Alluvial fan
fan- or cone-shaped deposit of sediment crossed and built up by streams
An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modern glaciation. They range in area from less than 1 square kilometer (0.4 sq mi) to almost 20,000 square kilometers (7,700 sq mi).
Alluvial fans typically form where a flow of sediment or rocks emerge from a confined channel and are suddenly free to spread out in many directions. For example, many alluvial fans form when steep mountain valleys meet a flat plain. The transition from a narrow channel to a wide open area reduces the carrying capacity of flow and results in deposition of sediments. The flow can take the form of infrequent debris flows like in a landslide, or can be carried by an intermittent stream or creek.
The reduction of flow is key to the formation of alluvial fans. If a river exits a mountain valley without any reduction in flow, it is more common to see the formation of an alluvial plain. The steepness of an alluvial formation depends on how much flow decreases when entering flat ground as sediment will be deposited further away from its source if river flow is high.
“Alluvial fan” enters the record as fan- or cone-shaped deposit of sediment crossed and built up by streams. Crown Archives preserves that source wording while asking what Alluvial, fan- and cone-shaped can confirm, complicate or overturn.
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