Plymouth lies between the River Plym to the east and the <answer>River Tamar</answer> 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 <answer>River Tamar</answer> forms the county boundary between Devon and Cornwall and its estuary forms the Hamoaze on which is sited Devonport Dockyard.
What river is to the west of Plymouth?
River Tamar