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<br />002486 <br /> <br />erode slightly (Figure 20). Post-dam high flows resulted in localized areas of deposition <br />between 1984 and 1990 and between March and April 1996. Despite these changes, <br />there was no significant change in the total area of pre-dam terrace, post-dam flood, or <br />fluctuating-flow deposits from 1984 to April 1996 (Figure 20). <br /> <br /> <br />Erosion and Deposition in Eddies <br />The eddies in Glen Canyon are not uniformly distributed throughout the reach <br />(Figure 1). Based on our surficial geologic mapping, we identified a total of 34 eddy <br />depositional zones between the dam and Lees Ferry, 20 of which are larger than 1000 m2. <br />In the first four reaches, eddies are rare but those that do occur tend to be large, resulting <br />in a much larger average EDZs than in downstream reaches (Table 5). In Reach 5, there <br />are approximately 2.6 EDZs per km, which is comparable with the frequencies observed <br />in some of the reaches downstream from Lees Ferry. Although there are a few large <br />eddies in Glen Canyon, most eddies are small (Figure 24). The cumulative frequency <br />distribution ofEDZs approximates a log-normal distribution, as is the case further <br />downstream (Schmidt et aI., 2002). <br />Like the gravel bars and channel-margin deposits, active eddy bars in Glen <br />Canyon became perched and stabilized with vegetation' as a result of bed degradation. <br />Figure 25 shows four nearby EDZs between 14.5 and 16.0 km downstream from Glen <br />Canyon Dam. These four EDZs are among the largest in Glen Canyon. The area of <br />exposed sand has not changed significantly between 1952 and 1984. Most of the area <br />that was mapped as clean sand in 1952 is mapped as a post-dam flood deposit in 1984. <br />These deposits consisted primarily of clean sand in 1952, and were densely covered by <br />vegetation in 1984. Examination of the repeat surveys of R-7 indicated that these bars <br />are currently at a lower elevation than in 1952 (Figure 9B), indicating net loss of sand <br />from the EDZs. Thus, despite negative changes in the volume of sediment stored in <br />eddies, the area of exposed sand has remained remarkably constant. <br />Changes at these sites are representative of the rest of Glen Canyon (Figure 26). The area <br />of high-elevation sand in eddies has decreased slightly since 1984, although this decrease <br />is not within our estimate of error. Meanwhile, the area of low-elevation sand has <br /> <br />36 <br />