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Gunnison River /Aspinall-Unit Temperature Analysis -Phase I Page 1 1. 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), and by diversion and return flow features primarily related to irrigation in the areas surrounding Montrose and Delta (Figure 1). Cool stream temperatures resulting from changes to the basin hydrology (Stanford, 1994) have been identified as a significant impediment to re- establishment of pikeminnow habitat in the Gunnison River near Delta (Osmundson, 1999). Records indicate that Colorado pikeminnow historically were found in the Gunnison River as far upstream as the Town of Delta (Quarterone, 1993), though recent studies indicate that pikemiruiow are largely confined to downstream reaches of the river (Valdez et al., 1982, Burdick 1995). Osmundson (1999) notes that areas upstream of Whitewater through the Town of Delta show no reduction in forage-size fish and that the floodplain area near Delta provides the "most diverse physical habitat conditions in the Gunnison River" between Hartland Diversion and the mouth. The Hartland Diversion dam is located above the city of Delta, just below the mouth of Tongue Creek. 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. Stream temperature in reservoir-regulated rivers is a function of several related variables. The "natural" mean water temperature is closely related to mean air temperature (Sinokrot and Stefan, 1993). Water released from a reservoir will tend to approach this natural or ambient water temperature as it travels downstream. The rate at which the waters warm, and the ability to achieve a specific temperature at a specific location, depends on release temperature, flow, and atmospheric conditions. In general, increasing reservoir release temperatures will result in warmer downstream temperatures. The relationship between release temperature and downstream temperature is nonlinear (e.g., a 1 °C increase in release temperature does not necessarily result in a 1 °C increase downstream) and is limited by the ambient atmospheric conditions. Reducing reservoir releases will also increase downstream temperatures. This is the result of a reduced volumetric heat capacity per unit surface area of stream, and of the slower rate at which the water travels downstream, thus increasing the time it is exposed to atmospheric heating. There are potentially two ways that downstream temperature control can be achieved in the Gunnison River. These are to 1~) increase the temperature of the water being released from the Aspinall Reservoirs, and/or 2) decrease the amount of water being released. Analysis of the potential for releasing warmer water is complicated by the physical characteristics of the Aspinall Unit reservoirs. If a temperature control device (TCD) Hydrosphere Resource Consultants