Tweed, river, 97 mi (156 km) long, rising in the Southern Uplands of Scotland. It flows E through S Scotland then NE, forming the Scotland-England border for 17 mi (27 km) before entering the North Sea at Berwick, NE England. The Tweed system drains most of SE Scotland; the Gala, Ettrick, and Teviot are its chief tributaries. In Scotland the Tweed waters a sheep-farming region and passes Peebles, Melrose, and Kelso. The Tweed also has rich salmon fisheries.