A ria or an estuary is formed when a river valley runs roughly at the right angles to the coast and the coast is therefore, a discordant coast.

The sides of the valley rise to higher land so that there is only a little low land at the bottom of the valley. As the sea land rises, seawater enters the lower parts of the valley and drowns it.

Rias are commonly along the coast of Guinea-Bissau, North western Madagascar, Congo river mouth and River Rokel in Sierra Leone.