Alluvial fans

An alluvial fan is a fan- or cone-shaped deposit of sediment crossed and built up by streams. If a fan is built up by debris flows it is properly called a debris cone or colluvial fan. These flows come from a single point source at the apex of the fan, and over time move to occupy many positions on the fan surface. Fans are typically found where a canyon draining from mountainous terrain emerges out onto a flatter plain, and especially along fault-bounded mountain fronts.

A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial fan.

As a stream's gradient decreases, it drops coarse-grained material. This reduces the capacity of the channel and forces it to change direction and gradually build up a slightly mounded or shallow conical fan shape. The deposits are usually poorly sorted. This fan shape can also be explained with a thermodynamic justification: the system of sediment introduced at the apex of the fan will tend to a state which minimizes the sum of the transport energy involved in moving the sediment and the gravitational potential of material in the fan. There will be iso-transport energy lines forming concentric arcs about the discharge point at the apex of the fan. Thus the material will tend to be deposited equally about these lines, forming the characteristic fan shape.

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