Estuary is a broad river mouth into which the sea flows. It is a narrow inlet which extends a few Kilometres inland. Its formation is explained as follows:-

Sides of the river rise high at the mouth due to increase of water in the ocean.

The rise in sea level forces water to enter the lower parts of the valley.

The valley becomes flooded and drowned.

Estuaries are wider seawards but decrease in width and depth inland.

Examples are R. Zaire at Matadi and Boma in Zaire, R. Rokel in Sierra Leone at Free town and R. Gambia in Gambia.