USING REMOTE SENSING TOOLS AND GIS TO MAP THE GEOLOGY OF THE FLOOR OF LAKE MEAD, NEVADA

Sidescan-sonar imagery and high-resolution seismic-reflection profiles were collected throughout Lake Mead to determine the surficial geology as well as the distribution and thickness of sediment that has accumulated in the lake since completion of the Hoover Dam in 1935. The seismic profiles show that post-impoundment sediment is deposited both in shallow water to form deltas off the mouths of rivers and creeks that enter the lake, and in the deep basins of the lake. In deep water, the seismic profiles indicate deposits are flat lying, fill the axes of former creek and river beds, and are thickest along the former Colorado River bed where they can reach a thickness of 40 m. In some places, the sidescan-sonar imagery shows channels on the surface of the post-impoundment sediment. These channels, coupled with the flat lying nature of the deep-water deposits, suggest turbidity currents were the mechanism of deposition.

The incorporation of these data into a GIS has facilitated the interpretation and visualization of the data, and the exchange of information with other agencies working on the lake. Comparison of the present lake-floor geology with the topographic maps compiled prior to dam construction shows the strong influence of the original topography on modern sedimentary processes in the lake. In addition, combining the sediment thickness data with the pre-impoundment topography of the area has helped resolve changes in the lake’s holding capacity. Thus, in addition to being a valuable research tool, the GIS provides information that is valuable to agencies responsible for water management and to the National Park Service who is responsible for navigational aids and marina placement on the lake.