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<br />40 <br /> <br /> <br />002490 <br /> <br />The changing pattern of bed degradation in Glen Canyon is consistent with the <br />transition from incised meander to debris-fan dominated canyon stream type. This study <br />has confirmed that presence of debris fans limits the downstream extent of degradation of <br />channel controls, precluding the possibility of bed-elevation lowering in Marble Canyon <br />and Grand Canyon. <br /> <br />Adiustment of Alluvial Deposits <br />The pattern of alluvial deposit change in Glen Canyon is consistent with the <br />dramatic bed lowering and reduced sediment budget. The major changes identified <br />between the 1952 and 1984 aerial photographs include approximately equal areas of (1) <br />sand deposit erosion, (2) perched sand deposits, and (3) perched gravel deposits. Erosion <br />of pre-dam terraces also occurred. Because the thickness of erosion of channel-side <br />deposits is known for only a few locations, it is not possible to estimate the volume of <br />that erosion comparable to the estimate of sediment evacuated from the bed. The only <br />well-constrained estimate for magnitude of channel-side deposit erosion is for the <br />terraces, of which there has been about 1.5 million Mg of erosion between 1952 and <br />1996. This assumes 5 m of erosion, based on the cross-section measurements at R-I1A. <br />There has been up to an additional 5 million Mg of erosion on all other channel-side <br />deposits (sand and gravel). This assumes 3 m of erosion from all low-elevation deposits <br />and 2 m of erosion from all high-elevation deposits. These estimates are also based on <br />the cross-section measurements, but are poorly constrained because very few of the <br />repeat cross-section measurements traverse these deposits. All estimates assume a dry <br />sediment weight of2650 kg/m3 and a porosity of35%. <br />While bed degradation has been progressive, changes in channel-side deposits <br />have been spatially and temporally variable. Comparison between the mapped changes <br />and the cross-section surveys suggests the widespread perching of alluvial deposits <br />accompanied the bed degradation that occurred during the 1965 channel-cleaning flows. <br />The perched deposits are areas that were part of the river's bed in 1952, but are now <br />elevated relative to the bed in the thalweg and the low discharge water surface elevation, <br />owing to degradation concentrated in a portion of the channel. These deposits decrease <br />