Saturday, 15 July 2006

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

Douglas Merkler, USDA NCRS, 5820 S. Pecos Rd., Bldg A Ste 400, Las Vegas, NV 89120 and Patrick Drohan, Univ of Nevada, Las Vegas, Dept of Geoscience, 4505 Maryland Pkwy Box 454010, Las Vegas, NV 89135.

The lower Virgin River Basin is a complex structural basin formed by Neogene extension in Nevada, Arizona, and Utah, U.S.A. There is a large volume of ground water in transient storage moving through the basin. Ongoing investigations to characterize the basin have determined that it is one of the deepest in the Basin and Range Province. The estimated depth to basement underlying the carbonate rock may be as great as five miles. Subsequent filling of the basin with the Muddy Creek Formation has created a deposit as thick as 6,000 feet. At about 5.5 mya, the Colorado River, and most 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 basin 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 provides modern day landforms, some of which are among the oldest geomorphic surfaces in North America. The Virgin River is the main surface-water feature in the lower Virgin River Valley; there is only one other perennial stream, Beaver Dam Wash. Throughout its course, however, it is intermittent with several sections becoming dry after spring runoff. Ephemeral drainages are evidence of surface-water runoff from the surrounding mountain blocks into the basin. Depending on storm patterns, flow often originates directly on the alluvial fans. Flash floods produce large debris loads, particularly from the Virgin Mountains south of the river. Several communities within the basin obtain municipal water supplies from the Muddy Creek Formation. Mesquite, Nevada, is the largest of these and one of the fastest growing communities in the United States. On the 10th of January 2005 warm rain began to fall on snow packs approching 300 percent of normal in Zion National Park, Pine Valley Mountains, Spring Mountains, and the Clover Mountains. Data collected by the USGS indicated record flows had occurred along the Beaver Dam Wash near Enterprise, Utah and near the Beaver Dam in northwest Arizona. Preliminary results of data collected for the Virgin River indicated the second highest flow on record had occurred near Littlefield, Arizona. Damage reports received at NOAA's NWS office in Las Vegas indicated 75 homes had to be evacuated in Mesquite subdivisions near the Virgin River. The Red Cross stated that as many as 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 flooding impacted several watersheds of the lower Colorado River drainage. This major flood on the Virgin River prompted a careful examination of the current growth and zoning along the Virgin River corridor and reflection of the historical flooding along one of the few perennial rivers in the Mojave desert. Early settlement and economic development of the Virgin River basin were affected to a large extent by the floods of the late 1800s and early 1900s. Flood-related loss of farmland and damage to irrigation structures, dams, and dwellings were continuing problems for early settlers of the region. The cost of replacing and maintaining dams, irrigation ditches, and property was an economic burden on every community. The agricultural history of the region was largely determined by floods and the resulting changes in stream channels. The causes of historic downcutting in the Southwest have been a topic of debate for more than 90 years. Overgrazing, climate change, and internal adjustments of the channel system unrelated to either climate or land use are the main explanations. Dendrohydrologic evidence was used to reconstruct and measure streamflows which were then used to interpret dated episodes of erosion and deposition from temporally recorded streamflows from the early 1900's. This data suggested that in prehistoric times, in the absence of man's influence, patterns of erosion of the Virgin River were contemporaneous with a high streamflow between A.D. 1200 and 1400 that was preceeded by drought conditions in the late A.D. 1100's. Low streamflow following A.D. 1400 until about 1880 coincides with deposition with in the channel. These variations are related to climate; human activity in historic times possibly increased runoff, but erosion would probably have occurred without human interference. Current regulation of streamflow by upstream reservoirs and structures will alter the geomorphic development of the Virgin River Valley and must be incorperated into regional plans for development along the river's corridor.

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