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FLOODING PROCESSES'AND ENVIRONMENTS ON ALLUVIAL FANS 45 Such evidence is found, for example, along many channelized alluvial fans that have been entrenched and rendered less active by changes of climate and hydrology. Former debris flow activity, or large meltwater or monsoonal Pleistocene floods, have often built fans of coarse sediment that were trenched by smaller floods soon after the climate change and now have channels that can contain all the stream discharges conceivable under current environmental conditions and are lined with sediment too resistant to be eroded rapidly. More equivocal conditions occur along channels that have undergone less extreme changes, but photographic and dendrochronological evidence indicates that channels on some, even fine-grained, fans do not aggressively undermine their root-reinforced banks, presumably because bank scour is not required by the combination of flood lows and sediment balance in the reach. Finally, even if the older surface is judged to be free from the risk of overbank inundation and undermining, the channel system upfan needs to be systematically surveyed for the potential of an avulsion that would lead flows across the older surface. This would require a combination of geomorphologic survey (for low banks, elevated channels, zones of sediment accumulation, bank erosion and channel shifting, and topographic lows on the fan surface) and calculations of the heights of conceivable floods and debris flows. On many alluvial fans, however, there is a relatively clear separation between older, higher, stable parts of fans, and channels or narrow inset floors trenched into the fan deposits. The kinds of field evidence that can be used for making such determinations are reviewed in Chapter 3, and valuable sources of information are listed in Appendix B. It is unfortunate that the problem of interpreting the significance of different ages of fan surface for alluvial fan flooding risk is often not reduced by a simple indicator or numerical index. However, with a systematic approach such as the example given above (identity the risk of overbank inundation, lateral bank erosion, and inundation by avulsion from upstream), most field situations can be classified as a basis for both regulation and choice of a method for estimating flood risk, as required by FEMA. Complicated situations, however, will tax any approach. REPORTS OF FLOODING ON ALLUVIAL FANS Documented accounts of flooding confirm that active fans function primarily as a terminus for water and debris, while relict fans and streams function to convey water and sediment. Appendix A summarizes accounts of flooding at 29 individual or groups of alluvial fans to illustrate a wide variety of flood conditions on streamflow, debris flow, and composite fans. Most of the alluvial fans are in the arid southwestern United States, and a few are in humid areas throughout the world. The accounts are for observed floods and consist of direct and indirect measurements and observations of flood characteristics and geomorphologic interpretation of flood remnants. Only a few maps of the extent of flooding are available, and direct measurements of flow depths and velocities on fans are rare. Opportunities for systematic collection of flood data on alluvial fans are also rare. These accounts demonstrate the complex nature of flooding on alluvial fans, ranging from flooding in stable channels to massive deposition of debris in urbanized areas. Accounts by different observers of a particular flood on a particular fan can depict different characteristics. This is because (1) observations from a single vantage point of the widespread and variable nature of fan flooding are limited and not desirable and (2) local conditions change during a flood