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<br />002487 <br /> <br />fluctuated, and neither an increasing nor a decreasing trend are suggested. These patterns <br />are consistent with observations for reaches downstream for this same period (Schmidt et <br />aI., 2002). <br /> <br />DISCUSSION <br /> <br />s''f- <br /> <br />Adiustment of the Bed <br />The timing of major bed degradation in Glen Canyon is directly related to <br />management decisions made in the operation of Glen Canyon Dam. The episode of most <br />rapid degradation occurred during a series of spike flows released 2 yr after dam closure. <br />Reclamation documents contain calculations that were carried out to predict the amount <br />of bed lowering these flows would be expected to produce, indicating these flows were <br />released with the knowledge and intention of bed lowering. The power plant was <br />designed to operate at maximum efficiency following degradation of the downstream <br />channel, the magnitude of which was predicted by the studies initiated in 1956. The <br />channel-cleaning flows achieved this degradation much sooner than might have occurred <br />under different circumstances. Thus, in the case of Glen Canyon and possibly many other <br />systems, the timing of bed degradation did not follow a natural progression of channel <br />adjustment, but was an engineering decision. <br />Certainly much of the variability that has been observed in rates of bed <br />degradation downstream from dams (Williams and Wolman, 1986) is related to local <br />geomorphic conditions and bed sediment characteristics (e.g. Xu, 1996). However, <br />management intentions may be also be an important source of variability. The degree of <br />human control over the timing of bed degradation exhibited in Glen Canyon was possible <br />because of the large magnitude of reservoir storage relative to annual runoff. The <br />storage to runoff ratio for the Colorado River upstream from Glen Canyon Dam is <br />approximately 2.3, contrasted to most reservoir storage systems in the United States, <br />which have storage to runoff ratios significantly less than one (Hirsch et aI., 1990). For <br />those systems with low storage to runoff ratios, managers have little control over the <br />release pattern and the timing of bed degradation. As the magnitude of storage capacity <br /> <br />37 <br />