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<br />002483 <br /> <br />the invasive saltcedar shrub (Tamarix sp.) has been present in the region since the 1930s <br />(Clover and Jotter, 1944), there has been a great increase in its abundance since 1952 <br />(Turner and Karpiscak, 1980). <br />This increase in the abundance of alluvial deposits occurred as active bars and bed <br />sediments became part of the flood plain as a direct result of bed degradation. Because <br />degradation has not been uniform across the channel, but has been concentrated in certain <br />locations, sand and gravel deposits became perched on the channel margins. <br />The shifts in the stage-discharge relations that occurred throughout most of Glen Canyon <br />resulted in decreased inundation frequencies for deposits that have not changed in <br />elevation. Thus, while sediment has been evacuated from the bed of the river, channel- <br />side deposits, which are relicts from the pre-dam era, are no longer inundated and are <br />preserved by stabilizing vegetation. <br /> <br />Erosion and Deuosition of Alluvial Deuosits <br />Erosion and Deposition: 1950s to 1984 <br />The best illustration of channel change is provided by those locations where cross-section <br />surveys and erosion-deposition maps both indicate changes on the banks. It is important <br />to note that the erosion-deposition maps do not provide an objective measure of bed <br />erosion because they only depict deposits that were above the water surface at the time of <br />photography. For example, in the vicinity ofR-11A bed degradation occurred across <br />much ofthe cross-section, but this degradation is not shown in the erosion-deposition <br />map (Figure 21). The maps do capture erosion in locations where channel-side deposits <br />present in 1952 have been replaced with lower elevation deposits or have become part of <br />the river channel. The cross-section measurements document the depth and the timing of <br />erosion while the maps provide the only means of quantifying the spatial extent of the <br />erosion. <br />The 1952 to 1984 erosion-deposition maps indicate apparent deposition on many <br />of the gravel bars. We refer to this as apparent deposition because the cross-section <br />measurements show that these deposits have not changed in elevation or have in fact <br />degraded (Figure 21). The appearance of deposition in these locations is a result of the <br /> <br />33 <br />