The term 'land at a river's mouth' can be summarized as a DELTA, which is a five-letter word. A DELTA is a landform created by the deposition of sediment that is carried by a river as it reaches its mouth. It is typically triangular in shape and is formed when the river splits into multiple distributaries, depositing the sediment between them. Deltas are often found in coastal areas and are important for providing habitats for many species of plants and animals.