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<br />26 <br /> <br /> <br />oozr76 <br /> <br />channel characteristics can be greater than longitudinal trends in the magnitude of <br />channel adjustment. <br />The locus of degradation varied considerably among the cross-sections (Figure 9). <br />Riffles tended to degrade differentially across the channel, often leaving part of the bed <br />near its pre-dam elevation, e.g. R-11A. In pools, degradation tended to occur across the <br />width of the channel, resulting in near uniform lowering, e.g. R-7. In nearly all cases, the <br />degradation was contained within the limits of the 1956 channel, thereby resulting in a <br />deeper and narrower channel. Exceptions to this pattern of narrowing occurred at R-IIA <br />(Figure 9) and R-4, where bank erosion caused channel widening. <br />The lowering of the bed has caused large shifts in stage-discharge relations that <br />are greatest near the dam and decrease downstream. The pattern exhibited by these shifts <br />in the stage-discharge relationships are, in fact, a better indicator of degradation of <br />channel controls than the actual measurements of bed topography at the cross-sections. <br />Stage-discharge relations were determined for each cross-section using all measured <br />water surface elevations. Linear trends were fit to each cross-section for periods where <br />the relation remained stable. For cross-sections where there were shifts in the stage- <br />discharge relation, most of the adjustment occurred by 1965, consistent with the period of <br />greatest bed degradation (Figure lOa). Stage-discharge relations at the downstream end <br />of the reach have been stable (Figure lOb). The downstream trend in the stage-discharge <br />adjustment was determined by evaluating the change in stage for a discharge of 150 m3/s <br />between the pre-1965 period and the post-1965 period. The change in stage decreases <br />from over 2 m at the upstream end of the study area to no significant change at R -1, 24 <br />km downstream (Figure 11). <br />The downstream limit of adjustment in the stage-discharge relation is about 20.2 <br />km downstream from the dam. The stage-discharge relation decreased at R-4, 20.1 km <br />downstream from Glen Canyon Dam and remained stable at R-2, which is 2.5 km farther <br />downstream. Between these two cross-sections are Cave Canyon and Fall Creek, two <br />tributaries with small debris fans (Figure 1). At low discharges (< 55 m3/s), there is a <br />small riffle at the mouth of Cave Canyon, which is the channel control for R-4. This <br />