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. . . e . . . . . . . . . :. . . . . . . . . . Ie . . . . . . . I. . I. . . I. . . . . . ... 8 02/23/00 Although active 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 active alluvial fan flooding now. Also, flooding may be occurring on inactive areas of the alluvial fan. In most of the United States, it is possible to identifY 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 possible is that the two time periods were identified and defined on the basis of 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. As a result of these climatic differences, flooding and sedimentation occurred at different rates and magnitudes during the Pleistocene and Holocene epochs. The impacts of these climatic changes on alluvial fan formation can be inferred from geologic, geomorphic, and soil data. A change in the rate of tectonic uplift along a mountain front can also result in abandonment of parts of alluvial fans. For example, a decrease in the rate of uplift at a mountain front relative to the alluvial fan could result in stream channel downcutting at the mountain front/alluvial fan apex over a period of time, As a consequence, the upper part of the fan would become entrenched, and the active area of deposition would shift downfan. 4A Identification of Active Areas The term active refers to that portion of an alluvial fan where deposition, erosion, and unstable flow paths are possible. If flooding and deposition have occurred on a part of an alluvial fan in the past 100 years, clearly that part of the fan can be considered to be active. This conclusion may be supported by historic records, photographs, time-sequence aerial photography, andengineering and geomorphic information, If flooding and deposition have occurred on a part of an alluvial fan in the past 1,000 years, for example, that part of the fan may be subject to future alluvial fan flooding. This conclusion may only be supported by geomorphic information, however. It becomes more difficult to determine whether or not a part of the fan that has not experienced sedimentation for more than 1,000 years really is active, that is, that there is some likelihood of flooding and sedimentation under the present climate. Becausethere is no clear analytical technique for making such projections of the estimates of the spatial extent of inundation, Stage 2 analysis involves systematically applied judgement and the combination of hydraulic computations and qualitative interpretations of geologic evidence concerning the recent history and probable future evolution of channel forms, as well as flooding and sedimentation processes. It must be kept in mind, however, that the intent of Stage 2 is to narrow the area of concem with regard to active deposition, erosion, and unstable flow paths over a period of time generally exceeding 100 years, Therefore, the combination of engineering and geomorphic analyses, both qualitative and quantitative, provide an indication of the approximate spatial extent of possible inundation over a relatively long time period (i,e., several thousand years). Stage 3 will determine the floodplain limits associated with the 100-year flood.