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<br />002489 <br /> <br />,...._---J <br /> <br />sections and the occurrence of differential erosion demonstrate that the longitudinal <br />profile that has developed as the riverbed eroded must be related to the distribution of the <br />bed sediment grain sizes. This indicates that the largest sediment buried in the bed was <br />located at the mouths of the largest tributaries, which have emerged as channel controls <br />as the finer sediment was evacuated from the bed. Therefore, occurrence of pool and <br />riffle segments exhibited in Glen Canyon appears to be related to the interaction between <br />tributary sediment delivery and mainstem sediment reworking, rather than a display of <br />classic pool-riffle morphology. <br />The bed lowering that established the modified post-dam water-surface profile <br />and resulted in perched gravel and sand deposits throughout the reach occurred in one <br />brief period, while sediment evacuation from the non-control sections is an ongoing <br />process. The recent post-dam high flows of the 1980s and 1996, which were probably <br />responsible for most ofthe sediment evacuation from the pools, did not cause further <br />lowering of the bed at channel controls. Based on these observations, it seems that future <br />lowering of the bed at channel controls, causing further shifts in the stage-discharge <br />relations, is very unlikely. Conversely, depending on the rate of tributary sediment <br />delivery and the size of supplied sediment, aggradation of channel controls is a <br />possibility. Aggradation of channel controls has been suggested for debris-fan dominated <br />canyons where floods have been eliminated (Graf, 1980; Webb, 1997). Continued <br />evacuation of sediment from pools should be expected during high power plant releases <br />and floods. It may, therefore, be useful to continue monitoring those cross-sections. <br />The volume of sediment that has been degraded from the bed in Glen Canyon is <br />large relative to post-dam sediment loads, but small relative to the pre-dam suspended <br />sediment loads. Approximately 18.4 million Mg of sediment has been excavated from <br />the bed of the Glen Canyon Reach (assuming the eroded sediment had a specific gravity <br />of2.65 and a porosity of 35%). This is 275 times the estimated annual post-dam input of <br />sediment of all sizes from ungaged tributaries in the study area, but just one-third the pre- <br />dam annual load of suspended sediment (sand and finer material) measured at Lees Ferry. <br /> <br />39 <br />