In order to properly define an “alluvial fan“, we must first define “alluvium“. Alluvium refers to clay, silt, sand, and/or gravel particles that are deposited by running water. Debris from flooding is also considered alluvium after it has been deposited. Therefore, an alluvial fan is a fan-shaped accumulation of alluvium deposited at the mouth of a ravine or at the juncture of a tributary stream with the main stream.
Alluvial fans generally form where a stream’s gradient decreases, when a fast moving tributary enters a larger, slower moving river, or where a stream flows into a lake. When this happens, the stream deposits course grained material that cannot be suspended by the flow. The deposition of course grained material then builds up and forces the stream to gradually build out in a conical or fan-shaped way. As this happens sand particles begin to fall out of suspension as they cannot be carried by the flow and the process continues until eventually the stream deposits its fine grained material farther out. The result is an alluvial fan.
A large scale alluvial fan in the Tibetan Plateau.