A river delta is a triangular river mouth with several branches, side flows and bayous. The delta is formed by the silt and terrain carried by the river flow along the river, and it little by little lenghtens the flow of the river towards the sea.
An estuary is a river mouth which resembles a funnel and which widens, deepens and opens toward the sea. An estuary is formed by a tidal bore, which ablates the riverbed and carries the silt out to sea.