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<br />002460 <br /> <br />76,000 Mg for the total tributary drainage area of the Glen Canyon study area. Based on <br />analysis of the Escalante River suspended-sediment data (this river drains similar <br />lithologies to those found downstream from Glen Canyon Dam), half of this load is <br />probably sand. Others have estimated the sand content oftributary inflow to be as low as <br />15% (Randle and Pemberton, 1987). <br />The difference between the estimated post-dam sediment delivery to the study <br />area and estimated average load at Lees Ferry indicate that a sediment deficit exists for <br />Glen Canyon, While such a deficit has certainly existed throughout the post-dam era, <br />there is uncertainty in its magnitude. <br />A brief description of the nature of pre-dam sediment transport in Glen Canyon is <br />necessary to provide context for understanding how flow regulation has completely <br />changed the character of this reach of the Colorado River. During the average pre-dam <br />year, sand exported from Glen Canyon accumulated in Marble and upper Grand Canyons <br />during the nine months (July-March) ofthe year when the discharge was typically lower <br />than about 250 m3/s (Topping et aI., 2000). Then, during the three months of higher <br />discharge during the snowmelt flood (April-June), this stored sand was exported from <br />Marble and upper Grand Canyons. This process led to pronounced annual hysteresis in <br />suspended-sand concentration and grain size at the Grand Canyon gage, located 141 Ian <br />downstream from Lees Ferry (Topping et aI., 2000). Although there were not large <br />changes in the loads of silt and clay, there were substantial differences in the loads of <br />sand as a function of discharge between the Lees Ferry and Grand Canyons gages. This <br />high degree of sensitivity to changes in sediment supply indicate that the amount of <br />background sediment storage in Marble and upper Grand Canyons was small relative to <br />the seasonal change in sediment storage. As the sand concentration decreased during the <br />snowmelt flood, the grain size of the sand in suspension coarsened leading to the <br />deposition of inversely graded flood deposits in Marble and Grand Canyons (Rubin et aI., <br />1998). <br /> <br /> <br />Topping et aI. (2000) showed that significant increase in the degree of sediment <br />supply limitation appeared to occur near Lees Ferry, thus significantly distinguishing the <br /> <br />10 <br />