Severn (sĕvˈərn) [key], Lat. Sabrina, one of the principal rivers of Great Britain, c.200 mi (320 km) long, rising on Plinlimmon Mt., W Wales, and flowing NE and E to Shrewsbury, W England, and from there SE, S, and SW—through an estuary—to the Bristol Channel. Worcester, Gloucester, and many smaller towns are on its banks. The tributaries include the Teme, Avon, and Stour. It is connected by canal with the Thames, Mersey, Trent, and other rivers. A railroad tunnel (opened 1886) more than 4 mi (6.4 km) long passes under the estuary. The Severn Road Bridge (opened 1966) is one of the world's longest (3,240 ft/988 m) suspension bridges. The river is an important transportation route. Because of the conformation of the estuary, a tidal bore occurs up to Gloucester.