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
Summary
A significant volume of sediment has accumulated in Boulder Basin and a lesser amount in Las Vegas Bay since the formation of Lake Mead. A detailed mapping of this sediment shows that it is not uniformly distributed throughout the lake. Instead, it occurs as flat-lying deposits that have filled only the deepest parts of the Colorado River valley and tributary valleys that were cut prior to the formation of the lake. These sediments are thickest in the floor of Boulder Basin where they are more than 20 m thick for large areas and locally exceed 35 m. The surface of the sediment filling Boulder Basin is shallowest at the eastern end of the study area in Boulder Canyon (about 128 m), and gradually deepens to the southwest to 147 m deep near Hoover Dam. The flat-lying nature of reflectors within the deposit, the gentle westward gradient of the surface of the deposit, and the presence of a shallow channel along the axis of Boulder Basin suggest that it is the result of turbidity currents that have flowed the length of the lake (Gould, 1951). The sediment that has accumulated in the floor of Las Vegas Bay also is limited to the thalweg of the former valley. Here sediment is less than 2-m thick, but probably also was transported by turbidity currents which followed the pre-existing axial valley. Understanding the distribution of sediment that has accumulated in Lake Mead since impoundment, and understanding the processes by which it is distributed through the lake are necessary information for monitoring the fate of pollutants that enter the lake.
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