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Crystal Reservoir Tesperature Study Page 2 ' September 23, 1988 mainframe computer. The Aquatic Systems Group, Fort Collins, Colorado, recently modified these programs for execution on IBM-AT compatible microcomputers (Bartholow, 1987). The model predicts mean daily (24-hour average), minimum night-time, and maximum daily water temperatures. The U.S. Fish and Wildlife Service used the temperature model to predict temperatures within the Upper Colorado River Basin, including the mainstem Colorado and Green Rivers, the Yampa River, and associated tributaries. The final model network included 78 nodes; a monthly time period was used. The ,absolute difference between observed and predicted average daily temperatures was__1.32°C for the entire network on an annual basis (Brewer, 1985). Theurer and Voos (1982) verified the model solution techniques and validated the temperature model using a network analogous to the one used by the Service (1984). Theurer et al. (1982) also used this network and calibrated the model to observed temperatures in the Upper Colorado River Basin. O'Brien and Miller (1984) used the temperature model to simulate weekly river temperatures through the Yampa River. The average mean error for all time periods after calibration was 0.14°C. STUDY METHODS S t u d y._ _D_e s_i..g_n The length of the growing season and the availability of tempera- tures suitable for growth of the Squawfish presuppose the develop- ment of a tool to evaluate management options which include manipulation of instream temperatures. One management option would be to operate dams and reservoirs that result in water temperatures suitable to growth and earlier spawning. The Service, .therefore, must determine if selective operation of reservoirs change instream temperatures enough to improve Squawfish habitat. S_ t_u d_y _ A r e_a To determine the effect of reservoir operations on instream temperatures, the Service developed a river network which included the Colorado River from the U. S. Geological Survey (USGS) gage near Cameo to the USGS Stateline gaging station and the Gunnison River from Crystal Reservoir to the Colorado confluence. The Plateau Creek on the Colorado River and the North Fork Gunnison and Uncompahgre Rivers were mainstem tributaries included in the study area (Figure 1).