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<br />22 <br /> <br /> <br />0024'72 <br /> <br />contour lines intersecting the respective deposit. In some locations, poor overlay between <br />the topographic map and the surficial geologic maps resulted in inaccurate elevations. <br />For example, a small position error may cause a narrow sliver of steep slope or cliff to <br />intersect an alluvial deposit and significantly alter the calculated average elevation for the <br />deposit. Polygons were examined individually and elevations were interpreted manually <br />where these errors occurred. Water surface elevations from the Reclamation cross- <br />section surveys and the US Geological Survey gage stations were included on the profile. <br />The longitudinal profile of deposit elevations shows that while there is a range of <br />elevations within each elevation category and some overlap between categories, the <br />categories are generally well separated with distinct mean elevations (Figure 4). The <br />average slopes of each category are similar but converge towards the downstream end of <br />the study area. This convergence is an expected result of the greater magnitude of bed <br />degradation and greater lowering of the stage-discharge relation that has occurred at the <br />upstream end of the study area. Note that the deposits mapped as pre-dam terraces in <br />1956 plot above the post-dam flood deposits. <br />Schmidt et aI. (2002) estimated error in identification of fluctuating flow and <br />post-dam flood deposits. This was done by comparing the area of sand surveyed (Hazel <br />et aI., 1999) with the area measured by mapping from aerial photographs for these two <br />elevation categories. This comparison was made for eight sites where both data are <br />available, including one site in Glen Canyon. Comparisons were possible for multiple <br />dates of overlapping survey and photographic data resulting in 26 comparisons. The total <br />area compared was 166,469 m2. The variance c? in the comparison between the surveyed <br />area and mapped area was 828,599 m2. We applied this estimate of variance to the Glen <br />Canyon data to calculate the standard error SE for deposit area in Glen Canyon as <br />SE = (J'...r;. <br /> <br />where (J' is the standard deviation and n is the number of deposits. Individual estimates <br />ofthe standard error were made for the total area of eddy, channel-margin, and all <br />deposits for each year of mapping (Table 4). <br />