What is an alluvial deposit at the mouth of river?

A delta is an alluvial deposit at the mouth of a river. Deltas form when a river loses its energy to carry its sediment and the sediment is deposited on the riverbed. This causes the river to split into multiple channels, which creates a fan-shaped deposit of sediment. Deltas are often fertile areas of land because they are well-watered and contain nutrient-rich soil.