Laserfiche WebLink
<br />00,24j32 <br /> <br />(Pemberton, 1976). Thus, Pemberton (1976) concluded that bed degradation was largely <br />complete by 1975. <br />The U.S. Geological Survey (USGS) records of discharge measurements made at <br />the Lees Ferry gage (station number 09380000) provide a long record of bed elevation at <br />the downstream end of Glen Canyon. These records were analyzed, in part, by Burkham <br />(1986) and Topping et al. (2000). Burkham (1986) examined a subset of those <br />measurements made between 1924 and 1984 and reported on long-term trends in channel <br />width, cross-section area, mean velocity, and maximum depth. He observed that, prior to <br />1940, the thalweg annually scoured during the spring snowmelt flood and then returned <br />tothe approximate pre-flood elevation during the summer and fall. Beginning in 1942, <br />the thalweg typically did not completely refill to the elevation of the preceding year, <br />resulting in a gradual trend of decreasing average bed elevation from 1942 to 1962 <br />(Burkham, 1986). Burkham (1986) suggested that this trend of decreasing average bed <br />elevation was in response to a trend of decreasing suspended sediment concentrations <br />during this pre-dam period. <br />The records analyzed by Burkham (1986) show scour of the bed during a May <br />1965 dam-bypass release, and that this scoured condition persisted through 1984. The <br />analysis of bed elevation from the Lees Ferry discharge measurement notes is, however, <br />complicated by the use of three different cableway locations. Burkham (1986) suggested <br />that the behavior of the sties was similar, based on characteristics of the stage-discharge <br />and velocity-discharge relations among the sites, and therefore treated measurements <br />made at the separate locations as one continuous time series. Because the location of the <br />cableway used for discharge measurements was permanently moved shortly after the <br />1965 scouring event, the exact response at that location has been unknown. <br />Topping et al. (2000) analyzed a more complete record of mean bed elevation for <br />one of the three Lees Ferry cableway locations for the pre-dam period 1921 to 1959. <br />Their analysis indicated no trend in mean bed elevation between 1921 and 1929 and a <br />very gradual, but statistically significant, decrease in mean bed elevation from 1929 to <br />1959. Topping et al. (2000) concluded that this trend was consistent with other lines of <br /> <br /> <br />12 <br />