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The magntude of these changes has likely varied in downstream reaches, depending on the <br />background geomorphic conditions of the reaches and on the availability of unregulated high <br />flow inputs from tributary basins. Because tributary flow inputs are limited between McPhee <br />Dam and the San Miguel River confluence, the effects of reduced high flows have likely been <br />concentrated upstream of the San Miguel. High flow inputs from the San Miguel River have <br />limited the magnitude of changes in geomorphically important flows in the lowermost reaches of <br />the Dolores River and have therefore likely limited the geomorphic changes associated with flow <br />modification. <br />Factors including bed gradient, confinement, and tributary influences mediate the sensitivity of <br />reaches between McPhee Dam and the San Miguel to flow-induced geomorphic changes. <br />Overall, the reaches that are most susceptible to geomorphic changes in the Dolores River are <br />those with shallow gradients and wide valley bottoms, including the reach from McPhee Dam to <br />the upstream end of Dolores Canyon (3-4 miles downstream of Bradfield Bridge) and portions of <br />the river flowing through Big Gypsum and Paradox Valleys. In addition, reaches downstream of <br />sediment-producing tributaries, especially Disappointment Creek, have likely experienced flow- <br />induced geomorphic changes due to reductions in the mainstem river's ability to rework and <br />transport tributary sediment inputs. <br />Other anthropogeiuc factors infhuencing geomorphic processes in the Dolores River have <br />historically included roads, grazing, and recreation. Valley-bottom roads constrain channel <br />migration along certain reaches, and grazing-induced reductions in vegetative cover have likely <br />increased sediment inputs to the lower Dolores River. Bail{ erosion associated with sport fishing <br />and rafting may locally increase sediment inputs. Overall, however, these activities have likely <br />had a small effect on geomorphic processes compared to reduction in the magnitude, frequency, <br />and duration of high flows caused by McPhee Dam. <br />The preceding analysis is intended to serve as the basis for future discussion of approaches to <br />improving ecological conditions in the Dolores River downstream of McPhee Dam. This <br />analysis suggests that management of high flows, subject to legal, operational and safety <br />constraints, has the greatest potential for maintaining or restoring geomorphic processes and the <br />aquatic habitats shaped by these processes. Geomorphologists from the Core Science Team will <br />work with the Dolores River Dialogue to determine the magnitude, frequency, and duration of <br />flows needed to maintain pool scour, rework coarse sediment, flush fine sediments, limit <br />vegetation encroachment and growth of bars, and perform other ecologically important <br />geomorphic work. <br />39 <br />