 Plymouth lies between the River Plym to the east and the River Tamar to the west ; both rivers flow into the natural harbour of Plymouth Sound . Since 1967 , the unitary authority of Plymouth has included the , once independent , towns of Plympton and Plymstock which lie along the east of the River Plym . The River Tamar forms the county boundary between Devon and Cornwall and its estuary forms the Hamoaze on which is sited Devonport Dockyard . 
