Wednesday, November 15, 2006

Flooding on the Virgin River, USA: Impacts and Historic Perspective.

Douglas Merkler, USDA - NRCS, 659 Del Prado Dr., Boulder City, NV 89005

The lower Virgin River Basin is a complex structural basin formed by Neogene extension in Nevada, Arizona, and Utah, USA. At about 5.5 mya, the Colorado River, and probably the Virgin River, were flowing into the Mesquite basin and formed Muddy Lake, an extensive body of water that extended from the base of the Clover Mountains southward into the Las Vegas basin. Subsequently, the Mesquite basin was breached and began draining into the Colorado River and eventually the Pacific Ocean. The subsequent lowering of base level after breaching caused a dramatic shift in deposition and erosion into and out of the Mesquite basin. Alluvial deposits record at least four cycles of incision and aggradation by the river and also record regrading of the surfaces between the river and the Virgin Mountains as their relative altitudes changed. This rapid erosion of soft sediments in the basin created modern day landforms, some of which are among the oldest geomorphic surfaces in North America. On 10 January 2005 warm rain began to fall on snow packs, approaching 300 % of normal in Zion National Park, Pine Valley Mountains, Spring Mountains, and the Clover Mountains. Data collected by the USGS indicated record flows occurred along the Beaver Dam Wash near Enterprise, Utah and near Beaver Dam in northwest Arizona. Damage reports received by NOAA indicated 75 homes were evacuated in Mesquite near the Virgin River. The Red Cross reported that  350 homes were affected by flooding in Overton; in Beaver Dam, Arizona, 22 homes were damaged or destroyed along with a portion of a main bridge.  This major flood on the Virgin River prompted a careful examination of current growth and zoning along the Virgin River corridor and reflection of historical flooding along one of the few perennial rivers in the Mojave desert.