Surficial Geology and Distribution of Post-Impoundment Sediment of the Western Part of Lake Mead Based on a Sidescan Sonar and High-Resolution Seismic-Reflection Survey, U.S. Geological Open-File Report 99-581, Title Page, text-only version

U.S. Geological Survey
Coastal and Marine Geology
Woods Hole Field Center

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Surficial Geology and Distribution of Post-Impoundment Sediment of the Western Part of Lake Mead Based on a Sidescan Sonar and High-Resolution Seismic-Reflection Survey

Based on U.S. Geological Survey Open-File Report 99-581

David C. Twichell, VeeAnn A. Cross, and Kenneth F. Parolski, U.S. Geological Survey, Woods Hole, Massachusetts

and

Mark J. Rudin, Department Health Physics, University of Nevada at Las Vegas, Las Vegas

This open file report includes the following pages: Introduction, Methods, Results, Discussion, Summary, and References and 16 Figures. Clicking on the Navigational Buttons Link, which is at the top and bottom on each page, will give a list of all these pages with links.

ABSTRACT

Sidescan sonar imagery and high-resolution seismic-reflection profiles were collected in Las Vegas Bay and Boulder Basin of Lake Mead to determine the surficial geology as well as the distribution and thickness of sediment that has accumulated in these areas of the lake since the completion of Hoover Dam in 1935 (Gould, 1951). Results indicate that the accumulation of post-impoundment sediment is restricted to the original Colorado River bed which runs down the axis of Boulder Basin from Boulder Canyon to Hoover Dam, and the old Las Vegas Creek bed that bisects Las Vegas Bay. The sediment cover along the original Colorado River bed is continuous and is typically greater than 10-m thick throughout much of its length with the thickness in some areas exceeding 35 meters. The flat-lying nature of the deposits suggests that they are the result of turbidity currents that flow the length of the lake. The sediment cover in Las Vegas Bay is much thinner (rarely exceeding 2 m in thickness) and more discontinuous. The source for these sediments presumably is Las Vegas Wash and a series of other ephemeral washes that empty into this part of the lake. The presence of sediments along the entire length of the Las Vegas Creek bed suggests that turbidity currents probably are active here as well, and that sediment has been transported from these streams at least 10 km down the axis of this valley to where it enters Boulder Basin. Alluvial deposits and rock outcrops are still exposed on large parts of the lake floor.

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This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.