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<br />002485 <br /> <br />similar among four of the five reaches (Figure 23). However, in Reach 2, the area of <br />erosion and the area of perched gravel deposits was more the twice that measured in any <br />of the other reaches. This is the reach with the widest alluvial valley (Table 3) and the <br />greatest area of pre-dam terraces in 1952. Within Reach 2, half of the erosion occurred at <br />one location: the terrace on river left at R-11A. The erosion at this location between <br />1952 and 1984 was the largest single change measured in the entire study area (Figures <br />9A and 21). This terrace was the largest fine-grained deposit in the study area in 1952, <br />and the remaining portion of that deposit was still among the largest terraces mapped on <br />the 1984 photographs. <br />The most pronounced difference between this terrace and other pre-dam terraces <br />that did not erode is its position relative to other channel features and the location of <br />maximum erosion on the channel cross-section. At R-11A, erosion of the bed was <br />greatest on the portion of the cross-section nearest the terrace. As this portion of the <br />channel lowered, the gravel bar on the right bank remained stable, concentrating flow on <br />the left side of the channel. This combination of bed degradation, which increased the <br />relief at the edge of the terrace, and concentration of flow near the terrace probably led to <br />the high rate of erosion of this deposit. In contrast, most of the other terrace deposits are <br />adjacent to lower elevation channel-margin gravel or sand deposits. In these locations, <br />bed degradation has typically been greatest in the center of the channel or on the opposite <br />bank. This is the case just 850 m upstream from R -11 A at R -12, where the pre-dam <br />terrace has been stable. This and the other observations suggest that the style and <br />magnitude of change in the alluvial deposits within the study area is controlled by local <br />reach characteristics more strongly than distance downstream from Glen Canyon Dam. <br />Tables summarizing the changes between aerial photograph intervals are included <br />in Appendix G. <br /> <br />Erosion and Deposition: 1984 to 1996 <br />Changes in the alluvial deposits in Glen Canyon between 1984 and 1996 were <br />small compared to the changes measured between 1952 and 1984. There has been little <br />bank erosion since 1984. Channel-side sand deposits and pre-dam terraces continued to <br /> <br />35 <br />