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walls, producing changes in bed sediment characteristics. These potential changes are discussed <br />below in the section on sediment dynamics. Reductions in sediment transport capacity also may <br />have reduced the frequency of pool scour and the reworking of gravel deposits on riffles, <br />resulting in an overall simplification of bed morphology. Further, the reduced frequency with <br />which the Dolores River attains discharges sufficient to overtop its banks suggests that channel <br />floodplain connectivity and flow-driven adjustment of floodplain processes has declined. The <br />magnitude of these changes has likely varied in reaches downstream of McPhee Dam, depending <br />on the background geomorphic conditions of the reaches and on the availability of unregulated <br />high flow inputs from tributary basins. Because tributary flow inputs are limited between <br />McPhee Dam and the San Miguel River confluence, the effects of reduced high flows have likely <br />been concentrated upstream of the San Miguel The San Miguel River is a large tributary with <br />miiimal flow regulation and continues to deliver high flows to the lower Dolores River. The San <br />Miguel River therefore likely dampens the effects of McPhee Dam and limits the magnitude of <br />changes in geomorphically important flows downstream of its confluence with the Dolores, as <br />illustrated by the IHA analysis discussed above. <br />Geomorphic changes associated with flow modification therefore have likely been smaller <br />downstream of the San Miguel than upstream. Factors including bed gradient, confinement, and <br />tributary influences mediate the sensitivity of reaches between McPhee Dam and the San Miguel <br />to flow-induced geomorphic changes. Overall, lower-gradient, unconfined reaches are those that <br />are most susceptible to geomorphic changes such as fining of bed material, channel narrowing, <br />and channel simplification. hi the Dolores River, such reaches include the reach from McPhee <br />Dam to the upstream end of Dolores Canyon (3-4 miles downstream of Bradfield Bridge) and <br />portions of the river flowing through Big Gypsum and Paradox Valleys. In addition, reaches <br />downstream of sediment-producing tributaries, especially Disappointment Creek, have likely <br />experienced flow induced geomorphic changes due to reductions in the mainstem river's ability <br />to rework and transport tributary sediment inputs. In contrast, high-gradient, confined, and <br />bedrock-controlled canyon reaches may be relatively less susceptible to geomorplc changes. It <br />is likely that even in these reaches, channel narrowing due to growth of bars and vegetation <br />encroaclunent, as well as fining of bed sediment, has occurred. <br />Sediment dynamics in the Dolores River basin <br />Sediment dynamcs, including the timing and magntude of sediment supply to streams, sediment <br />transport within streams, and the size, heterogeneity, and stability of substrates, have an <br />important influence on channel form and are an important driver of physical habitat structure <br />(Waters 1995). Sediment dynamics are especially important to bottom-dwelling aquatic <br />organisms, which are often associated with specific sediment enviroiunents (Allan 1995, Palmer <br />et al. 2000). Alteration of sediment regimes can degrade habitat through burial or scour, changes <br />in substrate heterogeneity and stability, and altered channel form. In addition, interactions <br />between alterations in flow and sediment regimes have important effects on aquatic biota and <br />implications for the conservation of many at-risk aquatic species (Osmundson et al. 2002, Hart <br />and Finelli 1999). The geology, climate, soils, and vegetation characteristics of the Dolores River <br />basin result in a substantial increase in sediment yield, in both absolute and per acre terms, from <br />the upper to the lower basin. Data from gaging stations indicates that total suspended sediment <br />36 <br />