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were made at intervals along each transect to describe the lateral <br />distributions and gradations of these parameters. Each stream cell was <br />defined in the IFG4 program with a surface area (defined by the distances <br />between transects and verticals), a substrate type, and an average depth <br />and velocity for various streamflows. The HABTAT4 program is now used in <br />conjunction with this information and the appropriate SI curve to develop <br />a habitat index value, termed weighted-useable-area (WUA), which <br />represents the availability of potential fish habitat as a function of <br />streamflow. The WUA value expresses the potential of each cell within a <br />stream reach to support a life stage of a given species. The WUA for a <br />stream reach is then determined by the equation: <br />WUA~,e=sum of C;.,, x Al <br />where WUAQ,s= the weighted-useable-area for a particular species and <br />life stage at a particular flow. <br />C1,.= the suitability index for the combined characteristics <br />of the cell (i), by the target species (s). <br />A1= the surface area of the cell. <br />It must be remembered that calculation of WUA does not completely <br />describe the quantity or quality of available fish habitat. It does, <br />however, provide a structured approach for utilizing streamflow dependent <br />microhabitat conditions to describe potential fish habitat in riverine <br />environments . <br />STUDY REACH ANALYSIS <br />The Green River Subbasin comprises about 116,000 1~n2 (45,000 mil) of <br />the 284,000 1~ (109,500 mil) occupied by the UCRB (Tyus et. al 1987). It <br />is the largest tributary to the mainstem Colorado River,~~and extends about <br />1 <br />19 <br />