The river's source is around 450 metres (1,480 ft) above sea level on Dartmoor, in an upland marshy area called Plym Head.
The course then changes to run southwards, between Plymouth and Plympton passing through the National Plymbridge Woods and under the ancient Plym Bridge.
Its upper tidal estuary is known as the Laira and the lower part is called Cattewater, which is navigable and leads towards Plymouth Sound.
The first section from Plym Valley from Marsh Mills station opened in 2001, with an extension to Lee Moor Crossing in 2008.
[9] The remainder of the line forms part of the Plym Valley Trail, a cycle route from Plymouth to Ilfracombe.