RiaA ria is a submergent coastal landform, often known as a drowned valley or drowned river valley. Rias are almost allways estuaries. Rias form when sea levels rise or plate tectonics cause coastal levels to fall. When this happens valleys which were previously at sea level become submerged.
The south coast of England is a submergent coastline, and contains many rias, including Portsmouth harbour, Southampton water, Poole harbour, River Exe and Plymouth Sound. Chesapeake Bay and Delaware Bay, on the east coast of the United States are rias.