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r Appendix I of the report explains the rationale for this shift in methodology. An analytical model, Physical Habitat Simulation (PHABSIM), was used for summer in stream flow recommendations for the July-September period. A flow recommendation of 700-1200 cfs was developed to maintain 95% of the total pool, riffle and run habitat for adult Colorado squawfish. PHABSIM has limitations when attempting to make flow recommendations for the remainder of the year. Since it was developed for use in higher gradient trout streams, its use is constrained in lower elevation, turbid waters like the Colorado system. As a result, direct observation of habitat use by fishes is not possible, and assumptions regarding the fishes' use of deep water habitats, "micro-habitats", backwaters, eddies, etc. must be made, leading inevitably to an over-simplification of the real world scenario. The Service hopes to gain greater insight into the relationship between the fishes and their habitats as additional methods are applied and further data become available. As an alternative to PHABSIM, the Service utilized observational data (i.e., conclusions about what habitat the fishes preferred based on their locations as fixed by radio- telemetry) and empirical relationships (relationships between numbers and flows) to develop the flow recommendations for fall, winter and spring. The concerns over PHABSIM and suggestions from CWCB staff (memorandum from E. I. Jencsok to George Smith, USFWS, October, 1988) are factors which caused the Service to develop flow recommendations using these methodologies. Sorinq CAoril-Junel Observations show that adult Colorado squawfish prefer complex habitats during the spring (see Figure 2). The term "complex habitats" refers to river segments with multiple channels, islands, bars, shifting substrates, and diverse velocity distributions. During spring run-off the fish seek sheltered environments where water is warmer and slower. It is a widely accepted fluvial geomorphologic theory that higher spring flows contribute to this "channel complexity" and prevent vegetative encroachment (channelization). Due to the importance of channel complexity, high peak flows are necessary to maintain this key element of the fishes habitat. Data show that high spring flows also increase spawning success and the subsequent abundance of young native fishes. For example, Figure 3 illustrates that the highest catch rate for young Colorado squawfish occurred in years when spring flows were high (1979, 1980) and were lowest in years when spring flows were relatively low (1981). High spring flows also reduce the density of predatory, competing non-native fish (Figure 4). When peak discharge at the Colorado/Utah stateline ranges from 30,000 to 40,000 cfs, the production of young squawfish in upstream sites is enhanced. Spring flow requirements for the razorback sucker are similar 3