GEOLOGIC MAP OF THE LAKE TAHOE BASIN, CALIFORNIA AND NEVADA

One objective of the California Geological Survey (CGS) is to prepare and maintain a database of geologic mapping throughout the state. This is done primarily through the Regional Geologic Mapping Project (RGMP) in cooperation with the U.S. Geological Survey (USGS) through STATEMAP. In 1997 President Clinton issued an executive order instructing federal agencies to establish an interagency partnership to coordinate and manage activities within the Lake Tahoe Basin. The Executive Order also called for the establishment of a linked natural resources GIS database. Subsequently a Lake Tahoe multi-agency data clearinghouse was established by the USGS. The existing digital geologic coverage contained many inconsistencies. A project to prepare a new digital geologic map and database was proposed with the goal of providing a seamless geologic database that could be used by planning and decision-making agencies in the Lake Tahoe Basin. These data can provide an important layer when performing GIS analyses throughout the basin. Although presented at 1:100,000 scale, the spatial resolution of the data is good to 1:62,500 scale and in some areas 1:48,000 and 1:24,000 scale. One striking feature of this geologic map is the incorporation of new USGS imagery of the lake bottom as well as the shaded-relief image on land. This new imagery provided a detailed picture of the lake bottom revealing some previously unidentified geologic features. Linear features that offset lake sediments are interpreted to be relatively young faults. The large embayment on the western shore is attributed to a mega-landslide. Both of these features raised questions as to the potential for a large tsunami-type wave that could cause considerable damage and loss of life around the lake. Investigations continue to evaluate these hazards and their potential.