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p eJ ~. . 1-4 ...... o <"'1 COLORADO RIVER SALINITY CONTROL PROGRAM (continued) 3 and the annual flow amounts to about 150 acre-feet (185 OOD m ). The climate at the geyser is desert-like with an average annual temperature of 52' F (110 C) and an average annual precipitation of 6 inches (150 mm). The vegetation in the geyser area is sparse with tamarisk and scattered cottonwood trees along the edges of the river and cactus, Brigham tea, greasewood, and shadscale elsewhere. The estimated annual removal of salt by the alternative plans is about 3,000 tons (2700 t), a relatively minor amount. Salinity concen- trations of the Colorado River at Imperial Dam would be reduced by an estimated 0.3 mg/L. The Definite Plan Report (DPR) , Environmental Assessment, and Negative Determination of Environmental Impact have been completed and were sub- mitted in June 1976. The plan is to collect and convey the flows to evaporation ponds about 3 miles (5 km) downstream. A compacted earth embankment would be constructed on the stream side of the geyser to collect and temporarily store the water from the eruptions. A plastic pipe would convey the water from the temporary storage pond at a uniform rate to the evaporation ponds. The pipeline would cross many small drainages and the Little Grand Wash. Bedrock may present some difficulties in laying the pipe. The evaporation ponds would be located on a typical river flood plsin of lean clay with lenses of silt, sand (sometimes clean), and gravel. The ponds would require a flexible polyvinyl lining to assure that no leakage back to the river occurs. About 2 miles (3 km) of the access road to the geyser would need improvement by grading, installing culverts, and possibly gravel surfacing. The access road to the evaporation ponds would also need some improvement. Construction on this unit has been delayed indefinitely due to high construction costs and low cost effectiveness. 4. Las Vegas Wash Unit Las Vegas Wash is a natural drainage channel providing the only sur face water out let for the enti re 2,193 square mi les (5680 km2) Las Vegas Valley. A drainage area of 1,586 square miles (4108 km2) directly contributes to the Wash which conveys stream runoff and wastewater to Las Vegas Bay, an arm of Lake Mead. Located in Clark County in Southern Nevada, the Las Vegas Valley contains the largest population center in the state. Three cities (North Las Vegas, Las Vegas, and Henderson) and other communities are drained by tributaries to Las Vegas Wash. Studies eval- uating salinity contributed by the wash are concerned mainly with the 10 mile (6 km) reach upstream of Las Vegas Bay. The wash flood plain and adjacent area support about 1,800 acres (730 ha) of halophyte, hydrophyte, and phreatophyte vegetation. 69