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that increases in the amount of preferred or optimum habitat increases carrying capacity and, barring other potentially limiting factors, results in an increase in population size. Four sub-reaches averaging 0.5 miles in length were selected as study sites within the 15-mile reach. The sites were selected based on results from a previous radiotelemetry study that indicated they were areas of high use by the endangered fish. To determine habitat preference for adult Colorado squawfish, the percent frequency of use of each habitat type by an individual fish within the study sites was compared with the relative availability of each habitat type within the study sites. Difference between frequency of use and availability provides a measure of the degree of preference for a given habitat type. For instance, if a fish is found in pools 90% of the time, but pools make up only 10% of the total water area within a reach of river that the fish occupies, preference for pools is indicated, i.e., the fish is selecting a habitat type in greater proportion than its availability would alone predict if selection of sites was random. Preference rating for pools for the fish in the example would be 0.8 (0.9 minus 0.1). This process is repeated for each fish for which location data were collected within the study sites. Preference rating for pools for each fish is then averaged to arrive at a mean preference rating for pools by squawfish in general. This exercise was repeated for each of eight habitat types that collectively made up the total water area of the study sites. Habitat frequency of use data were collected by radio tagging a sample of Colorado squawfish and razorback sucker and recording their location once weekly. When a location was made, the habitat type at the site of the fish was recorded as was the mean-column water velocity, depth and substrate type. Habitat availability data was collected by mapping habitat delineations on aerial-video prints of the study sites. A field crew mapped habitat units within the sites from on-the-ground vantage points while video of the sites was taped from a helicopter flown at a constant elevation. Once the habitat units were overlain on the video images and scanned into a computer, areas of individual habitat units could be measured and their percent contribution to the total water area of a study site calculated. This process was repeated at 11 different flow levels ranging from 557 to 11,200 cfs, as measured at the top of the 15-mile reach, to gain an understanding of how relative availability and total water area of eight habitat types change as a function of discharge level. ]Eddies, pools and deep backwaters were found to be the habitats preferred by adult squawfish during summer. Interestingly, these are habitat types that are relatively rare, comprising only a small proportion of the total water area. When summer water levels became very low in the reach, fish modified their behavior and were found in the more common slow and fast run habitats, or left the reach entirely by moving downstream below the confluence with the Gunnison River where more water was available. In winter, eddies, pools and backwaters were again found to be the preferred habitat types, with pools preferred most. Of the flow levels typical of summer and winter conditions, both recently and historically, the flow level evaluated here that provided the greatest amount of the preferred habitat was 1,630 cfs. This became the basis for the new summer and winter instream flow recommendation. Other variables that were examined included the suitability of depth at various flow levels and the interspersion or density of habitat types. In years with above average winter precipitation levels, a flow of 1,630 cfs is recommended for summer. In years of somewhat below average precipitation, when the ideal flow of 1,630 cfs would be difficult to meet, the recommendation could be relaxed to 1,240 cfs. At this flow, the area of preferred habitat would be reduced but the flow would not be so low as to compel the fish to modify their habitat selection or be forced from the reach. In years of drought (20% lowest xi