What are the areas of sediment deposit where a mouth reaches flat land at bottom mountain?

The areas of sediment deposit where a mouth reaches flat land at the bottom of a mountain are called alluvial fans. Alluvial fans are formed when a river or stream flows out of a narrow mountain valley and onto a flat plain. The water slows down and spreads out, depositing its sediment in a fan-shaped area. Alluvial fans are typically composed of sand, gravel, and boulders. They can be very large, extending for several kilometers from the mountain front.