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<br />002453 <br /> <br />downstream from Glen Canyon Dam, in kilometers. The dam is at RM -15.7 and the <br />Lees Ferry cableway, 25.4 kIn downstream from the dam, is at RM 0.0. <br /> <br />.~-w-- .' <br /> <br />Streamflow Reeulation <br />There are no significant water-contributing tributaries in the study area between <br />Glen Canyon Dam and the Paria River confluence (Figure 1). The streamflow measured <br />at Lees Ferry is, therefore, representative of the entire study area. A stream gage has <br />been in continuous operation at Lees Ferry since 1921. The gage and streamflow record <br />have been analyzed in detail by Topping et aI. (2003). Streamflow regulation at Lees <br />Ferry began with completion of the cofferdam at the Glen Canyon damsite in February <br />1959. Although the 130 million m3 storage capacity of the cofferdam (pemberton, 1976) <br />was too small to control floods, it likely caused reduced sediment concentrations in Glen <br />Canyon. The complete regulation of streamflow in Glen Canyon began with the closure <br />of Glen Canyon Dam in March 1963. Subsequently, the mean annual (2-yr) flood was <br />reduced by 63% from 2407 m3/s to 892 m3/s (Topping et aI., 2003). The post-dam <br />average flood is essentially the same as the capacity of the Glen Canyon Dam power <br />plant. Flows have exceeded power plant capacity only rarely since 1962 (Figure 2). <br /> <br />Sediment Supply <br />Like most reservoirs formed by large dams, Lake Powell is a highly efficient <br />sediment trap. Topping et al. (2000) analyzed the record of suspended sediment <br />measurements made at Lees Ferry and determined pre- and post-dam average loads of <br />fine sediment. For those pre-dam years with a complete sediment record, 1949 to 1962, <br />approximately 57 ='= 3 million Mg of fine sediment was transported past Lees Ferry each <br />year. Measurements from 1966 to 1970 indicate a post dam mean annual load of 0.24 ='= <br />0.01 million Mg, a reduction of more than 99%. The post-dam sediment load is derived <br />from the bed and banks of the river in the Glen Canyon study reach and ephemeral <br />tributaries that drain the highly erodible Mesozoic sedimentary rock formations. <br />The post-dam sediment yield from tributaries between Glen Canyon Dam and <br />Lees Ferry has been independently estimated by Webb et aI. (2000) using regional <br />sediment yield equations and other empirical methods. They estimated an annual load of <br /> <br />9 <br />