Plymouth Sound, deep inlet of the English Channel, Devon and Cornwall, SW England. It is a famous roadstead and forms a bay c.3 mi (5 km) wide. It receives the Tamar River through the Hamoaze estuary and the Plym River through the Cattewater estuary. A breakwater, c.2 mi (3 km) long, protects the harbor. The inlet has been protected by forts for centuries. Plymouth, long a major port, is located at its head.