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<br />. <br /> <br />ABSTRACT <br />The 25-km reach of the Colorado River immediately downstream from Glen <br />Canyon Dam is a classic example of a channel whose bed has degraded and armored in <br />response to flow and sediment regulation caused by a dam. Although the evacuation of <br />bed sediment from this reach was reported upon nearly 30 yrs ago (Pemberton, 1976), the <br />complete array of channel adjustments has not been described previously. This study <br />uses the abundance of historical data available for this river segment and field <br />measurements made in the last decade in a comprehensive analysis of channel change. <br />We add 25 years to the record of previously reported bed elevation measurements, such <br />that the long-term trend in channel bed adjustment can now be understood. These <br />measurements are supplemented with (1) new analyses of long-term records of bed and <br />bank elevation at U. S. Geological Survey stream gaging stations, (2) mapping of <br />historical aerial photographs that depict changes in the channel-side alluvial deposits, and <br />(3) a pre-dam sediment budget for Glen Canyon. These data depict in detail the <br />processes of the transformation of the Colorado River in Glen Canyon from an alluvial <br />sand-bedded river with a large reservoir of fine-sediment storage to a pool-and-riffle, <br />gravel-bed trout stream. <br />Although bed degradation began when the cofferdam was installed in 1959, the <br />greatest proportion of degradation occurred in 1965 when the U. S. Bureau of <br />Reclamation intentionally released high flows from the dam in a series of pulses intended <br />to scour the reach downstream from the dam. This event evacuated a reach-average of <br />2.6 m of sediment from the center of the channel. Continued erosion occurred in the <br />downstream half of the study area during emergency power plant-bypass releases of the <br />mid-1980s. There were small adjustments in bed elevation in the 1990s, but the channel <br />bed today is much the same as it was 15 years ago. The total volume of bed sediment <br />evacuated from the study reach is equivalent to one-third the pre-dam annual sediment <br />load and is two orders of magnitude greater than estimated post-dam sediment inputs to <br />the reach. The average size of bed material has increased from 0.2 mm in 1956 to over <br />20 mm as measured in 1999. <br /> <br />. <br /> <br />. <br /> <br />4 <br />