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Mapping Sediment Distribution and Thickness in Lake Mohave
The project scientists focused first on Lake Mead, behind Hoover Dam on the Colorado River, about 25 mi east of Las Vegas, NV (see article in June 2001 issue of Sound Waves). Recently, they have turned their attention to Lake Mohave, about 50 mi south of Lake Mead. The north and south sections of Lake Mohave are narrow, about 1 mi wide, and are bounded by the canyon walls of the Colorado River. The main basin of the lake is the middle section, about 8 mi long and 5 mi wide. Bathymetry, sidescan-sonar, and Chirp subbottom data were collected to map the thickness and distribution of sediment that has accumulated since the construction of Davis Dam and the impoundment of Lake Mohave in 1953. The resulting maps will guide selection of sites for sediment sampling and analysis, which, in turn, will help scientists understand how pollutants from urban and agricultural runoff might be distributed throughout the lake. The sidescan-sonar data yield images of the surface of the lake floor, revealing rock, sediment, geomorphic features, and manmade debris. The Chirp subbottom data yield high-resolution seismic-reflection profiles that show sediment layers and bedrock features down to about 10 to 20 m below the lake floor. The data collected in Lake Mohave will be used to construct a sidescan-sonar mosaic (swaths of sidescan-sonar data joined to produce a continuous image) of the lakebed and a sediment-thickness map of post-impoundment deposits (sediment deposited after the lake was created). The information will be shared with USGS hydrologists and made available to several other government agencies and universities involved in studies of the lake (UNLV, the Southern Nevada Water Authority, the Bureau of Reclamation, and the National Park Service).
A leased houseboat was rigged with side-mount transducers for the Chirp subbottom system and an A-frame to tow the sidescan sonar (see photo, above). Dust storms with winds gusting from 50 to 60 mph delayed our start by a few days. The crew quickly made up for lost time by working and staying overnight on the houseboat. This strategy also enabled us to reach some of the more remote areas of the lake. Preliminary interpretation of the data shows that pre-impoundment features dominate the lakebed and that post-impoundment deposits are not as extensive as they are to the north, in Lake Mead. The shallow areas of Lake Mohave are predominately alluvial-fan deposits and bedrock ledges; both are clearly observable in the sidescan-sonar data. The sidescan-sonar and Chirp subbottom records also clearly define the pre-impoundment banks of the Colorado River. Sand waves are still visible on the riverbed where post-impoundment sediment is absent. Sand dunes are still preserved along the banks of the river channel. Post-impoundment deposits do not exceed a few meters in thickness. Lake Mead, in contrast, has deposits as much as 30 m thick. Major sources of sediment are not obvious in Lake Mohave; however, there are localized deposits from flash floods in the many tributaries that enter the lake. Although the lake has a short history, there was a significant flash flood in Eldorado Canyon in 1974 that caused nine fatalities and considerable property damage. Witnesses observed a large flood surge at the canyon mouth, carrying debris, cars, and trailer homes. Our sidescan-sonar records clearly show some of the debris from this flood (see image above). The Chirp subbottom profiles show a localized deposit near the canyon mouth. We will be mapping the deposits in this area to estimate how much sediment entered the lake during this event.
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in this issue:
Gulf of Maine GIS Workshop for Teachers Florida Caribbean Science Center Open House Communicating Science in a Virtual World Continental-Shelf Territory Rights New Woods Hole Chief Scientist University of Minnesota Visitor
Timely Publication for Gulf of Mexico Mercury Concerns |
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U. S. Department of the Interior | U.S. Geological Survey
cover story:
