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• Humpback Chub Adults at Black Rocks WUA at Black Rocks varied by a factor of less than 2 throughout the entire flow range (Figure 12). The Black Rocks hydraulic simulation model was only run to a low flow of 2000 cfs (a very low recurrence flow at the Colorado-Utah State line USGS gage). Even at this flow the Black Rocks reach offered more than 40,000 ft2 of WUA for every 1000 linear feet of stream, which is nearly half the total surface area of the reach. At flows between 8,000 and 15,000 cfs, over 60,000 ft2 of WUA was available. Clearly, based on the rather broad habitat suitability curves for depth and velocity and the fact that the substrates repre- sented in the curves are essentially the only ones available at the Black Rocks reach, flow changes are not likely to have a major effect upon adult humpback chub WUA at Black Rocks. Analysis of Flow Time Series I— The Colorado and Green Rivers have been influenced quite differently by water developments. The present vs. natural comparisons (using percent exceedence plots) show a proportional reduction in the occurrence of all flow levels on the Colorado river (Figure 13). In contrast, the Green River has experienced a similar reduction in flow, with proportion- ally greater losses in the high flow range (Figure 14). • The timing of depletions on the Colorado River appears to have changed slightly with present level peak flows occurring more often in May than in June, an event expected with major reservoir storage taking place on the runoff peak (Figure 13). Peak flow on the Green River simulation appears to have been shifted from June to July (Figure 14) during about half the simulated years. These timing shifts may simply be j the result of simulation techniques in the CRSS model, but could in actual occurrence cause significant impacts upon reproductive life history of the endangered fishes. WUA times series for YOY squawfish were produced at the Ouray, Mineral Bottom, and Potash sites because these habitats had been identi- fied as good rearing locations. The results from the Ouray and Potash reaches were probably more reliable than those from Mineral Bottom because of the latter"s very low and narrow range of suitable discharges. The effects of severe water depletion upon YOY habitat at the Ouray reach were variable among months from May through September (Figure 15). May, June, and July YOY habitat appears to have been profoundly improved by reduction in flow during those months. August and September results, however, indicate habitat depletions due to flow reductions in the lower range of suitable flows. During August, the depletions caused little change in physical habitat, but during September, reductions were typi- cally so severe that the pre-development regime provided almost 50 percent more habitat at the 50th percentile exceedence levels and 300 • percent more at the 80th percentile level. While these results were based on simulated data, they did clearly demonstrate that, even for immature fish which preferred slow, shallow water, flow levels must be maintained within a prescribed range if suitable habitat is to be provided. 19