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essary conditions for fan formation. D~ferences in climate and geology have produced mostly active fans in regions such as Nevada and Cal~ornia, and inactive fans in southern Arizona. Active fans exhibit braided channels and erratic fiowpaths typical of a young fan formation. Fans in southern Arizonatend to exhibit more predictable channels and flood paths due to their geologically older setting. "has been estimated that 15 to 25 percent of the arid West is covered by fans. How A,,, At/uvial Fans Fo,m"d? Alluvial fans build up over the course of geologic time through this general sequence: 1. Rock and soil are eroded from the mountain watershed; 2. These sediments are transported through stream chan- nels via storm-generated floodwaters; FORMATION OF AN "IDEALIZED" ALLUVIAL FAN 1. Streamflow from intense rainstorms emanates from the confined channel of a mountain canyon and proceeds onto the relatively flat valley below. The canyon outlet forms the APEX of the fan, which represents the point of highest elevation on the fan. 2. Flow leaving the apex spreads onto the uppermost portion of the alluvial fan surface via a single high-velocity channel. This singular channei will either follow a pre- existing path cut from past flood events, possibly deepening the channel in a proc- ess called entrenchment, or cut a new path downslope. Flood hazards in this CHAN- NELIZED ZONE of the upper fan region can be severe due to the high velocity of flow, the presence of debris from the wa- tershed, and the unpredictable location of flowpaths. 3. The floodfiow which has been confined in mountain ravines emerges onto the wide, flat valley slope, and progressively loses veloc~y while depos~ing sediment as it travels downfan. The sediment deposits are generally narrow and steep at the head of the valley, broadening as they spread out onto the valley floorin a wedge or conical shape. The natural processes responsible for fan formation have frequently been observed to occur in a specific sequence and range of location along the length of the fan. For the sake of illustration, these processes will be presented in a generalized fashion to portray the formation at the "idealized" alluvial fan. In reality however, wide variations in the physical characteristics of each fan system affect the actual location, exlent and sever~y of these flood processes. Variations occur between areas exhibiting differences in climate and geology, as well as among fans within the same valley. 1. APEX 3. As the single channel flow encounters the flatter slope olthe mid-fan area, it widens and becomes shallower, losing veloc- ity and depositing sediment and debris. Materials that become deposited into previously-cut channels can backfill the old streambeds, leading to the abrupt development of new chan- nels in a process called avulsion. The erosion/deposition proc- esses include channel braiding, where singular flows split and rejoin as channels are alternately cut and filled with sediment. These BRAIDED ZONE processes occur erratically, creating random, unpredictable flow patterns. 2 4. SHEET FLOW ZONE Rgur.3 4. Toward the base of the fan, called the TOE, water veloc~ies are further reduced as the fan surface becomes more uniform, its slope flattens and water inMrates the soil surface. In this portion of the fan, SHEET FLOW (shallow, overland flow) is common, though flow velocities may remain high. Adjacent fans which have fNmed along mountain fronts tend to con- verge near their bases, producing alluvial APRONS, or zones of coalescence.