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24 3. The 15-mile reach provides an important refu<3e for endangered fishes should a catastrophic event cause a loss of populations in the Gunnison River or in the Colorado River below the Gunnison River confluence. FLOW RECOMMENDA~'IONS FOR THE 15-MILE REACH The 15-mile reach has experienced major agricultural water depletions for many years. During late summer and early fall, this reach can be severely dewateied. Although it experiences major water depletions, the 15-mile reach is viewed as critical to the recovery of Colorado River populations of Colorado and razorback sucker. The development of .recommended flows in the 15-mile reach, therefore, was identified as important to achieving recovery of these species. The Service first developed flow recommendations in 1.989 and 1991. The summer flow recommendations (Kaeding and Osmundson 1989) were developed using the Instream Flow Incremental Methodology. The spring and winter flow recommendations (Osmundson and Kaedir~g 1991) were based on other methodologies. Serious shortcomings of the IFIM approach in developing flow recommendations for endar.~gered fish in the Upper Colorado River led the Service to initiate a ne:w study for determining recommended summer and winter flows. Using new information obtained from this study as well as that collected by other researchers, the Service updated and refined it.s earlier flow recommendations (Osmundson et al. 1995). . The Service's 1989 flow recommendations for summer were developed by modeling microhabitats based on depth, velocity, and substrate measurements at a site thought to be representative of the reach. The Service's 1991 flow recommendations for winter consisted of a tentative recommendation that flows not fall below historic levels; this recommendation was based on the assumption that historic conditions provided adequate winter habitat for adult fish. In refining these flow recommendations for summer and winter, the Service determined which habitat types (pools, riffles, etc.) were preferred by the fish during these seasons and then determined the flow level at which the preferred habitat types are maximized in area. This approach, as with other instream flow methodologies, assumes 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. Earlier Service flow recommendations for spring were based on information on how the decrease in magnitude of aprir.~g flows could negatively affect endangered fish reproduction and survival. Data were provided which showed that low spring runoff resulted in lower larval production. The explanation for this relatior.~ship was that high flows are periodically needed to build cobble bars and flush fine sediment from the gravel/cobble substrates used by for spawning.