Surficial Geologic Mapping and Associated GIS Databases for Identification of Alluvial Fans

Abstract

The California Geological Survey (CGS) has completed a geographic information system (GIS)–based compilation of geologic maps of Quaternary age and older deposits in southern California. The compilation provides a consistent classification of surficial deposits within a 10 county area of interest to the Department of Water Resources (DWR) and the governor9s Alluvial Fan Task Force (AFTF). During the past two decades, rapid population growth in southern California has resulted in increased pressure for residential development on alluvial fans. Due to unique flood hazards associated with alluvial fans, the AFTF reviewed the state of knowledge regarding alluvial fan floodplains and developed recommendations specific to floodplain management. The CGS compilation was designed as a regional-scale planning tool to assist DWR, the AFTF, and local communities in evaluating hazards and locating future development on alluvial fans. The goal of the compilation was to represent southern California9s complex geology in a consistent user-friendly format that depicts areas of most recent alluvial fan deposition on a regional scale. The Derivative GIS Database merges new mapping and existing digital geologic data into a common seamless format that differentiates alluvial fan deposits, related Quaternary deposits, and various older deposits into 40 derivative units at a scale of 1: 100,000 for the entire mapped area. Original source data compiled at various scales and detail are retained in the Source GIS Database.