River in Britain, which rises on the slopes of Plynlimon, in Ceredigion, west Wales, and flows east and then south, finally forming a long estuary leading into the Bristol Channel; length 336 km/208 mi. The Severn is navigable for 290 km/180 mi, up to Welshpool (Trallwng) on the Welsh border. The principal towns on its course are Shrewsbury, Worcester, and Gloucester. England and South Wales are linked by two road bridges and a railway tunnel crossing the Severn (see Severn Bridge). A remarkable feature of the river is a tidal wave known as the ‘Severn Bore’ that flows for some miles upstream and can reach a height of 2 m/6 ft.
The Severn rail tunnel was built 1873–85. The first of the road bridges to be built opened in 1966, carrying the M4 motorway linking London and South Wales. When a second road bridge was opened in 1996, the M4 was moved to this crossing, and the old stretch of motorway was renamed the M48. In 1998, the Severn Estuary was contaminated with the radioactive isotope of hydrogen, tritium, and the radioactive isotope of carbon, carbon-14. The isotopes were discharged from a medical isotope production plant in Cardiff.
Course From its source, the Severn passes east through Powys and enters Shropshire near the Brythen Hills. Southeast of Shrewsbury, the river passes through Ironbridge Gorge, ‘cradle of the Industrial Revolution’ and now a tourist attraction. Thereafter, it runs through Worcestershire and Gloucestershire, widening considerably after it passes Newnham. The Severn is navigable by larger ships (of around 8,000 tonnes) as far as Sharpness, and by smaller vessels (up to 700 tonnes) to Gloucester, while barges of 350 tonnes capacity can negotiate its upper reaches as far as Stourport. The Severn is connected with the rivers Trent and Mersey via the Staffordshire and Worcestershire Canal, and with the canal network around Birmingham via the Worcester and Birmingham Canal, which joins the Severn at Worcester. Between Gloucester and Sharpness, the treacherous nature of the riverbed necessitated the construction, in 1827, of the 26 km-/16 mi-long, lock-free Sharpness and Gloucester Ship Canal. Tributaries of the Severn include the Teme, Stour, Wye, Vyrnwy, Tern, and Avon; in total, the river basin covers an area of 11,420 sq km/4,409 sq mi.
Crossings Between 1873 and 1885, a rail tunnel was dug underneath the Severn near Chepstow, running for a distance of 7.2 km/4.4 mi, from New Passage to Portskewett; this crossing greatly facilitated travel between Bristol and the Welsh capital Cardiff. A road suspension bridge (see Severn Bridge) was opened nearby, from Aust to Beachley, in 1966; this crossing carries the main M4 motorway linking London and South Wales and is subject to payment of a toll by users. Because of the increase in traffic volume, construction of a new road bridge was started in 1992 and completed five years later.
Hydroelectric power In 1933, a committee recommended the construction of a hydroelectric power station on a river barrage at English Stones reef, which would utilize the tidal flow of the Severn. This plan, which was interrupted by World War II, was revived in 1945, when engineers confirmed the practicability of the scheme and projected an output of some 2,190,000,000 kWh. However, no tidal power plant has yet been built.