We strongly recommend that a rigorous modeling effort be undertaken, with a particular focus on how thermal regimes in the three Aspinall reservoirs could change with installation of a temperature control device (TCD). The results of phase I of the Gunnison River /Aspinall Unit Temperature Study indicate that Aspinall Unit construction and operation has had a significant impact on water temperatures at Delta, and that warmer release temperatures during the summer would in most cases translate into warmer temperatures in the river near Delta. The findings also indicate that a TCD on Blue Mesa Dam is likely to be the best approach to achieving warmer releases from Crystal Dam. Although modified annual flow patterns have also impacted water temperatures at Delta, complications arising from physical and Hydrosphere Resource Consultants