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• provided the capability to quickly calculate WUA for a time series of monthly discharges without running the HABTAT program with its high associated computer cost. WUA vs. Ascending Flow The WUA vs. flow graphs presented here are primarily for use in describing general relationships and cover only one life history stage of two species. They are best interpreted as indicators of general flow ranges, i.e., as aids in determining whether low, medium, or high regime discharges are most suitable for a life history stage. They should serve as tools in the development of flow recommendations, not as sources of inviolable "minimum" flow requirements. Green River The two sand-substrate reaches, Mineral Bottom and Ouray refuge provided dramatically higher WUA for YOY squawfish than did the Gray's Canyon reach (Figures 6 through 8). Mineral Bottom and Ouray were quite consistent in offering high WUA in the 2000-3000 cfs range both for squawfish YOY. The Gray's Canyon reach, probably because of its large substrate and severe velocity conditions offered far less habitat, most of which was probably around the edges of the wetted area. The selection of a site below a major rapids was influenced strongly by access and measurability. The more distinctive Gray's Canyon aquatic habitats were the deep, rather slow reaches where steep cliffs bordered one channel margin and broad sandbars the other. Measurement of such a site (several of which are accesible below Rattlesnake Rapids) is suggested as a valuable addition to the current sites. Colorado River As on the Green River, two Colorado River sites (Black Rocks and Potash) provided good WUA for Colorado squawfish YOY while the Moab site had significant WUA only for a limited range of low flows (Figures 9 through 11). The Black Rocks reach gained in adult humpback chub habitat as flow increased, probably because of the large secondary channel area which became inundated at higher flows. The same effect was probably true for squawfish YOY, in this case resulting in two peaks of WUA. At Potash, a similar bimodal relationship was apparent; this condi- tion has been noted in other large rivers in the basin and probably resulted from the stepped cross-sectional configuration and the estab- lishment of two distinct habitats and different flow levels. The Moab reach, because of its simple, incised channel characteris- tics, provided little shallow, slow water and subsequently, very little YOY habitat. The center channel areas were of uniformly high velocity, however, and provided good adult habitat only at very low flows. • 15