The mouth of a river is where it meets: the sea, a lake or a larger waterway and ends its journey.
Deposition often happens where a river channel enters the still water of a lake or the sea. If sediment is deposited faster than it is carried away, it builds up and forms a raised area called a delta. As the river flows across the delta, it splits into lots of channels. The most famous deltas are the Nile delta and the Mississippi delta in the USA.