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34 ALLUVIAL FAN FLOODING ; " !, , ~f ~ . t io 11 FIGURE 2-4 Oblique aerial pholograph of an alluvial fan of the western slopes of the McDowell Mountains in central Arizona showing recent separation and braiding of channels downfan. The topographic apex is at the lower edge of the scene. Floodflow can inundate much of the area in Ihe scene except for a few ridges of old-fan remnanls. Courtesy ofR W. Hjalrnarson. fans of sediment are deposited. At the toe of most fans, sheetflood that has a relatively low sediment concentration because of deposition on vegetated surfaces can once again gain an erosive capacity if it is concentrated into a number of swales and small channels before entering trunk streams that drain the entire mountain front. On streamflow fans where the sediment balance has turned negative, either at present or for some period in the recent past, the channels are deeper because sedimentation on their floors and margins is replaced by incision. The flow and sediment conveyance capacity increase because form roughness is less in the absence of aggressive bar growth. Thus, many of these channels are incised below the surrounding fan surface, and avulsion occurs less frequently or not at all in the current climatic and hydrologic regime. The separation of the flow into diverging, smaller channels is reversed, and one or a few trunk streams convey the floodflow to the toe of the fan, Because these major conduits are incised they are not so frequently diverted by mid-channel bar deposits and they do not shift across the fan as quickly as those on actively accumulating fans. Instead, they tend to gather local runoff generated on the fan surface because the rills and small channels produced by such runoff repeatedly erode toward the stable trunk channel. The channel network is slightly convergent downfan, and mapped contours show upfan re-entrants that reflect