COMPILATION OF SURFICIAL GEOLOGIC MAPPING IN SOUTHERN CALIFORNIA

The California Geological Survey (CGS), with funding from the Department of Water Resources (DWR), has completed an update of a GIS-based compilation of high-resolution geologic maps of Quaternary age and older deposits in southern California. The compilation provides a consistent classification of surficial deposits that cover approximately 35,000 square miles of a 10-county area currently of interest to DWR and the Governor’s Alluvial Fan Task Force (AFTF), including San Bernardino, Riverside, Los Angeles, Ventura, Santa Barbara, San Luis Obispo, Kern, Orange, Imperial and San Diego counties.

Over the past two decades, alluvial fans have experienced rapid growth and high development pressures in southern California. Due to the unique flood hazards associated with alluvial fans, the AFTF was charged in 2007 to review the state of knowledge regarding alluvial fan floodplains and develop recommendations that would be specific to alluvial floodplain management. The CGS geologic compilation was designed as a regional-scale planning tool to assist DWR, the AFTF, other state and local agencies, local communities, consultants, and geologists in evaluating hazards as well as locating future development on alluvial fans.

The project merges new mapping and existing digital geologic data by various authors at CGS and the U.S. Geological Survey into a common seamless format that normalizes and differentiates alluvial fan deposits, related Quaternary deposits, and various older deposits into 40 derivative units at a scale of 100,000 for the entire area. Quaternary surficial deposits are divided into four main age categories: late Holocene=most recent; Holocene to late Pleistocene=young; late to middle Pleistocene=old; and middle to early Pleistocene= very old. CGS Special Publication 217 (SP 217) includes both a GIS-based dataset and a Portable Document File (PDF) version of the 100,000 scale derivative maps. An online web map viewer is in progress. The data is provided to assist elected officials, local floodplain managers, representatives from various governmental agencies, developers, environmental groups, consultants, geologic investigators, and the public in the rapid identification of areas subject to previous and potential future flooding and other geologic hazards on alluvial fans and floodplains.