Severn (Lat. Sabrina), after the Thames, the largest of the rivers of England, rises, 1500 feet above sea-level, from a chalybeate spring on the eastern side of Plinlimmon, 12 miles west of Llanidloes, in Montgomeryshire, North Wales. Flowing eastward to Llanidloes, to which town it retains its original British name of Hafren, it afterwards flows north-east past Newtown (465 feet) and Welshpool to the eastern boundary of Montgomeryshire, then east-south-east past Shrewsbury and Bridgnorth in Salop, and finally southward through Worcester and Gloucester, in which last it begins to form the estuary that merges in the Bristol Channel (q.v.). It is navigable for barges to Welshpool (180 miles). Its entire length is 210 miles (from source to mouth as the crow flies only 80). The chief affluents are the Terne and the Upper and Lower Avon on the east, and the Teme and Wye on the west. A canal 18 1/2 miles long, and navigable for vessels of 350 tons, extends from Gloucester to the upper portion of the estuary; and in 1891 the improvement of the navigation to Worcester was begun. The Montgomery Canal extends from Welshpool to Newtown, and other canals connect with the Thames, Trent, Mersey, etc. In some of the reaches below Gloucester, especially near Newn-ham, the tide, which flows with great velocity, produces a bore (locally termed hygre) or wave sometimes 5 or 6 feet high. The railway twice crosses the estuary - near Berkeley by a viaduct (1879), 1194 yards long, and near Chepstow by a tunnel (1873-85), 4 1/3 miles long.