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.