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2 The Federal Emergency Management Agency (FEMA) is the nationwide leading agency in regulating development in flood plains. In the 1970's, FEMA identified alluvial fans that present the most likely hazard to public safety and adopted a methodology to delineate flood hazards on them for the past several decades. The FEMA alluvial fan model has been used to map flood hazards and set design criteria on alluvial fans throughout the United States. For the last decade, the California Department of Water Resources has been concerned about the lack of accepted standardized alluvial fan definitions and the controversy among arid west communities on the effectiveness of FEMA's alluvial fan flooding definition and procedure for risk assessment. It has aroused criticism and considerable opposition from many flood plain managers and some communities that participate in the National Flood Insurance Program. The criticism is due to the fact that the original methodology received only minor technical attention regarding the hydraulic, geologic, and geomorphic processes associated with alluvial fans. It is recognized that the evolution of, and flooding processes on, alluvial fans are highly variable in time and space. Thus, there is a need for a flexible approach in identifying, evaluating, and regulating an alluvial fan. However, based on the National Research Council recent Alluvial Fan Flooding report (NRC, 1996), FEMA has reevaluated their approach to modeling alluvial fan hazards and is making great progress in improving the process for regulating development on alluvial fans. This study will complement FEMA's efforts in this regard. Need for Definition and Classification The controversy over FEMA's definition and methodology encouraged the California Department of Water Resources to look for more logical and process-based definitions to describe and differentiate among the types of alluvial fans. The lack of well-defined definitions to identify the characteristics of alluvial fans and a thorough understanding of their complexities can lead to misinterpretation or misidentification of potential flood hazards. In order to protect the public there is a need to develop new technical, process-based definitions and a classification system, based on reasonably simple and straight forward processes, to describe and differentiate among the types of alluvial fan flooding, This is needed by the State to keep pace with the increased rate of development into previously underdeveloped areas and for providing information for protecting future construction from potential alluvial fan flooding. Recognizing the need for these standards, the California Department of Water Resources has requested that the Corps of Engineers, Sacramento District, develop criteria and a procedure for identifying and classifying alluvial fans, This work has been separated into two distinct phases. The first phase is to prepare a report summarizing the current information regarding alluvial fans and then to develop a tentative standard definition and base classification system to be used in identifying and classifying alluvial fans throughout the State. The second phase will be to complete statewide identification and classification of these alluvial fans. This report completes the first phase of the work. Study Methodology The methodology used in this study included (1) surveying California counties and other agencies to determine the current knowledge and practices regarding alluvial fans; (2) researching and reviewing the published literature and internet sources regarding alluvial fans;