The River Plym is a river in Devon, England. Its source is some 450m (about 1500 ft) above sea level on Dartmoor, in an upland marshy area called Plym Head. From the upper reaches, which contain antiquities and mining remains, the river flows roughly southwest and enters the sea at Plymouth Sound. The river passes to the east of (and gives its name to) the city of Plymouth — the settlement was historically known as Sutton. Its tidal estuary is known as the Laira and the final part is called Cattewater. It is approximately 30 km (about 20 mi) long.