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<br />002477 <br /> <br />riffle is the furthest downstream channel control for which degradation due to dam <br />operations is known to have occurred. The next riffle downstream at Fall Creek may <br />have degraded, but we were unable to reoccupy the cross-section at R-3, to determine <br />whether the stage-discharge relation had changed. The next significant channel control <br />downstream is the riffle opposite the mouth of the Paria River, and stability of the stage- <br />discharge relations at cross-sections downstream from Fall Creek indicate that that <br />channel control has been stable (Figure 11). <br />Prior to closure of Glen Canyon Dam, the average gradient through the study area <br />was similar across a broad range of discharges (Figure 7). The reach average gradient <br />was 0.00034 and 0.00037 at discharges of79 and 2067 m3/s, respectively. The onset of <br />bed degradation, which has been greatest at the upstream end of the study area, caused <br />the reach-average gradient to decrease to between 0.00025 and 0.00028. While reach- <br />average gradient has decreased, differential bed erosion has caused the local gradient to <br />become steeper in some locations and less steep in others. <br /> <br />Magnitude of bed degradation relative to pre-dam scour-and-fill <br />Upper Lees Ferry Cableway <br />Combined, the USGS discharge measurements and the Reclamation and GCMRC <br />cross-section surveys provide a 79-yr record of bed elevation for the Upper Lees Ferry <br />Cableway. Our time series differs from that of Topping et aI. (2000) by the inclusion of <br />all available post-dam data and from that of Burkham (1986) by segregating the data by <br />measurement location. The time series shows annual scour and fill of up to 7 m and a <br />downward trend in the annual maximum of the minimum bed elevation (Figure 12). This <br />trend begins in about 1940 and was previously identified by Burkham (1986) and <br />Topping et aI. (2000). During this period, the bed never scoured lower than 941.5 m, <br />which was the maximum depth of scour measured in 1929. Thus, the 1940 to 1959 trend <br />of decreasing bed elevation is a result of the cross-section failing to completely fill rather <br />than progressively deeper scour. Topping et aI. (2000) concluded that this pattern is <br />indicative of a slight pre-dam trend of increasing sediment-supply limitation in Glen <br />Canyon. <br /> <br />27 <br />