WSP12655 - Laserfiche WebLink

Home Browse Search

of all fine sediment is assumed to have accu- mulated on the bed. Analysis of pre-dam historical photographs, and estimation of the fine-sediment storage potential of eddies, indicates that large fluctuations in sand storage also occurred in eddies, thus indicating that a significant proportion of seasonal storage also occurred there. Thus, it is unlikely that more than 50% of the total fme sediment storage occurred in the channel. It is likely that less than 30% of the channel bed stored significant amounts of sediment. Despite the different patterns of scour and fill, the two gages had similar long-term trends of degradation (Topping et aI., 2000b). There was a small, but progressive, decrease in bed elevation at each gage after each year's snow- melt flood, because the average bed elevation at low flows was never quite as high as that of the previous year. This decrease in bed eleva- tion was progressive between the 1920s and the 1960s (Topping et aI., 2000b). The degra- dation rate was between 2.5 and 3.0 crnlyr for the periods between 1929 and 1944 and be- tween 1945 and 1959, respectively, at the Lees Ferry gage (Fig. 16) and 1.6 crnlyr for the period between 1922 and 1962 at the Grand Canyon gage (Fig. 17). Topping et al. (2000b) argued that progressive degradation was caused by a long-term decrease in the amount 600 of fine sediment supplied from the upper I Colorado River basin, rather than a long-term change in flow regime. 002333 I '~- l' 0 .Jr I 150 300 I Meters main channel (Table 5). The range in esti- mates of bed elevation change depends on how much of the bed is assumed to have stored fine sediment. The smaller value assumes that fine sediment was stored everywhere but in rapids. The larger value assumes that fme sediment was stored in 30% of the main channel. Both predictions are less than was measured at the Lees Ferry and Grand Canyon gages. The only scenarios where annual average bed scour and fill approximate the magnitude measured at the two gages is if30% of the channel is assumed to have stored fine sediment and 50% or more 5.2 Post-Dam Changes of the Bed in Glen Canyon The bed of the Colorado River within 5 kIn from Glen Canyon Dam began to degrade between 1956 and 1959, soon after the coffer- dam was constructed. There was extensive degradation of pools and riffles within 20 kIn from the dam, caused by the 1965 channel cleaning flow. Subsequently, there was little degradation of riffles, but some pools contin- ued to degrade. The net effect of degradation was the transformation of a sand bed to a gravel bed throughout the 25 kIn of Glen Canyon. 5.0 Changes in the Topography of the Main-Channel Sed 25