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<br />that is operating with the environmental constraints imposed by the recently completed Operation <br />of Glen Canyon Dam Environmt:ntallmpact Statement. a document developed with the <br />participation of the Sierra Club and other environmental groups along with water and power <br />users, recreational interests, and state and Iribal governments, <br /> <br />End of Response <br /> <br />Impacts of drawing down Lake Powell <br />to allow exposure of Glen Canyon <br /> <br />I - With respect to David Brower's concept to drain Lake Powell completely, the following can <br />be noted: <br /> <br />a - The spillway discharge tunnels, to which are connected the river bypass tunnels <br />used during construction, have a "flip bucket" elevation of 3 158 feet, <br />approx.lmately 14 feet below the original streambed elevation of 3172 feet at the <br />dam, and a tunnel bottom elevation of 3 134 feet near the flip bucket. The flip <br />buckets would likely be removed to allow free flow of both water and sediment <br />through the tunnel. Thus with the bypass tunnels open, the lake would be <br />completely drained, even the dead storage which cannot be released from the <br />hollow jet outlet tubes that were used for the spike flow test. <br /> <br />b - The river bypass tunnels have a diameter of 41 feet. With the concrete plugs <br />removed, the dam would function much like during the initial construction period <br />when the tunnels were used to bypass the entire Colorado River flow, Under an <br />assumption of non-pressurized, open channel flow, the tunnels have a combined <br />release capacity of about 100,000 cfs, Greater amounts could be discharged if <br />Lake Powell refilled enough to pressurize the tunnels (about 10 to 20 feet of <br />head), <br /> <br />c - Peak flows on the Colorado River at Lees Ferry typically occur in Mayor June and <br />have a broadly based shape, Statistical analysis revealed that the 1 DO-year peak <br />flows are about 155,000 cfs, the 20-year peak flows are about 125,000 cfs, and the <br />average peak flows are about 65,000 cfs, This means that the rive bypass tunnels <br />probably could pass all but about 10% of the flows in all years, Only yean such <br />as 1952, 1957, 1983. and 1984 would there be more water than the tunnels <br />could pass. In those cases, there would be some filling (10 to 20 feet) of the <br />reservoir, creating some head on the tunnel resulting in river flows of about <br />120,000 to 130,000 cfs. <br /> <br />5 <br />