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Formation

Alluvial fans usually form where a confined feeder channel exits a mountain front or a glacier margin. As the flow exits the feeder channel onto the fan surface, it is able to spread out into wide, shallow channels or to infiltrate the surface. This reduces the carrying power of the flow and results in deposition of sediments.

Flow in the proximal fan, where the slope is steepest, is usually confined to a single channel (a fanhead trench), which may be up to 30 meters (100 ft) deep. This channel is subject to blockage by accumulated sediments or debris flows, which causes flow to periodically break out of its old channel (nodal avulsion) and shift to a part of the fan with a steeper gradient, where deposition resumes. As a result, normally only part of the fan is active at any particular time, and the bypassed areas may undergo soil formation or erosion.

Alluvial fans can be dominated by debris flows (debris flow fans) or stream flow (fluvial fans). Which kind of fan is formed is controlled by climate, tectonics, and the type of bedrock in the area feeding the flow onto the fan.