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Open-File Report 2009–1150

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## 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

| ABSTRACT
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
-  **OFR2009-1150**

**For additional information contact:**DirectorU.S. Geological  SurveyWoods Hole Coastal and Marine Science Center384 Woods Hole RoadQuissett CampusWoods Hole, MA 02543(508)548-8700
http://woodshole.er.usgs.gov/
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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 http://pubs.usgs.gov/of/2009/1150/.)

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### Contents

Title Page

List of Figures and Tables

Conversion Factors

Abstract

Introduction

Methods

Lake-Floor Geology

Discussion

Data Catalog

Acknowledgments

References Cited

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U.S. Department of the Interior |		U.S. Geological Survey		                                    URL: http://pubs.usgs.gov/of/2009/1150/		Page Contact Information: GS Pubs Web Contact		Page Last Modified: Thursday, 04-Feb-2010 09:21:17 EST