Alluvial plains are areas that are formed by deposits of sediment carried by water. Some synonyms of alluvial plains include floodplains, delta plains, and coastal plains. Floodplains are formed by the periodic flooding of rivers, which deposit sediment onto the nearby land. Delta plains are formed at the mouth of a river as it reaches and spreads into a larger body of water, depositing sediment into a fan-shaped pattern. Coastal plains are formed by the gradual deposition of sediment along the coast by the action of waves and tides. All of these types of plains are rich in nutrients and are often used for agriculture.