Lower Tamar Lake is located on the Devon-Cornwall border, in England, near Thurdon. It was constructed in the 1820s to supply water to the Bude Canal. It is now a nature reserve. It is also a popular fishing lake. The reservoir has been formed by damming the River Tamar. To the north (upstream) is the newer Upper Tamar Lake, a reservoir used for public water supply. The county boundary originally ran along the (natural) route of the River Tamar, but was re-aligned along a (now disused) re-routing of the river to the reservoir's west side, placing the lake in Devon.