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FLOODING PROCESSESJlND ENVIRONMENTS ON ALLUVIAL FANS 43 meteorologic events, and between episodes as climate fluctuates or as debris accumulates slowly for centuries after a period of debris flow activity. In general, debris flow activity is most frequent on the upper parts of a composite fan, but a large, dilute, and therefore weak debris flow may remain channelized and mobile for tens of kilometers, and therefore traverse the zone that is dominated by streamflow activity. However, through geologic mapping of distinct debris flow and alluvial deposits it is usually possible to define and map the probable nature of the various types of deposition (bouldery, lobate debris flow deposits; channelized debris flow deposits with bouldery levees; tabular sheets of debris flow deposits; bars of streamflow gravel; and sheets of gravely or sandy streamflow deposits) and thus the maximum extent of debris flows on a composite fan. After examining the debris flow generation potential of the source area, it is also possible to make an approximate calculation of the maximum conceivable transport of debris flows downfan. It is also possible through geologic mapping and dating of deposits and examination of source areas to determine whether the debris flow activity that may have built most of a fan is still active or will generate much smaller debris flows than during the period of intense fan building. An important issue for the prediction of flood risk on fans affected by debris flow is that the rearrangement of the channel system by a debris flow can cause a long-term change in the flooding hazard during stream flooding. Furthermore, after a debris flow has deposited large volumes of sediment on the upper parts of a fan, streamflow in the same or later events will spread the sediment far downfan, causing channel instability and sedimentation that reduce the flood conveyance capacity of channels. CHANGE OVER TIME Fans form over thousands to millions of years, during which time environmental conditions affecting their formation change more or less continuously. For this reason, they are always evolving, although parts ( reaches) of them may temporarily attain a steady state in which their channel gradients can pass the supplied sediment and no further deposition occurs in that reach, Even in that state, the channels can shift across the fan and remold its surface by erosion and deposition. However, part or the whole of the fan may become a zone of greatly diminished accumulation or in the extreme case may become a zone of net erosion with the entire channel system entrenched. The fan surface may thus be weathering and eroding, or it may have attained a steady state after a period of incision. However, some runoff and channel patterns develop on the fan after it ceases to be a zone of net accumulation. Changes in the sediment balance and its spatial distribution can occur because of (1) externally imposed changes (e.g., of climate or land cover influencing sediment supply and transport capacity or of tectonism influencing channel slope and therefore transport capacity); (2) long-term evolution of the fan shape (especially its gradient) as it evolves over time in response to the sediment accumulation; and (3) shorter-term changes in form due to sediment accumulation (e.g., channel shifting accompanying bar accumulation, channel avulsion, raising of channel above the surrounding surface, deposition and channel filling by a single debris flow, runs of wet years caused by weather fluctuations, which may cause accelerated channel widening or shifting or reactivation of sediment sources producing debris flows that have been dormant for a long time).