Alluvial fan

An alluvial fan is a fan-shaped deposit of soil and rocks. It forms where a fast-flowing river spreads out over a flat plain. They are usually found in between mountain ranges that are crumbling away. If it rains a lot, the fan area will usually flood.
Alluvial plain.
An alluvial plain is a mostly flat landform where sediment is deposited over time. The cause is the same: one or more rivers coming from highland regions. A floodplain is the smaller area over which the rivers flood at a particular period of time, and the alluvial plain is the larger area, where floodplains have shifted over geological time.