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44 ALLUVIAL FAN FLOODING Even if the fan is still a zone of active accumulation, the locus of that deposition may vary between fans or on the same fan during its lifetime. Distributaries develop most frequently where aggradation is intense and thus may shift the centers of active sedimentation across or along the fan. Deposition also may shift down the fan if the uplift rate in the mountain source exceeds the deposition rate at the fan head. This can cause trenching of the source canyon and the upper fan or intense episodes of debris flow and stream transport associated with increased runoff. In the latter cases, the channel may be entrenched below the upstream part of the fan surface and all deposition and attendant channel shifting or change occurs on the lower portions of the fan. Appreciation of such changes is important for understanding flood hazards on fans because (1) some of the changes occur sufficiently quickly to affect channels on timescales relevant to engineering analyses, so that it is important to know whether a particular fan is undergoing a change at the present time, and (2) geologically recent, but no-longer active, processes may leave a morphologic signal on the fan or in the source basin that must be interpreted in order to refine estimates of recent changes in flood hazard. Fans grow in a variety of ways and the thickness generally increases at the slope transition formed at the fan toe as it is progressively buried (Hooke and Dorn, 1992). Deposits near the apex commonly are remobilized and redeposited farther down the fan on and below the fan toe. French (1995) gives a method of estimating the depth of sediment deposition at these slope transitions on fans. The most permanent deposition typically begins at the toe and propagates both up the fan and below the toe where the slope typically diminishes. The evolution of the fan surface causes a difficult problem for the interpretation of field evidence concerning alluvial fan flooding and for the prediction of future flood risk. For example, if a part of a fan surface has not been disturbed by flooding or erosion for 15,000 years, its surface will have become weathered and covered by a soil-profile and vegetation (described in Chapter 3). The surface of such a fan will be very different from the surface of a nearby channelized and actively evolving area. An important geomorphologic and hydrologic question for flood risk analysis is whether the older surface has evolved out of the flood zone or whether it simply has not been flooded for 15,000 years because random channel migration across the fan took the locus of flooding and sedimentation far from the site for that length of time. If, for example, the active sedimentation zone is now migrating into the older surface through lateral bank erosion, or lies at an elevation only a few meters below that surface (i.e., within the range of flood stage), or if upstream of the site there is an opportunity for avulsions that could lead channels or sheetfloods across the older surface, then the older surface lies within the zone subject to alluvial fan flooding. If, by contrast, channels have become entrenched during the past 15,000 years for one of the reasons given above, the elevation difference between the recently active sedimentation zone and the older surface may be greater than any flood or debris flow stage conceivable in the current regime of climate, hydrology, or land use in the source area. In this case, "overbank" flooding is not possible. Maximum conceivable flood heights, albeit approximate because of the assumptions that must be made about potential changes of bed elevation, are predictable through methods described by Burkham (1988) for floods and Whipple (1992) for debris flows. If field inspection reveals that the margin between the older and the recently active surfaces is armored with bouldery or cohesive sediment, and if the ages of trees along this margin as well as the aerial photographic record (now approximately 50 years old) indicate little or no migration of that boundary, then one's confidence is increased that lateral bank erosion does not threaten the site.