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001083 Final Environmental Assessment-Chapter 3-Affected Environment and Environmental Consequences Dam Removal: Removing the dam would result in sediment deposits being washed downstream. Sediments are deposited in the riverbed as river velocities slow down. The geometry ofthe river near the dam, the steepness of the river bottom, and the constriction caused by Interstate 70 and the railroad tracks keep the velocities higher that what is commonly found behind dams. Surveys of the river bottom upstream from the dam revealed a thin layer of sediments behind the dam, but due to the water velocities, most of the river bottom is composed of gravels and cobbles (Collins, 1999). The manger 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 conducted a sediment study in the area above the dam. To ensure that the study addressed Clifton Water District's concerns, the District reviewed the sediment study proposal. This identified volume and composition of the sediment (USGS, 2000). If dam removal was selected, additional sampling and monitoring may be necessary. Ute Water Conservancy District Pump Plant-Spring Flooding Issue: Effects of each alternative on spring flooding of Ute Water pumping plant. Existing Conditions: The Ute Water pump plant historically flooded when river flows were high and the Colorado River exceeded elevation 4,732 feet. In recent years, Ute Water constructed a concrete retaining wall to an approximate elevation of 4,739.8 feet to protect the pump plant from flooding. The estimated 100-year to 500-year flood events at the dam are 44,500 cfs 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 the top of their retaining wall in 1983 (elevation 4,738 feet). Ute Water placed sand bags on top of the wall as a precautionary measure, and subsequently has 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. Conventional Fish Ladder: The fish ladder would be designed so it would have no effect on flood flows in the Colorado River. Downstream Rock Fish Passage: The fish passage would also be designed so it would have no effect on flood flows in the Colorado River. Downstream Rock Fish Passage with Whitewater Recreation Features: Same as the Downstream Rock Fish Passage Alternative. 30