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<br />001638 <br /> <br />. <br /> <br />. <br /> <br />character of pre-dam sediment supply of Glen Canyon from than of Marble and Grand <br />Canyons. Unlike the Grand Canyon gage, very little hysteresis was evident in either <br />suspended-sand concentration or grain size at the Lees Ferry gage. The seasonal scour <br />and fill in Glen Canyon at Lees Ferry appeared to be controlled mainly by reach <br />geometry and was not substantially influenced by depletion in the upstream supply of <br />sediment during snowmelt floods, as at the Grand Canyon gage. Also, very little change <br />in the bed-sediment grain size occurred during pre-dam snowmelt floods at Lees Fen)', <br />unlike at the Grand Canyon gage, where the bed sediment coarsened substantially during <br />these floods. Moreover, the pre-dam flood deposits sampled in Glen Canyon did not <br />ubiquitously coarsen upward with respect to sand grain size, unlike deposits sampled <br />further downstream in Marble and Grand Canyons. Topping et al. (2000) therefore <br />concluded that the Colorado River behaved much more like an equilibrium sand-bedded <br />channel in Glen Canyon than it did in the more sediment supply-controlled Marble and <br />Grand Canyons. This hypothesis is supported by an analysis of the pre-dam sediment <br />budget for Glen Canyon (Appendix A). <br /> <br />. <br /> <br />Previous Geomorphic Investieations in Glen Canyon <br />Annual and seasonal patterns of sediment transport and storage in the Colorado <br />River in Glen Canyon were drastically changed with the completion of the cofferdam at <br />the Glen Canyon damsite on February 11, 1959 and closure of the gates of Glen Canyon <br />Dam on March 13, 1963. Pemberton (1976) summarized bed degradation in Glen <br />Canyon measured between 1956 and 1975, and compared these measurements with <br />predictions made 20 years earlier (U.S. Bureau of Reclamation, 1957). The 1957 study <br />predicted that a gravel bar approximately 6 km downstream from the dam and the riffle at <br />the mouth of the Paria River would act as controls on the depth of bed degradation and its <br />downstream extent. Pemberton (1976) found that the measured net degradation of 9.87 <br />million m3 in the 25-km study area was only slightly greater than the predicted net <br />degradation of 8.26 million m3 and that stability had been achieved by 1975 through bed <br />armoring at gravel and cobble bars acting as channel controls. The median size of the <br />armor layer at these bars was equal to or larger than the predicted armoring size <br /> <br />11 <br />