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FLOODING PROCESSES AND ENVIRONMENTS ON ALLUVIAL FANS 33 Field evidence is an important source of information on the nature and intensity of the sedimentation processes that built the fan and is therefore critical for refining estimates of the nature, frequency, physical controls, and engineering significance of the flood risk on any fan. The morphology of the fan surface and the character of the deposits visible in the sides of channels indicate the relative contribution of water flows and debris flows to the flooding hazard on various parts of the fan. It can also be established whether changes in the governing geographical factors have changed the nature and distribution of the flood hazard during the history of the fan. It is important to realize that although fans vary in geometry and flooding characteristics because of the various combinations of their controlling factors, it is not always necessary to regress to a default assumption that there is no way to reduce uncertainty in the prediction of flooding processes on them. The copious field evidence available provides a means of reducing uncertainty about flood behavior, if it is properly interpreted. Streamflow Fans On fans that are actively forming from water-borne sediment alone, channels are usually braided, or multithreaded, from the apex. Deposition occurs on the channel bed in the form of bars on the margins or in the centers of channels. Rapid erosion of channel bed and banks is possible because of the loose, unconsolidated nature of the sediments. Thus, rapid erosion or deposition along a channel reach can alter the flow conveyance capacity during a single flood, Bank erosion and lateral bar formation can force the channel to shift, while both bed aggradation and mid-channel bar formation can force water overbank and into new paths, so that channels divide and streams episodically abandon one or both channels. Channels may shift dominantly as a result of the accumulation oflateral bars, in which case they do not build up their bed or banks above the level of the surrounding surface. In addition to this gradual channel migration, sudden changes in flow path (avulsions) can occur due to overbank flooding. Even quite large and well-defined channels can be abandoned if a flood breaches one of the channel banks and water flows overbank in depressions between old bar deposits on the fan surface, often eroding a deep channel headward up to the source channel, which is then diverted. Particularly large, kilometer-scale changes in the positions of flow paths and active sedimentation zones can occur without the channel occupying or shifting across intermediate positions if the channelized and the overbank flow cause sediment to be deposited within and close to the channel, raising the bed and the channel margins above the surrounding fan surface (Figure 2-4). Breaching of the elevated banks in a large flood can allow the flow to travel toward the lower areas between channels or along the fan margins. Small shifts near the fan head can cause dramatic changes in channel position farther down the fan. As one proceeds down the fan, the channels separate more frequently than they join, so that on average the channels diverge and diminish in width, depth, and discharge along a general flow thread during anyone flood event. Despite the decrease in discharge, the reduction in width, depth, and gradient can force water overbank in many floods, and thin, unchannelized, relatively uniform expanses of water can cover large areas (sheetflood). Sufficiently far down some fans, most of the runoff occurs as a sheetflood, either generated locally on the fan or forced overbank by the diminishing conveyance capacity of the channels. The sheetflood itself is irregular with zones of concentrated flow giving way downslope to divergences and shallowing at which small