WASTEWATER-INDUCED URBAN EROSION IN LAS VEGAS VALLEY, NEVADA

Population in Clark County, which includes the city of Las Vegas, has grown from about 5,000 in 1930 to 2 million in 2009. Las Vegas Wash became a natural conduit for valley wastewater, excess irrigation runoff, and storm runoff since settlement began in 1905. Urban growth was accompanied by commensurate increases in wastewater discharge: mean-daily flow steadily increased from 1 cfs in 1955 to 160 cfs in 1989 and to 275 cfs in 2004. Continually increasing wastewater base flows created a series of sequential impacts on the wash floodplain and channel that completely altered the desert stream hydrology and valley landscape. Before 1950 wastewater from Las Vegas supported expanding riparian vegetation. As valley fill sediments saturated, the floodplain became dominated by wetlands, and perennial streamflow began in 1955. Increasing wastewater-dominated streamflow initiated two headcuts by 1969 that expanded upstream during the 1970s. Monthly sediment data show that channel erosion occurred during times of both wastewater flow and during storm runoffs. The headcuts drained marshes and shallow groundwater from the floodplain and became integrated into a large incised channel during several summer floods in 1984. From 1985 to 1999 wastewater discharges progressively increased and channel erosion continued unabated. Digitized and rectified aerial photosets of lower Las Vegas Wash were used to measure channel changes and to calculate eroded sediment volumes. Average channel widths increased from 2.4 m in 1975 to a range of 33 m to 48 m in 1999. Sediment eroded from the lower wash from the time of valley settlement to 1975 was approximately 0.12M m³. From 1975 to 1999 6.63M m³ of sediment was eroded from lower Las Vegas Wash and deposited into Las Vegas Bay of Lake Mead––the largest volume of 20^th century erosion in the US attributed primarily to urbanization. Since 2000 a combined group of agencies has implemented engineering and management plans to stabilize the channel and prevent future floodplain erosion, to channelize and to attenuate storm runoff through the urbanized basin, and to enhance the floodplain wetland ecosystem at a cost of >$2B.