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<br />002479 <br /> <br />Selected plots of the cross-section at the Lower Cableway show that the bed has <br />degraded along much of the channel width, creating a more rectangular cross-section than <br />existed in the pre-dam era (Figure 14). While the thalweg elevation in January 2000 is <br />similar to the elevation during pre-dam floods, there has been 2 to 5 m of erosion along <br />more than 100 m of the cross-section that never scoured during pre-dam floods. Thus, <br />the area of degradation at this cross-section is large even though the depth of degradation <br />is less than for the Upper Cableway. <br /> <br />Channel-width chant!es at the Lower Lees Fer" Cablewav <br />Measurements made at the Lower Cableway record changes in channel width <br />from 1929 to present. Such a record is not available for the Upper Cableway where <br />channel width has changed owing to rockfall (Topping et aI, 2003), but not alluvial <br />deposition or erosion. The Lower Cableway traverses a large sand deposit on the left <br />bank, and changes in channel width at this location resulted from aggradation and <br />degradation of that deposit. <br />Since the first Lower Cableway measurements in 1929, there have been episodic <br />decreases in channel width, illustrating a pre-dam cycle of deposition and erosion of <br />channel-side deposits. The process of narrowing we describe is similar to that described <br />by Allred and Schmidt (1999) wherein bank-attached bars form or migrate into the cross- <br />section, aggrade and become colonized by riparian vegetation, and are never scoured by <br />subsequent floods. Figure 15 shows the channel width for each discharge measurement <br />plotted in relation to the Lower Cableway water surface elevation. The data are divided <br />into five time periods and describe a progressive decrease in channel width. From May <br />1929 through most of June 1935, the relation between width and water surface elevation <br />was stable throughout the range of discharges measured. Between June 20, 1935, and <br />May 11, 1936, there was a three-meter decrease in width, establishing a new trend in the <br />relation that was stable through 1938. In June 1938, the channel narrowed by more than <br />2 m in one day, establishing a new trend at elevations above 952 m. A final episode of <br />narrowing occurred in May 1948. Time series of channel width can only be shown for <br />narrow elevation intervals, which removes the effect of water surface elevation on the <br /> <br />29 <br />