What is the difference between deltas and alluvial fans?

Answer 1

Deposits by Rivers

As water moves, it carries sediments with it. Any time moving water slows down, it drops, or deposits, some of the sediment. As the water slows down, fine particles fall to the river's bed. Larger stones quit rolling and sliding. Deposition creates landforms such as alluvial fans and deltas. It can also add soil to a river's flood plain. In Figure 10 you can see these and other features shaped by rivers and streams.

Alluvial Fans

Where a stream flows out of a steep, narrow mountain valley, the stream suddenly becomes wider and shallower. The water slows down. Here sediments are deposited in an alluvial fan. An alluvial fan is a wide, sloping deposit of sediment formed where a stream leaves a mountain range. As its name suggests, this deposit is shaped like a fan. You can see an alluvial fan in Figure 8.

Deltas

A river ends its journey when it flows into a still body of water, such as an ocean or a lake. Because the river water is no longer flowing downhill, the water slows down. At this point, the sediment in the water drops to the bottom. Sediment deposited where a river flows into an ocean or lake builds up a landform called a delta. Deltas can be a variety of shapes. Some are arc shaped, others are triangle shaped. The delta of the Mississippi River, shown in Figure 9, is an example of a type of delta called a "bird's foot" delta.

A delta forms when a river empties into a larger body of water. An alluvial fan forms on land where a river emerges from a mountainous area and flows out onto a more gently sloping plain. The decrease in slope causes the river to decelerate and deposit sediment, forming an alluvial fan.