Here's why:
* Alluvium: This refers to the sediment transported and deposited by a river.
* Fan: The shape of the deposit often resembles a fan, with the apex at the mouth of the river canyon and the wider base spreading out onto the plain.
As the river flows from the mountains onto the flatter plain, its velocity decreases dramatically. This causes the river to deposit its load of sediment, forming a fan-shaped accumulation. Alluvial fans are common in arid and semi-arid regions where there is a high sediment load and the river flow is often intermittent.
