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adult fish but also the backwater nursery areas critically important to <br />the young. Perhaps most importantly, production of young is highest in <br />summers following springs with peak flows that result in 30,000 to 40,000 <br />cfs at the State line. There also appears to be a strong relationship <br />between years of reduced peak flows and abundance of non-native minnows, <br />including red shiner, a species believed by many researchers to have a <br />negative effect on the rare native fish. <br />If recent changes to the natural hydrograph have had a negative impact on <br />populations of Colorado squawfish in the Colorado River as we have con- <br />cluded, similar changes would be expected to produce similar results in <br />other river systems. Indeed, when we plotted recent and historic mean <br />monthly discharge data for the San Juan River, where populations of Colo- <br />rado squawfish have also seriously declined (Platania 1990), a similar <br />alteration of the natural hydrograph was revealed (Fig. 10). As in the <br />Colorado River, spring flows have been greatly reduced in the San Juan <br />River; in addition, unlike the Colorado River, average base flows during <br />the remainder of the year have increased. In the Green River, Colorado <br />squawfish populations have not declined as appreciably as they have in the <br />other two river systems; spring electrofishing data from the Interagency <br />Standardized Monitoring Program indicates that catch rates of adult Colo- <br />rado squawfish in the Green River are about five times as high on average <br />as they are in the Colorado River (U.S. Fish and Wildlife Service 1987b, <br />1988, 1989, 1990). Plotting recent and historic mean monthly discharge <br />reveals that reduction of spring flows in the Green River has not been <br />nearly as significant as in the other two rivers (Fig. 10). Change in mean <br />peak flow is also much less in the Green River than in the other two <br />35