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Continued, ongoing research which will refine existing flow recommendations and integrate new information into the process is an explicit element of the Recovery Program (USFWS 1987). In Colorado, the Colorado Water Conservation Board is responsible for appropriating instream flows. In 1988, the 15-mile reach (RM 171-185) of the upper Colorado River was identified as one of the highest priority areas for instream flow protection. In May 1989, the USFWS provided recom- mendations for flows needed for rare fish in the 15-mile reach during the summer months (July- September); in April 1991, recommendations for the remainder of the year (October-June) were provided (see Kaeding and Osmundson 1989 and Osmundson and Kaeding 1991). Approach Used for Summer and Winter Recommendations The earlier summer recommendations were largely aimed at providing flows that would optimize adult squawfish physical habitat within the reach and to boost water temperatures during July to promote growth of young squawfish within and downstream of the reach. To relate flows to adult squawfish habitat, an analytical model was used; this was the Physical Habitat Simulation System (PHABSIM-2), a technique often employed within the larger Instream Flow Incremental Method- ology (IFIM) described by Bovee (1982). Untested assumptions and other limitations associated with utilizing IFIM', along with some site- specific problems (e.g. representativeness) prompted USFWS to initiate an alternative method of tying flows to habitat needs. Results from such a method would either substantiate the PHABSIM- 2 output, identify discrepancies, or serve to refine the earlier recommendations. It was hoped that if this new technique showed promise, it could be used throughout the upper basin as a standard- ized means of developing flow recommendations. The approach used in this study to determine the best summer and winter flows for the reach was to identify which habitat types were preferred by the fish and then determine the flow level at which the amount or total area of those types is maximized. To determine habitat preference, frequency of use of habitat types by the fish was compared with relative availability of those habitat types. To determine the total area of those preferred habitats at different flow levels as well as relative availability for fish use, habitats were mapped with a combination of on-the-ground mapping and aerial videography. Stage and stream-bed cross sections were also monitored to determine at what flow level insufficient depth might become limiting. Like IFIM, this method attempts to quantify changes in habitat with changes in discharge. Also like IFIM, it assumes that, given the alleviation of other potentially overriding limiting factors, area of preferred habitat is correlated with standing ' The Recovery Implementation Program is an interagency consortium of Federal, State and private groups whose mission is to recover four endangered fish (Colorado squawfish, razorback sucker, bonytail and humpback chub) in the Upper Colorado River Basin while providing for future water development to proceed in compliance with the Endangered Species Act. 'Problems associated with utilizing IFIM for the endangered Colorado River fish have been summarized by Kaeding and Osmundson (1989), Osmundson and Kaeding (1991) and Stanford (1993). 2