Severn

Severn, river, south-western Great Britain. Britain's longest river, it rises in the Plynlimon Mountains, central Wales, and flows into England, then south until it forms a large estuary that widens into the Bristol Channel. The river is about 354 km (220 mi) long and is navigable for about 290 km (180 mi), from the Bristol Channel to Welshpool, Wales.

It originally flowed into the Dee, but during the last Ice Age (the Pleistocene epoch) was diverted by glaciation into its present course. It flows through Shrewsbury in Shropshire before turning south-east through the Severn Gorge, where the swiftness of its waters were important in providing the power that made the area the cradle of the Industrial Revolution. Symbolic of this, the river flows under the world's first iron bridge, erected by Abraham Darby III in 1779 and designated a UNESCO World Heritage Site in 1986. Continuing south, the Severn flows through the cathedral city of Worcester, near which it is fed by the Teme, and Tewkesbury, where it receives the Avon. From there it carries on to Gloucester, where the Severn becomes tidal. About 25 km (15 mi) below Gloucester the river begins to widen considerably.

The river is an excellent source of salmon with its tributaries abounding in trout. Canals connect the Severn with the Thames, Trent, and Mersey rivers; a deep-water canal parallel to the river connects Gloucester with the lower reaches of the Severn because of the difficulty of navigating this section. This is caused by the tidal bore that sweeps up the estuary into the river twice each day. A railway tunnel under the estuary, about 6 km (4 mi) long, connects Bristol with southern Wales. In 1966 a suspension bridge, originally part of the M4 motorway, was completed over the estuary with a 990 m (3,240 ft) main span, and a second bridge, a cable-stayed design with a central span of 456 m (1,496 ft), and now the route of the M4, was officially opened in June 1996. A long-term strategy for sustainable development of the Severn estuary, which aims to ensure balance between the environmental, economic, and social aspects, was published in late 2001 by the Severn Estuary Partnership (SEP).