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001556 Chapter 3 - Affected Environment and Environmental Consequences constriction caused by the highway and railroad tracks keep the velocity higher than what is commonly found behind a dam. Surveys of the river bottom upstream from the dam revealed a thin layer of sediment behind the dam, but due to the water velocities, most of the river bottom is composed of gravels and cobbles (Collins, 1999). The manager of Clifton Water District has said the District's main concern is knowing what to expect and when. They need to know what sediments exist, their composition, volume, and when the sediments would reach their river diversion. Consequently, Reclamation and the U.S. Geological Survey are conducting a sediment study in the area above the dam. To ensure that the study addresses Clifton Water District's concerns, the District reviewed the sediment study proposal. This study will identify volume and composition of the sediment. Field work for the study has been done, but the laboratory analysis has not been completed. Once available, all information will be provided to the District, and results will be reported in the Final EA. Timing of the dam removal would be coordinated with the Clifton Water District to avoid water quality problems. Ute Water Conservancy District Pump Plant - Spring Flooding Issue: Effects of each alternative on spring flooding of Ute Water pump plant. Existing Conditions: The Ute Water pump plant historically flooded when river flows were high and the Colorado River exceeded elevation 4,732 feet. Ute Water constructed a concrete retaining wall to an approximate elevation of 4,738 feet to protect their pump plant from flooding. The estimated 100- and 500-year floods at the dam are 44,500 and 52,800 cfs, respectively (Norval, 1998). The highest recorded flow in this stretch of the Colorado River was 36,000 cfs in 1983. According to Ute Water, the river elevation at that flow was just below their retaining wall (4,738 feet). They placed sandbags on top of the wall as a precautionary measure, and subsequently have raised the wall to elevation 4739.8 feet. Impacts No Action: The No Action alternative would allow Ute Water to operate their pump plant as they have historically. However, the installation of 4-foot-high flashboards on the dam, as described in the 1990 FERC license, would increase the risk of flooding at the pump plant. Fish Ladder with Hydropower Plant: The fish ladder would be designed so it would not have any affect on flood flows in the Colorado River. Adjustable flashboards on the dam (as proposed in the 1996 FERC license amendment) could be lowered to reduce the risk of flooding at the pump plant. Fish Ladder without Hydropower Plant: The fish ladder would be designed so it would not have any affect on flood flows in the Colorado River. 24