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UJIG87 Revised Supplemental Draft Environmental Assessment-Chapter 3-Affected Environment and Environmental Consequences Existing Conditions: The Clifton Water District provides domestic water to . about 30,000 residents in the Grand Valley. Using the Colorado River as their source of water, Clifton Water District produces potable water that exceeds drinking water standards (Clifton Water District, 1997). The District's diversion is approximately 6 miles downstream from the Price-Stubb Diversion Dam. Impacts No Action: Water quality would remain unchanged ifno fish ~assage is constructed. Conventional Fish Ladder: Fish ladder construction could cause a temporary increase in erosion and sediment, but impacts are expected to be minor. Construction would occur when the Colorado River is low and a temporary cofferdam would be used to divert water away from construction areas. Downstream Rock Fish Passage: Temporary effects on water quality are predicted to be greater than the Conventional Fish Passage Alternative since more of the construction activities take place in the river channel. However, implementation of best management practices and construction during low river flows would minimize negative impacts. Temporary cofferdams would also assist in minimizing effect on water quality. Operation of the fish passage would have no effect on water quality. Downstream Rock Fish Passage with Whitewater Recreation Features: Effects would be similar to the Downstream Rock Fish Passage Alternative. 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 of the 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 teach 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 ofthe sediment (USGS, 2000). If dam removal was selected, additional sampling and monitoring may be necessary. 28