When large amounts of alluvium are deposited at the mouth of a river, a delta is formed. The river slows down at the mouth, so it doesn’t have the energy to carry all the silt, sand, and clay anymore. These sediments form the flat, usually triangle-shaped land of a delta. Examples of deltas are the Nile River Delta in Egypt and the Mississippi River Delta in the U.S. state of Louisiana.

Source: National Geographic