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18. The FWS recently completed a preliminary run of its Physical Habitat Simulation model (PHABSIM) developed by the Instream Flow Group, Fort Collins, Co. The output of the model predicts the quantity of simulated habitat for various life stages of Colorado squawfish and other fishes at different flows. The model has been refined to include depth, velocity, and substrate data, and other empirical data gathered in field studies conducted by FWS. The results of the preliminary outputs are difficult to interpret because hydrologic cross sections were not specifically selected to include some critical habitats. FWS field work and model output tend to support one another in a general way although it must be recognized that the model output has yet to be validated in field tests. Examples of model output for Ouray, Utah are presented in Table 4 and Figures 13 and 14. Monthly mean historic and present flows were simulated along with target flows recommended to optimize Colorado squawfish nursery habitat. Simulated target flows were derived from habitat vs. flow curves formulated from field research data accumulated by CRFP and other sources. Although some differences are evident between field observations of nursery habitat and the amount of habitat predicted by the model, the peaks and dips in habitat availability under different flows suggests that caution be exercised in recommending flows until the model output can be varified with empirical field data. FLAMING GORGE DAM The impact of Flaming Gorge Dam on Colorado squawfish recruitment is unknown, and the operation of the dam is controversial. In general, the effects of hydropower storage projects on downstream riverine environ- ments usually result from altered flow and temperature regimes. Discharge is usually decreased from the natural seasonal maximum with this water being stored and released later to increase discharge from the natural seasonal low flows. In addition, power peaking operations result in daily flow variations which did not exist under natural conditions. Because deep reservoirs thermally stratify, the depth from which water is withdrawn determines the temperature of reservoir releases (the temperature of water released through hydropower turbine penstocks can be controlled if the project is fitted with a multi-level outlet structure). Flaming Gorge Dam is a hydropower storage project located on the Green River at RM 407.0. Original penstock intakes for three hydropower units are located at the 190 ft depth at normal pool elevation (6040 ft msl). The dam was retrofitted with a selective hydropower intake device allowing water to be withdrawn from any depth between 20 and 190 ft at normal pool elevation. The temperature of water released from the dam is regulated for enhancement of the trout fishery that exists in the downstream tailwater, and the target temperature for the warmer months is 55-560F. In the cooler months, thermal conditions prevailing in the reservoir produce water temperature in the tailwater which may be slightly higher than under natural conditions.