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• desirable to determine the flow requirements within these reaches for YOY squawfish. Such flow requirements result from analysis of both physical habitat and biologic data; the results presented in this phase of the report only define the species requirements with respect to depth, velocity, and substrate. 3. Certain habitat types, based both on PHABSIM results and actual fish captures, offer little suitable habitat at any normal flow, but may serve as refugia during extremely low-flow periods. In such cases, passage from normally suitable reaches to these areas might allow short-term survival during such severe conditions as were seen in 1977 or 1981. Also, such habitats, usually characterized by poor physical diversity, may be selected as potential habitat improvement locations. 4. To assure reproductive isolation between humpback and roundtail chubs at Black Rocks, May mean monthly dis- charges should be maintained in the 10,000 to 13,000 cfs range; June flows, at least at the historic mean monthly level, appear to be desirable. 5. The analyses of flow and WUA time series for natural vs present level depletions demonstrated a generally positive • effect upon squawfish YOY. However, the apparently narrow range of suitable discharges at the Green River nursery sites (especially Mineral Bottom) resulted in effects ranging from high positive to high negative, depending upon the month involved. It is quite possible that fur- ther unspecified depletions in either river might result in generally negative effects. At present, the discharge ranges suggested should be adhered to. 6. The apparent suitability of low-regime flows for YOY should not encourage neglect of temperature and sediment transport consequences. The 3,500 cfs flow found to provide excellent YOY habitat at Ouray and Mineral Bottom would not be a suitable peak flow to ensure maintenance of sand-transport equilibrium throughout the Green River. Similarly, the 7000-8000 portion of the suitable Potash YOY range, if allowed to become peak flow in the Colorado, would not ensure sand-transport equilibrium, and might alter the Black Rocks-Westwater temperature regime enough to cause loss of reproductive isolation between humpback and roundtail chubs. LITERATURE CITED • Bovee, K. D., and R. T. Milhous. 1978. Hydraulic simulation in instream flow studies: theory and techniques. U.S. Fish and Wildlife Service, Instream Flow Information Paper 5, FWS/OBS-78-33, Fort Collins, CO. 130 p. 33