Alluvial fan

An alluvial fan is like a big pile of rocks and dirt that forms when water moves really fast and carries stuff along with it. Imagine you and your friends are floating down a river on inner tubes. Sometimes the water is calm and slow, but other times it gets really rough and starts moving really fast. When the water moves fast, it can pick up rocks and dirt from the riverbed and carry it with it.

Now let's say you all float around a bend in the river and suddenly the water becomes calm again. When the water slows down, it can't carry all the rocks and dirt anymore and it starts dropping them on the riverbank. Over time, as more and more stuff gets dropped, a big pile starts to form. That big pile is called an alluvial fan.

Alluvial fans usually form where rivers meet flat areas like a plain or valley. They can be really big, sometimes even stretching for miles and miles. People sometimes even build towns or cities on alluvial fans because the soil is really fertile, which means it's good for growing crops. But they also have to be careful because alluvial fans can be dangerous during heavy rainstorms. The fast-moving water can cause flash floods, where the water suddenly rises and rushes down the fan, picking up everything in its path.