How do alluvial fans form?
Where ephemeral, intermittent, or perennial streams flow out of narrow, constricting, upland canyons onto the less confining, open lowland plains, the channels become wider and shallower, causing them to slow. The decrease in velocity reduces the stream's competence and its capacity—that is, the size and amount, respectively, of load it can transport—thus causing deposition. As velocity continues to decrease, the sediment carried and deposited decreases in size. As the stream begins to build a fan it also shifts laterally around deposited clasts and sediment, sweeping back and forth. In some cases, debris flows can build up fans rather than, or in addition to, stream action.
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