Click on figure for larger image.

Figure 1. Location map showing the names of features referred to in the text, the bounds of the lake at full capacity, and the sidescan-sonar coverage. White boxes indicate other figure locations. Areas of low acoustic backscatter show as dark tones and areas of high acoustic backscatter show as light tones on the grayscale sidescan-sonar imagery.

Lake Mead is a large reservoir located on the Colorado River in the Southwestern United States that started to fill in 1935 after completion of the Hoover Dam (fig. 1).  Water from the lake is used for municipal, agricultural, and industrial purposes in the States of Arizona, California, and Nevada.  Important information for management of the reservoir and its water resource includes understanding the geology of the reservoir floor and the amount of sediment that has accumulated there.  To address these questions, the U.S. Geological Survey (USGS) used sidescan-sonar and high-resolution seismic-reflection techniques to map much of the lake during field programs in 1999, 2000, and 2001.  Detailed bathymetry for large parts of the lake was provided by the Bureau of Reclamation.  The geophysical and bathymetric datasets are reported in Twichell and others (2003), and the distribution and thickness of the post-impoundment sediment and the processes of deposition are reported in Twichell and others (1999, 2001, and 2005) and Zybala (2004).  Missing from these earlier studies is an interpretation of the surficial geology of the entire part of the lake that was surveyed. While the distribution of post-impoundment sediment has been mapped, the geology of the remainder of the lake floor had not been completed in any of these earlier reports in a GIS-compatible format.  The sidescan-sonar imagery collected during these three field programs covers approximately 70 percent of the lake floor as of 2001, when lake level stood at approximately 363 m above sea level.  Here we expand the interpretation of the lake-floor geology beyond just showing the extent of the post-impoundment sediment, providing a complete interpretation of the surficial geology of the parts of the lake floor imaged by sidescan sonar (fig. 1). The interpretation presented in this report uses the sidescan-sonar imagery as a basemap but draws on the seismic-reflection data, lake-floor bathymetry, and geologic maps of Arizona and Nevada to complete the interpretation.