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002480 1. INTRODUCTION Hydrology ofthe 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.~~-: ' water:IllIkIMS _ml_'A~ft'Uffit'fleg~i~\h~e~ett identi~tw)iIi~~' ,m..edtrn8'fH;6~stablimm_ ofi'ilamninMWbaMtanfi;ttre~"~aeatDella (_~~:f~). Results of Osmundson's work indicate that increasing mean water temperatures at Delta by I Oc in June, September and October, and by 2 oc 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 pike minnow. 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. 2. RESULTS AND RECOMMENDATIONS ~.'~. ,...L-"'i_~"'~~' _................_*.~.... ~1~.li~\Gfthe ,,~~ . ~ :paI_ J]<:nD WtfiI8l:Jkt::dIta. 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. 1ft"~als&' i~:tftMft;'~_~MIsa~js Hke~ tobethe:l'el'hi~ID a.chi~~g w~;~;~~Dam. Although modified annual flow patterns have also impacted water temperatures at Delta, complications arising from physical and institutional constraints would severely limit the effectiveness of a flow-based temperature management approach. Data Collection. An extensive data collection program was completed during the summer of 2001. Much of the data required to conduct model development and calibration during phase II of the project were obtained, These data include meteorological, hydrological, and water temperature time series data, as well as