What is an Alluvial Fan in Geography?

An alluvial fan forms due to a sudden and dramatic decrease in the speed of a river.

1. The Fast Flow: A stream or river flows rapidly down a steep mountain slope or through a narrow canyon. Because the water is moving incredibly fast, it has the energy to pick up and carry large amounts of heavy sediment (alluvium).
2. The Sudden Stop: Eventually, this fast-moving mountain stream exits the narrow valley and empties out onto a flat, open plain.
3. The Deposition: Because the water is suddenly no longer confined by the canyon walls, it spreads out widely. As it spreads, its speed and energy drop instantly. It can no longer carry the heavy rocks and gravel, causing them to drop out of the water and pile up in a spreading, fan-like shape.

Alluvial fans are most commonly found in arid and semi-arid desert regions where sudden, intense flash floods wash massive amounts of debris down dry mountain canyons. Famous examples are found in Death Valley, California, and at the foothills of the Himalayas in the Indo-Gangetic plain (where they form the porous Bhabar belt).

People often confuse alluvial fans with river deltas because they both involve water depositing sediment in a fan shape.

• Alluvial Fan: Forms on dry land, specifically where a steep mountain stream hits a flat plain.
• Delta: Forms in water, specifically where a slow-moving river meets an ocean or a lake.