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<br />The WUA for a stream reach is then determined by the equation: <br />WUAa.s = sum of C1.S x ~ <br /> <br />where <br /> <br />WUAa.s <br /> <br />= the weighted-useable-area for a particular species and <br />life stage at a particular flow. <br /> <br />C~ = the sUitability index for the combined characteristics of <br />the cell (i), by the target species (s). <br /> <br />~ = the surface area of the cell. <br /> <br />It must be remembered that calculation of WUA does not~completely describe the <br />quantity or quality of available fish habitat. It does, however, provide a <br />structured approach for utilizing streamflow dependent microhabitat variables <br />as an index to describe potential fish habitat in riverine environments. <br /> <br />III. ' STUDY REACH ANALYSIS <br /> <br />The Green River sub~basin comprises about 116,000 km2 (45,000 mi2) of the <br />284,000 km2 (109,500 mi2) occupied by the UCRB (Tyus et. al 1987). It is the <br />largest tributary to the mainstem Colorado River, and extends about 800 linear <br />kilometers (500 mil to its headwaters in the Wind River Range of Wyoming (Tyus <br />et. al 1987). <br /> <br />Frqm 1955 to 1961 prior to the closure of Flaming Gorge Dam, the mean monthly <br />flows at the Jensen gage, RM 305, ranged from a low of 533 cfs in December, to <br />a high of 26440 cfs in June. From, 1962 to 1986, the mean monthly flows have <br />ranged from a low of 346 cfs in October, to a high of 24110 cfs in May <br />(Appendix A). <br /> <br />A. RUDle Ranch Study Site <br /> <br />The Ruple Ranch study site, 3282 ft in length, is located between RM 332.3 and <br />333.0. It morphologically represents sub-segment E of Segment 3, from Little <br />Rainbow Park upstream to the Cove just below Whirl Pool Canyon (RM 326.8- <br />334.0). The area encompasses Little Rainbow, Rainbow, and Island Parks, and <br />is dominated by multiple channels and numerous vegetated islands (Figure 6). <br />The river meanders with a low gradient (slope=0.00097) through an alluvial <br />flood plain resting on Navajo sandstone. The west edge of the river joins a <br />broad flood plain vegetated by sagebrush (Artemisia sp.), saltbush (Atriolex <br />sp.), rabbitbrush (Chrysothamus sp.), a variety of grasses, and a few <br />cottonwood trees (Pooulus sp.). The east edge is bound by a steep, sparsely <br />vegetated bank which lies against sheer walls of sandstone. <br /> <br />The channel stability of the Ruple Ranch study site is presently beinganalyzed <br />by the FWS Division of Water Resources, Region 6. The preliminary evaluation <br />of the channel processes, location and extent of islands, bars, vegetation, <br />topography, geology, and land use suggests that persistent channel narrowing <br />has ceased and a new equilibrium condition has been attained in the Green <br />River (Regional Hydrologist, Division of Water Resources Region 6, memo to <br />Keith Rose, Project Leader, Endangered Species, Grand Junction, 3 December <br />1987). Some changes were noted, however, in alluvial reaches of the Island <br /> <br />15 <br />