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<br />24 <br /> <br /> <br />0024'74 <br /> <br />is likely that most of this degradation occurred in May 1965. During the first two years <br />of dam operations, releases from Glen Canyon Dam were extremely low to increase the <br />level of Lake Powell. It is unlikely that significant bed lowering took place during this <br />period of low flows. In May 1965, the emergency bypass facilities Get tubes) were used <br />for the first time in conjunction with the left-bank diversion tunnel. In that month, flows <br />of up to 1645 m3 Is were released in a series of several short spikes (Figure 2). Although <br />the stated purpose of these releases was to test the bypass facilities, it appears that <br />accelerating the process of bed degradation near the dam was a secondary objective. <br />Several Reclamation documents, including a 1957 degradation report, make it clear that <br />degradation of the bed in Glen Canyon was anticipated and incorporated in power plant <br />design (U.S. Bureau of Reclamation, 1957). Moreover, engineer's notes in Reclamation <br />files indicate that the 1965 high releases were intended as a "channel-cleaning" flow <br />designed to scour the bed in Glen Canyon to achieve the optimum tailwater elevation <br />downstream from the dam (Appendix E). Not only did these "channel-cleaning" flows in <br />1965 scour sediment from Glen Canyon, they also scoured approximately 16 million Mg <br />of fine sediment from Marble and upper Grand Canyons (Topping et aI., 2000; Rubin and <br />Topping 2001). <br /> <br />Continued degradation and the magnitude of sediment evacuation <br />All of the cross-sections in the study area degraded between the beginning of dam <br />construction in 1956 and the most recent survey in 2000. However, the rate of bed <br />lowering decreased markedly following the 1965 channel-cleaning flows. The rate of <br />bed degradation was lowest between 1965 and 1975, when dam releases were maintained <br />at or below power plant capacity. Between 1975 and 1990, most cross-sections were <br />relatively stable but R-lO and R-l degraded (Figure 5). Most of this degradation <br />probably occurred during the large floods of 1983-86. The 1983 cross-section <br />measurement was made in October, several months after that year's peak discharge. <br />Degradation occurring during the 1983 flood is recorded in the 1975-1983 measurement <br />interval, and degradation occurring during the 1984-86 floods is recorded in the 1983- <br />