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Alluvial Fans

Alluvial fan is an outspread, gently sloping mass of sediment deposited by a stream where it issues out of the mouth of a narrow canyon draining from and upland area. Alluvial fans are a result of a buildup of sediments or alluvium —a general term for unconsolidated sediments deposited by flowing water on stream channel beds, flood plains, and on alluvial fans. (The term alluvium applies to stream deposits of recent times and it does not include subaqueous deposits, such as in lakes or undersea.) Viewed from above, an alluvial fan typically has the shape of an open fan with the apex being at the mouth of the canyon. Alluvial fans are common in arid to semi-arid regions, but can be covered with forests in the California Coast Ranges. Alluvial fans may merge together to form an apron-like slope along the base of a mountain front.

Badwater alluvial fans

This aerial view show alluvial fans in Death Valley that have built up at the mouths of canyons draining from the Black Mountains (to the right). The fan-shape forms from the gradual filling and shifting of stream channels as the fill with sediment carried by flowing water from infrequent rainstorms in the desert.

Alluvial fans are common landscape features throughout the western states, particularly in the mountainous desert regions of the Great Basin and southern California where bedrock and alluvium are well exposed due to the general lack of vegetation cover. However, alluvial fans can develop in wetter regions (such as in the western Coastal Ranges) and may be covered with forests.

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