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<br />001700 <br /> <br />Revised Supplemental Draft Environmental Assessment-Chapter 3-Affected <br />Environment and Environmental Consequences <br /> <br />Conventional Fish Ladder: Construction of a fish ladder around the existing <br />diversion dam would have little or no effect on the stability of the Tunnel No.3 landslide <br />provided there is not an overall increase in the river water surface elevation. <br /> <br />Downstream Rock Fish Passage: Construction of the downstream rock fish <br />passage would have no effect on the stability of the Tunnel No.3 landslide. <br /> <br />Downstream Rock Fish Passage with Whitewater Recreation Features: Same <br />as the Downstream Rock Fish Passage alternative. <br /> <br />Dam Removal: Removal of the Price-Stubb Diversion Dam would change river <br />dynamics upstream ofthe dam in the vicinity of the Tunnel No.3 landslide. A <br />preliminary scour study conducted by Reclamation's Technical Service Center (Lyons, <br />1998) shows the average flow velocity ofthe river wouldincrease in the reach from the <br />diversion dam upstream to the Colorado River Siphon. However, this study indicated no <br />channel degradation would be anticipated since there is no extensive area of sediment <br />deposition upstream of the dam. <br /> <br />In the preliminary study, assumptions were made concerning the composition of the <br />riverbed. A more formal study was subsequently conducted, and riverbed samples were <br />taken and analyzed. In addition, scuba divers conducted a survey of the deeper portion of <br />the riverbed upstream from the dam (Collins, 1999). The results of these studies <br />fundamentally agreed with the initial study, except they anticipate the removal of about 2 <br />to 3 feet of fine materials that have been deposited behind the dam. It is believed that <br />under the existing conditions, these materials are flushed annually during spring runoff, <br />and are re-deposited after the higher flows subside. <br /> <br />Another study completed by Reclamation's Technical Service Center specifically <br />analyzed the effects of dam removal on the stability ofthe Tunnel No.3 landslide (Pabst, <br />1999). Detailed geologic information is limited for this slide and a monitoring program <br />is in place. The main conclusion from this study was that dam removal should not have a <br />negative impact on slide stability assuming no river scour occurs. Lowering the river <br />water surface would cause a lowering of the water table within the landslide mass, which <br />would slightly increase landslide stability. A rapid drawdown of water surface or an <br />overall increase in water surface would contribute to instability of the landslide. Since <br />dam removal would occur during low flow conditions, and the dam would be breached in <br />a controlled manner, a rapid drawdown of the river surface would not occur. <br /> <br />Ownership of Dam and Lands <br /> <br />Issue: Before any modification to the dam and site could be made, permission <br />would be needed from the dam and adjacent land owners to access the site and/or use <br />their lands and facilities. <br /> <br />41 <br />