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interstices of coarser bed materials such as gravels and cobbles. Coarser sediments (e.g., <br />gravelsize sediment) deposited in the mainstem from side canyons form tributary bars, and most <br />of the rapids in the Lower Dolores River are produced by such depositional features (USDI <br />BLM, 1990). Whereas at least some size fraction of these bar deposits would likely be mobilized <br />on an annual basis by high flows under unregulated conditions, these coarse materials are likely <br />rarely transported under current conditions, potentially causing steepening of rapids. This <br />potential change in the stability of tributary bar deposits is analogous to the conditions in the <br />Colorado River in Grand Canyon associated with reductions in peals flows by Glen Canyon <br />Dam. The effects of flow modification-induced changes in downstream sediment characteristics <br />may have been exacerbated by increases in sediment supply above natural levels due to <br />agriculture and other land uses. A 1972 report suggested that overgrazing had depleted <br />vegetation cover and increased soil erosion in many arid rangeland areas (USDA, 1972). <br />Because of the dry climate and slow rate of recovery of vegetative cover in this area, grazing <br />induced increases in erosion likely persist in the lower Dolores River basin. Bai~lc erosion <br />associated with sport fishing and rafting may locally increase sediment inputs. Roads and other <br />land uses have also likely increased sediment inputs to river channels, but these sources are <br />likely minor compared to natural sediment yields. Overall, grazing, recreation, roads, and other <br />land-use activities have likely had a small effect on geomorphic processes compared to the <br />effects of flow modification. <br />Key data gaps <br />As mentioned above, much of the information provided here about geomorphic conditions in the <br />Dolores River and about potential human effects on those conditions is based on previous reports <br />and other remote methods. Data are needed on bed substrates sizes and cross-section geometry <br />(especially repeat surveys of previously measured cross-sections) in order to develop <br />scientifically based estimates of the discharge magnitudes required to mobilize sediment of <br />various size fractions and perform other geomorphic work. Additional historical analysis of <br />repeat sequences of aerial photographs in relation to flow data would elucidate relationships <br />between geomorphic processes and high-flow magiutude, frequency, and duration, although <br />available aerial photographs may be of insufficient resolution for such analysis. Data on <br />geomorphic conditions in canyon reaches is especially sparse due to the inaccessibility of many <br />of these reaches. <br />Summary <br />This analysis suggests that reductions in high flows below McPhee Dam are a primary cause of <br />geomorphic changes in the Dolores River downstream of the dam. The primacy changes that <br />have occurred due to flow modification have likely included: <br />• changes in channel dimensions, including narrowing and reduced depth; <br />• growth of lateral and mid-channel bars associated with reduced sediment <br />• mobility and encroachment of riparian vegetation; <br />• changes in the size, embeddedness, and mobility of bed sediments; <br />• simplification of bed morphology; and <br />• reduced channel-floodplain connectivity. <br />38 <br />