Here's why:
* Alluvium is a general term for loose, unconsolidated sediment that has been transported and deposited by a river or stream.
* Mountain canyons are often carved by rivers, which carry sediment eroded from the mountains.
* Gentle slopes at the base of mountains are ideal locations for deposition as the river's energy decreases and it loses its ability to carry the sediment.
So, when the river reaches the gentler slopes, it drops the sediment it has been carrying, creating alluvial fans or alluvial plains.
