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60 ALLUVIAL FAN FLOODING marked by a topographic trough or ridge. Although it is difficult to separate young deposits on one fan from similar age deposits on a coalescing fan, it sometimes is possible to distinguish them based on different source-basin rock types. For example, Bull (1963, 1964) defined fan boundaries in central California using contour maps, aerial photographs, and tests of the gypsum content of core hole samples. Bull found that the gypsum content of fan deposits derived from drainage basins underlain predominantly by clay-rich rocks was about five times that of fan deposits from drainage basins underlain predominantly by sandstone. Boundaries of many alluvial fans are defined on 7.5-minute series orthophoto base maps by the NRCS. In the U.S. Southwest, typical NRCS soil series for alluvial fans include the Ramona, Soboba, Kinburn, and Anthony. Large areas where material from stream banks is freshly deposited and partly reworked during floods also are mapped, and smaller areas are identified as part of a particular series where the reworked material is located. For small alluvial fans less than about .8 km2 (.3 m?), the detail of the mapped soil units on the 7.S-minute soil map series may not be sufficient to show many distributary channels and the fan boundaries. Soil maps used in conjunction with aerial photographs are an excellent means to define fan boundaries. The nature and extent of alluvial fan flooding can be partially determined from published topographic, soils, and geologic maps and other sources of data. However, the committee emphasizes the importance of a field inspection by a qualified professional with experience and technical knowledge of geomorphology, slope stability, avalanche potential, flood hydraulics, flood hydrology, sedimentary facies, and alluvial fan processes. The general use of secondary information and the importance of field information is described in this chapter and in the examples described in Chapter 4. STAGE 2: DEFINING THE NATURE OF THE ALLUVIAL FAN ENVIRONMENT AND IDENTIFYING THE LOCATION OF ACTIVE EROSION AND DEPOSITION Most alluvial fans have parts that are active and parts that are iI/active, Alluvial fan floodil/g occurs 01/ active parts of alluvial fans, In Stage 2, evidence is obtained that identifies areas of potential flooding. This step narrows the area of concern for Stage 3, which is the specification identification of the extent of the 100-year flood. Although alluvial fan flooding has occurred on all parts of an alluvial fan at some time in the geologic past in order to construct the landform itself, this does not mean that all parts are equally susceptible to alluvial fan flooding now, In fact, in most of the United States it is possible to identity parts of alluvial fans that were actively constructed during Pleistocene time (about 2 million to 10,000 years ago) and parts that have been active (i.e., flooded) in the Holocene (the past 10,000 years). The reason that this broad distinction generally is straightforward and simple in practice is that the two time periods were identified and defined on the basis of different climatic conditions, The Holocene epoch is a time of interglacial warm conditions, whereas the Pleistocene epoch was marked by repeated full glacial, cool conditions alternating with warm interglacials like that of the Holocene (Figure 3-3). During glacial times, ice masses expanded and advanced, evaporation was low, and in the dry western U.S. ground water tables and stream discharges were high relative to interglacial times. As a result of these climatic differences, flooding and sedimentation occurred at different rates and magnitudes during the