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EXECUTIVE SUMMARY Introduction This report presents the results of a U.S. Fish and Wildlife Service (FWS) habitat evaluation study for a 15-mile segment of the Colorado River. Utilizing new information provided here as well as that collected by other researchers, recommendations for mi tream flows provided in earlier FWS documents is updated and refined. The river segment in question, hereafter referred to as the ' 15- mile reach' is viewed as critical in recovering Colorado River populations of endangered Colorado squawfish (Ptychocheilus lulus) and razorback sucker (Xyrauchen texanus). Previous recommendations for flows during summer (July-September) in the 15-mile reach were provided by Kaeding and Osmundson (1989). In that report, flow levels were recommended based on output from the Physical Habitat Simulation System (PHABSIM), a model often employed within the larger Instream Flow Incremental Methodology (IFIM). Summer use of the reach was considered most valuable as habitat for adult Colorado squawfish and the PHABSIM model was used to predict what flow level would maximize the amount of river consisting of microhabitats with a set of depth, velocity and substrate characteristics often selected by adult squawfish. FWS later provided recommendations for the 15-mile reach for the remainder of the year, i.e., the winter and spring months (Osmundson and Kaeding 1991). In determining flows for the winter months, use of comparatively deep water by both species during winter led FWS to conclude that flow needs would be greater than during summer. Until more data could be collected or analyzed the interim recommendation was for current or historic flow regimes to be maintained. For the spring months, FWS concluded that the greatest value of high flows, typical of the spring runoff period, was the year-round benefits provided by the scouring and flushing action of the flood waters, i.e., channel maintenance, removal of embedded fine sediments from gravel and cobble substrates, control of vegetation encroachment, entrainment of organic debris from the floodplain into the channel, control of otherwise prolific non-native fish , etc. Of particular importance was a relationship that was noted between reproductive success of Colorado squawfish and peak spring flows within a given range of magnitudes. In addition, FWS linked the spawning and nursery habitat needs of razorback sucker to the ability of the river to flood its banks during the spawning period. Summer and Winter Flows Serious shortcomings of the IFIM approach used in developing flow recommendations for the endangered fish in the upper Colorado River led FWS to initiate anew study for determining recovery flows for the non-runoff, summer and winter periods. Rather than modeling microhabitats based on depth, velocity and substrate measurements at a site thought to be representative of the reach, the approach used here was to determine which habitat types (pools, riffles, etc.) were preferred by the fish and then determine at what flow level such preferred types are maximized in area As with other instream flow methodologies, the underlying assumption is x