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l to this is when an extremely deep trench is formed by major changes in the watershed such as climate changes or depletion of erodible materials. Such a channel may be stable over geologic time. Within the human time scale, many fans are not uniform and exhibit stron~ geomorphic biases which confine and direct the flood flow paths. The key question that must be asked in these cases is as follows. What is the likelihood that, within our planning period (typically lOa years for flooding). the present observed channel pattern on a fan will be substantially changed? If this probability is large (as when biases and entrenchments are minor or large volumes of sediment are produced in the watershed), the fan can be considered as uniform and the entire fan surface can be delineated as within the flood prone area. If the prob&bility is small. then delineation of the flood prone area must .take existing fan topo~raphy into account. High areas or areas far away from a stable entrenchment would then be delineated as areas of low flood hazard, while areas directly down-fan from a channel or within an existing flow path would be high flood hazard areas. The geometry of flood paths and the dynamics of flood flows have been observed, both in the field studies and in the physical model results, to change substantially with distance from the fan apex. The flow is initially confined to a single channel, which increases in width and decreases in depth as it moves away from the fan apex. The flow continuously loses velocity due to friction effects and loses volume to infiltration. At some point, the ability of the flow to transport sediment falls below that required to carry the sediment suspended in the flow and substantial deposition occurs. Down fan from this point channel braiding begins and rapidly increases flow path width with distance from the apex. This braiding and spreading process 41