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4 '' INTRODUCTION The U.S. Fish and Wildlife Service (Service) is currently evaluating the long term decline of the Colorado Squawfish within the Upper Colorado River Basin. Research Biologists from the Service have concluded that water temperature may be one reason this decline has occurred. Because the Service must work to conserve the endangered Squawfish, a pilot study was completed to evaluate the Instream Water Temperature Model as a tool for predicting river temperatures given either historic or synthetic hydrologic, meterologic, and stream geometry conditions. The purpose of this paper is to describe the methods, analyses, and results from the pilot study. LITERATURE REVIEW C o l._o_r_a_d_o _S_qu_a_w_f_i_s h P o p u 1_a_t i_o n s Kaeding and Osmundson (1988) developed the hypothesis that slow growth and increased early-life mortality have partially caused Squawfish decline in the Upper Colorado River Basin. They found a positive correlation between temperature suitability and first-year Squawfish growth and recommended water management to enhance instream temperatures for growth. Kaeding, et al. (1985) suggested that water temperature, as it affects Squawfish growing season length, is a limiting resource; they concluded that reductions in available temperature necessary for growth would detrimentally affect Squawfish populations. Kaeding and Osmundson {1988) suggested that Colo.Fado River flows be manipulated to improve water temperatures in the "15-mile reach" of the Upper Colorado River Basin for maintenance and enhancement of Squawfish populations. I n_s t_r_e a m ___F 1_ o.w.._._T a m.p a ,r a_t_u, r_e . _M o_d e 1 The Instream Flow Temperature Model was developed by the Service's ~ National Ecology Center, in cooperation with the U.S. Soil Conserva- tion Service (Theurer, et al., 1984). The model predicts instream water temperatures based on hydrologic, meterologic, and geometric stream conditions. The model is applicable to any size watershed and can handle up to 365 time periods. Features included in the model are described in detail by Theurer, et al. (1984). The primary model components are 1) a solar model that predicts the solar `~~ radiation penetrating a water surface; 2) a meteorology transfer model that transfers meteorological data from a weather station to any point in the basin; 3) an equilibrium water temperature model using heat flux components; 4) a dynamic temperature, steady flow heat transport model; and 5) validation and calibration protedures for certian physical parameters when water temperatures are available at interior points. The model programs were originally written for execution on a Cyber