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delivered downstream. However, because of reductions in peak flows, both the Colorado River <br />and the Gunnison River have lost some of their capacity to carry sediment. Changes in transport <br />capacity over the long term have caused sediment to accumulate in the channel, causing it to <br />become narrower and less complex overall. Van Steeter and Pitlick [1998] report that between <br />1937 and 1993 the main channel of the Colorado River narrowed by an average of about 20 m, <br />and one quarter of the area formed by side channels and backwaters had been lost. <br />Although water-management activities have caused persistent, long-term changes in the <br />hydrology of the Colorado River, the potential exists to coordinate reservoir operations in the <br />upper basin to periodically augment spring snowmelt flows and enhance peak discharges in the <br />15- and 18-mile reaches. The function and importance of peak flows were summarized in the <br />recommendations given previously by Pitlick and Cress [2000]: <br />• Flows equal to or greater than 1 /2 the bankfull discharge are needed to mobilize <br />gravel and cobble particles on a widespread basis, and to prevent fine sediment from <br />accumulating in the bed. Flows greater than 1/2 the bankfull discharge also transport <br />between 65 and 78% of the annual sediment load of the Colorado River. Flows <br />greater than 1/2 the bankfull discharge thus provide several important geomorphic <br />functions, assuming they occur with sufficient frequency. In the 20-year period from <br />1978 to 1997, daily discharges equaled or exceeded 1/2 the bankfull discharge an <br />average of about 30 days per year. Given these results and supporting information <br />about what these discharges accomplish, we recommend that flows equal to or greater <br />than 1/2 the bankfull discharge should occur with an average frequency of at least 30 <br />days per year. <br />3