The Tamar is a river in south west England, that forms most of the border between Devon and Cornwall. A part of the Tamar valley is a World Heritage Site due to its historic mining activities.
The Tamar's source is less than 6 km from the north Cornish coast, but it flows southward and its course runs across the peninsula to the south coast. The total length of the river is 61 miles. At its mouth, the Tamar flows into the Hamoaze before entering Plymouth Sound, a bay of the English Channel. Tributaries of the river include the rivers Inny, Ottery, Kensey and Lynher on the Cornish side, and the Deer and Tavy on the Devon side.
Wikipedia https://en.wikipedia.org/wiki/River_Tamar
Coordinates 50°25'40.557" N -4°11'56.491" E