Delta plains are commonly known as floodplains or alluvial plains. These plains are formed by the deposition of sediment carried by rivers over time. Other synonyms for delta plains include estuarine plains, river valleys, and lowland areas. These synonyms are used to describe the same landform that lies at or near sea level, and are usually characterized by fertile soils that make them ideal for agriculture. Delta plains are found in many parts of the world, including the Nile Delta in Egypt, the Indus Delta in Pakistan, and the Mississippi Delta in the United States. Regardless of the name used to describe them, delta plains are vital habitats for many species of plants and animals.