An alluvial fan is a fan-shaped formation of debris at the base of a mountain range. These are usually found in dry areas, and are similar to deltas, but without the body of water. They are formed when a creek, river, or wash exits a steep canyon and enter a flatter area. Most of the sediment, rocks, and other debris carried by the water are dropped here. After one area is piled with debris, the watercourse will shift or fan out, creating the charactaristic shape.

These are mainly found in arid areas because in wet areas, the sediment is less likely to be able to accumulate below the mountains. They are common at the base of desert mountains, such as the White Mountains and the Panamints of Death Valley.

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