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<br />002491 <br /> <br />in abundance in the downstream direction as the degree of degradation-induced shift to <br />the stage-discharge relation decreases. Although the largest changes in the alluvial <br />deposits occurred by 1984, some deposits have eroded or aggraded since 1984. <br />Although there has been a significant amount of terrace erosion, approximately <br />75% of the area of pre-dam terraces remains intact. The lower elevation fluctuating-flow <br />and post-dam flood alluvial deposits may be adjusted to the current post-dam flow and <br />sediment regime. Although there has been a significant amount of erosion of these <br />deposits, the rate of erosion has been declining since 1984 and has been balanced by <br />some deposition. The large eddy-deposited sand bars and channel-margin deposits that <br />do exist are largely stabilized by dense thickets of riparian vegetation. Moreover, these <br />deposits are rarely inundated by dam operations and in many cases protected by gravel or <br />cobble armor. The persistence ofthe few remaining bare sand deposits demonstrates the <br />efficiency of these sediment traps and indicates that the sediment input from the ungaged <br />tributaries in Glen Canyon, however meager, appears to be sufficient to maintain these <br />deposits. <br /> <br />CONCLUSIONS <br /> <br />1. The majority of bed lowering and sediment evacuation occurred during the <br />channel cleaning flows of May 1965. This degradation was anticipated and <br />planned by dam managers. <br />2. The magnitude of degradation of channel controls decreases systematically <br />downstream from Glen Canyon Dam to a distance of 20.6 km, below which <br />channel controls have been stable. <br />3. The magnitude of degradation in pools does not vary systematically with distance <br />below the dam. <br />4. Riffles and channel controls have not degraded significantly since the 1965 <br />channel-cleaning flows, whereas sediment evacuation from pools has continued to <br />present. The total magnitude of sediment evacuation is about 18.4 million Mg, <br />exceeding the predicted magnitude by about 30%. <br />5. The dropping stage-discharge relations have decreased the inundation frequency <br />of deposits leaving pre-dam channel-side sand deposits and portions of the <br />exhumed gravel bed perched above the range of post-dam normal power plant <br />discharges. This has caused in increase in the area of exposed alluvium and <br />reduction of channel width by about 6%, despite the large magnitude of net <br />sediment evacuation from the reach. <br /> <br />41 <br />