The deposit that forms where a river leaves a mountain range is called an alluvial fan.

Here's why:

* Alluvium: This refers to the sediment carried by a river.

* Fan: The shape of the deposit resembles a fan, spreading out from the narrow river channel at the mountain's base.

As a river flows down a mountain, it carries sediment and debris. When the river exits the mountains and enters a flatter area, its speed decreases. This decrease in speed causes the river to deposit its sediment load, forming the alluvial fan.