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Open-File Report 2009–1150

Surficial Geology of the Floor of Lake Mead (Arizona and Nevada) as Defined by Sidescan-Sonar Imagery, Lake-Floor Topography, and Post-Impoundment Sediment Thickness

By D.C. Twichell and V.A. Cross


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Sidescan-sonar imagery collected in Lake Mead during 1999-2001, a period of high lake level, has been used to map the surficial geology of the floor of this large reservoir that formed upon completion of the Hoover Dam in 1935.  Four surficial geologic units were identified and mapped: rock exposures and alluvial deposits that existed prior to the formation of the lake and thin post-impoundment sediments (< 1 m) and thick post-impoundment sediments (> 1 m) deposited since the lake formed.  Exposures of rock are most extensive in the narrow, steep-sided sections of the lake, while alluvial deposits are most extensive on the gentle flanks of the broader basin sections of the lake.  Post-impoundment sediment is restricted to the floors of the original river valleys that now lie below lake level.  These sediments are thickest in the deltas that form at the mouths of the Colorado River and its tributaries, but cover the entire length of the valley floors of the lake.  This sediment distribution is consistent with deposition from turbidity currents.  Lake level has dropped more than 30 m between collection of the sidescan imagery and publication of this report.  During this time, thick delta deposits have been eroded and redistributed to deeper parts of the lake by turbidity currents.  While present-day post-impoundment sediment distribution should be similar to what it was in 2001, the thickness may be greater in some of the deeper parts of the lake now.

Posted January 2010

For additional information contact:
U.S. Geological Survey
Woods Hole Coastal and Marine Science Center
384 Woods Hole Road
Quissett Campus
Woods Hole, MA 02543

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Suggested citation:

Twichell, D.C., and Cross, V.A., 2009, Surficial geology of the floor of Lake Mead (Arizona and Nevada) as defined by sidescan-sonar imagery, lake-floor topography, and post-impoundment sediment thickness: U.S. Geological Survey Open-File Report 2009-1150, CD-ROM. (Also available at


Title Page

List of Figures and Tables

Conversion Factors




Lake-Floor Geology


Data Catalog


References Cited

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