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Fieldwork

Mapping Sediment Distribution and Thickness in Lake Mohave


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Leased houseboat, converted into a research vessel, moored at one of the overnight sites on Lake Mohave
Research duty: Leased houseboat, converted into a research vessel, moored at one of the overnight sites on Lake Mohave. The A-frame on the stern was used to tow an Edgetech DF-1000 sidescan sonar. The side mount on the starboard bow contains the transducers for a Knudsen Chirp subbottom profiler.
From April 13 to 29, Ken Parolski, Dave Foster, and Mark Capone of the Woods Hole Field Center (WHFC) completed a high-resolution geophysical survey in Lake Mohave, on the Arizona and Nevada State line along the Colorado River. The fieldwork is part of an ongoing project led by Dave Twichell (WHFC) and Mark Rudin of the Health Physics Department at the University of Nevada, Las Vegas (UNLV), to address such issues as water quality and reservoir capacity in Lakes Mead and 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).

Sidescan-sonar image showing debris deposited in Lake Mohave from a 1974 flash flood
Imaging the bottom: Sidescan-sonar image showing debris deposited in Lake Mohave from a 1974 flash flood. Vertical line in center shows where two swaths of sidescan-sonar data have been joined.

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.


Related Sound Waves Stories
Lake Mead Mapping Completed
June 2001

Related Web Sites
Woods Hole Field Center
U.S. Geological Survey (USGS)
Health Physics Department
University of Nevada, Las Vegas (UNLV)

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Mapping Lake Mohave

Research Delmarva Coastal Bays

Glen Canyon Dam

Outreach Career Options for Jr. High School Girls

Coastal Hazards Lecture

Earth Day at Lowry Park Zoo

Practical Applications of GIS

Gulf of Maine GIS Workshop for Teachers

MarineQuest 2002

Florida Caribbean Science Center Open House

Meetings CMGP Knowledge Bank

Communicating Science in a Virtual World

Continental-Shelf Territory Rights

Awards Kvenvolden Honored

Staff & Center News Student Employees

New Woods Hole Chief Scientist

Two New Employees at WHFC

University of Minnesota Visitor

Publications Special Issue of Marine Geology on USGS Monterey Bay Research

Timely Publication for Gulf of Mexico Mercury Concerns

June Publications List


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