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<br />002475 <br /> <br />1990 measurement interval. Between 1990 and 2000, many of the cross-sections <br />aggraded slightly, during this period dominated by low power plant capacity flows. <br />At the time ofthe 1956 cross-section measurements, the bed of the Colorado <br />River in Glen Canyon was mostly sand and the average bed surface size was about 0.2 <br />mm (Figure 6). Underlying the sand at varying depths was a layer of gravel with an <br />average size of about 20 mm. Throughout the study area, the entire thickness of the sand <br />layer and a significant thickness ofthe underlying gravel have been evacuated (Figure 7). <br />This is shown at those locations where the depth to gravel in 1956 was measured at or <br />near one of the monitoring cross-sections. Up to 7 m in thickness and 50% of the total <br />volume of material evacuated between 1956 and 2000 was derived from the underlying <br />gravel layer. <br />A total of approximately 10.7 million m3 of sediment was eroded from the study <br />area (Figure 8), exceeding the volume of degradation predicted at the time of dam <br />construction (Pemberton, 1976) by about 30 percent. In the upstream 10 km, the <br />accumulated volume of degradation has not changed significantly since 1965. At <br />distances greater than 10 km downstream, the volume of degradation increased <br />significantly after 1983 and was concentrated at cross-sections 10 and l1A. In both <br />cases, the degradation occurred along the margins of the channel and not in the thalweg. <br />Thus, continued degradation has evacuated sediment from the reach without affecting the <br />longitudinal profile of minimum bed elevation. <br /> <br />Spatial pattern of bed adjustment and development of the post-dam longitudinal profile <br />Based on observations in the field and inspection of the cross-sections and the <br />post-dam longitudinal profile, cross-sections were divided into those that occurred in <br />pools and those that occurred at or near riffles. On average, the magnitude of degradation <br />in pools exceeded that measured in riffles by a factor of five. The magnitude of <br />degradation in riffles decreases with distance downstream from the dam. However, the <br />magnitude of degradation in pools does not systematically decrease (Figure 5). This <br />demonstrates that local variation in channel adjustment owing to local variation in <br /> <br />25 <br />