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upon physical habitat (depth, velocity, substrate and cover) of altered flow regimes are determined using the PHABSIM system. • In this system, depth, velocity, substrate and cover at stream study sites characterized by multiple transects are predicted using hydraulic simulation techniques. Values for these parameters are com- pared with habitat suitability curves for each species in question, to obtain relative preferences or weighting factors. The product of the weighting factors is applied to the surface area of the study reach to obtain a Weighted Usable Area (WUA) for the given species at the modeled flow level. WUA is an approximation of the relative usability of the stream reach; in some cases (notably for certain salmonids) it correlates well with the actual adult biomass. In most cases, changes in WUA should be thought of as an indicator of the amount of preferred physical habitat combinations, and as such a guideline for evaluation of various flow levels or regimes. The specific regimes to be analyzed for this report were to depict present and past flow regimes and resulted in: 1. WUA for flows during some past period (before measurable depletions) to be known as "natural" flows: 2. WUA for a time period under present depletion levels, to be known as "present" flow levels, and • 3. WUA for actual measured flows for the period between 1960 and 1980 (to be known as "actual" flow levels). Each of the analyses were to be of mean monthly flows. The "present" and natural flows were to be simulated using the Bureau of Reclamation Colorado River Simulation System (CRSS) model. Instream Water Temperature Model Because water temperature changes in the Green and Colorado Rivers were considered significant factors in the decline of the endangered fish species, a model was developed to determine effects of altered flow and reservoir release temperatures upon water temperatures throughout the Upper Colorado River Basin (UCRB). The desired capabilities were to 1) determine steady-state basin-wide temperature patterns which might result from various monthly flow levels; 2) determine site specific temperature regimes resulting from predicted flow regimes and 3) determine expected monthly maximum temperatures at various observation points. The original Statement of Work specified use of the temperature component of the SSAM IV water-quality model developed jointly at the IFG and Utah State University. As discussed later, this model served as a basis for development of a temperature model which allowed accounting for several data deficiencies unique to the UCRB. Specifically, these were • 1) scarcity of meterologic monitoring stations; 2) extensive shading due 2