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<br />6 <br /> <br /> <br />002456 <br /> <br />INTRODUCTION <br /> <br />Bed degradation is a common response downstream from large dams (Petts, 1979; <br />Galay, 1 983;Williams and Wolman, 1984), and the 25-km segment of the Colorado River <br />immediately downstream from Glen Canyon Dam has degraded and armored <br />dramatically in response to the water and sediment regulation (Pemberton, 1976), <br />Although the magnitude, and in some cases, the rate of bed degradation can be predicted <br />with reasonable results in pre-impoundment studies (e.g. Komura and Simmons, 1967; <br />Pemberton, 1976), changes in channel width and the distribution and character of <br />channel-side alluvial deposits are typically more complex and generally less predictable <br />(e.g. Benn and Erskine, 1994; Grams and Schmidt, 2002). Degrading reaches <br />downstream from dams in the semi-arid western United States have been shown to <br />exhibit both trends of increasing and decreasing channel width (Williams and Wolman, <br />1984). Friedman et aI. (1998) demonstrated that channel narrowing was the dominant <br />response in braided reaches downstream from dams in the western Great Plains, while <br />meandering channels had more stable widths, but exhibited a reduced rate of channel <br />migration. Processes of channel narrowing have been described in detail for aggrading <br />reaches (Everitt, 1993) and reaches where the bed is stable (Allred and Schmidt, 1999; <br />Grams and Schmidt, 2002). The specific interaction between stream-bed elevation and <br />channel width was recently investigated by Friedman et aI. (1996) who describe channel <br />narrowing subsequent to bed degradation following extreme floods on unregulated Plum <br />Creek, Colorado. <br />Although the effects of operations of Glen Canyon Dam on channel-side alluvial <br />deposits have been studied in reaches downstream from Lees Ferry (e.g. Schmidt et aI., <br />1999; Hazel et aI., 1999), the Glen Canyon reach has been largely excluded from <br />previous geomorphic investigations, because most of these studies have been conducted <br />with the objective of evaluating or monitoring the condition of campsites used by river <br />float trips, which begin at Lees Ferry, or habitat for endangered fish species, which do not <br />occur in the Glen Canyon study area, There are, nevertheless, important resources in <br />