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others; at the same time, high flows may be the only available means by which to reduce production of several prolific and potentially detrimental introduced species. Reduction of Historic Peak Flows During recent times, spring flows in the Colorado and Gunnison rivers have been significantly reduced as a result of upstream water development . projects, primarily transmountain diversions and reservoir storage. Large changes in the upper Colorado River first began during the 1943-1953 period when various storage and diversion facilities related to the Colo- rado-Big Thompson Project were being put on line. Closure of the first dam, Green Mountain, was in fall 1942; closure of the last western slope dam associated with the project, Willow Creek, was in spring 1953 (Water and Power Reasources 1981). Though other projects built since this period have further impacted Colorado River flows, we felt this was a reasonable block of years to use for separating historic from recent periods for comparison purposes. When we compared yearly discharge records for the 36 years following this period with the 41 years prior to this period for which records exist, we found that the mean peak flow in the 15-mile reach is now only 56 percent of the pre-development mean (Table 4, Fig. 6 and Appendix Table I). Major changes have also occurred in the Gunnison River basin, which affects flows of the Colorado River downstream of the 15-mile reach. During the 24 years since completion of Blue Mesa and Morrow Point reservoirs in 1966, the mean yearly peak flow at the mouth of the Gunnison River has been reduced to 52 percent of that which occurred prior to completion of the f'_rst major water project in the Gunnison River head- waters (Taylor Reservoir in 1937). 27