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<br />38 <br /> <br /> <br />002488 <br /> <br />relative to runoff volume increases, managers have an increasing flexibility with release <br />patterns and, therefore, rates of bed degradation. <br />The different channel elements in the Glen Canyon reach exhibited different <br />patterns of degradation, both during the period of greatest bed lowering in 1965 and <br />during the period of more gradual degradation and sediment evacuation that followed. At <br />most of the riffles, nearly all of the degradation occurred during the 1965 channel- <br />cleaning flows and the magnitude of that degradation decreased systematically <br />downstream. This is supported by the systematically decreasing shift in stage-discharge <br />relations with distance from the dam. On the other hand, significant bed degradation and <br />sediment evacuation from pools has been variable throughout the reach and has continued <br />through 2000. This difference in behavior between hydraulic controls and the bed at <br />other locations explains the difficulty that has been encountered in efforts to predict the <br />magnitude of degradation based on distance downstream from dams (e.g. Williams and <br />Wolman, 1984). In this study, we have documented degradation of pools more than 6- <br />km downstream from the downstream limit of degradation of channel controls. Schmidt <br />et aI. (2002) report pre- to post-dam bed degradation at cross-sections in Marble Canyon <br />more than 60 km further downstream. <br />Previous investigations of bed adjustment at Lees Ferry (Burkham, 1986; Topping <br />et aI., 2000) focused on the role of decreasing supply of fine sediment in causing bed <br />degradation. The data we have analyzed show that both fine and coarse sediment have <br />been evacuated from the bed in Glen Canyon, indicating the sediment imbalance for this <br />reach applies to gravels, cobbles, and sand. <br />The disparity of degradation rates between riffles and pools has also led to the <br />adjustment of the river's longitudinal profile. Each steep segment in the profile is located <br />at a large gravel bar and the three steepest riffles occur at the mouths of tributaries. <br />These tributaries, Honey Draw, an unnamed tributary at RM -9, and Water Holes <br />Canyon, were identified by Webb et al. (2000) as potentially significant contributors of <br />sediment to the Colorado River by either stream flow. None of these tributaries has a <br />large debris fan, and only Honey Draw has a small debris fan. The development of steep <br />