What is a fan-shaped area at the mouth of river?

A fan-shaped area at the mouth of a river is called a delta. A delta is formed when a river carrying sediment flows into a body of water, such as the ocean or a lake. As the river slows down, the sediment it is carrying is deposited, building up a fan-shaped plain. Deltas are often important agricultural areas, as the soil is fertile and there is a good water supply. Some of the world's largest deltas include the Nile Delta in Egypt, the Ganges Delta in India, and the Mississippi Delta in the United States.