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<br />002470 <br /> <br />deposits. The pre-dam high sand and the post-dam flood deposits correlate and are <br />referred to as high-elevation deposits. The terrace category includes all deposits mapped <br />as terraces on either the pre- or post-dam photographs. <br />The study area was divided into five 5-km reaches for the purpose of generalizing <br />longitudinal patterns of channel adjustment indicated by the surficial geologic maps. <br />Post-dam average gradient in these reaches varies from 0.0001 to 0.0005 and post-dam <br />average channel width ranges from 156 to 216 m (Table 3). <br /> <br />....~\ <br />:~=:'~ <br />~~=-::~ <br /> <br />Characterization of Eddy Deposits <br />Eddy sand bars are numerous in reaches ofthe Colorado River downstream from <br />Lees Ferry (Schmidt and Graf, 1990), but also occur within Glen Canyon. Eddies form in <br />channel expansions downstream from constrictions created by flow obstructions. These <br />obstructions are most commonly caused by tributary debris fans but may also be caused <br />by bedrock outcrops and other bank irregularities. Downstream from Lees Ferry, storage <br />in eddies accounts for a significant proportion of the total fine sediment storage area <br />(Schmidt et aI., 1999; Schmidt et aI., 2002). Because the actual area of recirculating flow <br />is a function of discharge, it is necessary to quantify eddy size by some other objectively <br />defined measure. We use the method described by Schmidt et aI. (1999) that defines <br />eddy size based on the historical extent of sand within each eddy for a given reach. This <br />area is the union of all contiguous deposits within a recirculation zone as mapped from all <br />years of available aerial photography. The value of this metric, which we refer to as the <br />eddy depositional zone (EDZ), is a function of both the sand storage condition and the <br />water surface elevation at the time of the aerial photography. Because the same <br />photographs are used throughout a given study reach, this bias is consistently applied. <br />The frequency of large EDZs, defined as those larger than 1000 m2, varies from 2.2 to 5.0 <br />per km in reaches where detailed mapping has been completed in the first 120 km <br />downstream from Lees Ferry (Schmidt et aI., 2002). Their analysis included photographs <br />taken in 1935 that we did not use, because most of Glen Canyon is obscured in shadow. <br />The 1935 photographs were taken at low discharge in the sediment accumulation season <br />when many eddies were filled with sand, and the deposits mapped from these photos <br /> <br />20 <br />