ACCELERATED MODERN EROSION IN LAS VEGAS WASH, CLARK COUNTY, NEVADA

The second half of the 20th century saw the unprecedented population growth of Las Vegas Valley from about 127,000 in 1960 to over 1.6M in 2003. Urban growth drastically changed the surface hydrology across the valley and resulted in steadily increasing wastewater runoff though Lower Las Vegas Wash to Lake Mead. During the 1950s-60’s the initial wastewater releases created broad wetlands on the valley floor; however, continually increasing sediment-free wastewater resulted in several discontinuous headcuts by the late 1960s. By 1984 these headcuts integrated into a deep channel during a series of El Nino-generated storms. Aerial photosets of Lower Las Vegas Wash from 1975, 1989, and 1999 were digitized and rectified to characterize modern channel changes and eroded sediment volumes. Volume calculations are based on DEMs and channel reconstructions where no DEMs exist. Average channel widths increased from 8 feet in 1975 to about 120 feet in 1999. The amount of sediment eroded from the time of initial valley inhabitation to 1975 was approximately 135,000 cubic yards. From 1975 to 1999 more than 65 times that amount of sediment was eroded from the wash into Las Vegas Bay of Lake Mead. The majority of the sediment was removed from 1984-1989, which indicates that once the wash was destabilized through the loss of vegetation and incision, storm events were able to accelerate erosion by continued channel downcutting and widening. Between 1989 and 1999, continued urban growth resulted in increased storm runoff that further eroded channel walls. The changes in channel dimensions and erosion destroyed wetlands, valley bottom property, and local roads. In addition, reconstruction of a bridge and the valley’s main water supply pipeline were also required. Since 2000, state and local agencies have worked together to implement a comprehensive engineering and management plan, at a cost of several tens of millions of dollars, to stabilize the channel and enhance the wetland ecosystem.