The term DEPOSIT AT A RIVER'S MOUTH can be summarized as the process of sediment accumulation at the end of a river, where it meets the ocean or another body of water. This process is known as DELTA formation, which is a five-letter word that can be used to describe the phenomenon. DELTA formation occurs when sediment is carried by a river and deposited at its mouth, forming a triangular-shaped landmass. This sediment is usually composed of sand, silt, and clay, and can be found in many coastal areas around the world.