The River Tweed (Uisge Thuaidh in Gaelic) (156 kilometres or long) flows primarily through the Borders region of England and Scotland. It rises on Tweedsmuir at Tweed's Well near where the Clyde, draining northwest, and the Annan draining south also rise. "Annan, Tweed and Clyde rise oot the ae hillside" as the Border saying has it. It drains the entire Borders region. Its lower reaches mark the Scottish border with England for 27 kilometres near Berwick-upon-Tweed. The Tweed is one of the great salmon rivers of Great Britain.
The Tweed valley is also a very interesting area when it comes to looking at the glacial history of Britain. The valley floor on which the river currently flows is a drumlin field and is the relic of a paleo ice stream that flowed through the area during the last glaciation.
Tweed tributaries include: