SEVERN (Welsh, Hafren, Roman, Sabrina), a river of Wales and England. It rises on the north-east side of Plynlimmon, on the south-west border of Montgomeryshire, and flows with a nearly semicircular course of about 210 TM to the Bristol channel. Its course is at first in a south-east direction, and for the first 15 m. it flows over a rough precipitous bed. At Llanidloes it bends north-east, passing Newtown and Welshpool; this part of the val ley bearing the name of the Vale of Powys. It receives the Vyrnwy near Melverley, and forms a mile of the Welsh border, and then turning eastward enters Shropshire and waters the broad plain of Shrewsbury, after which it bends southward past Ironbridge and Bridgnorth to Bewdley in Worcestershire. In Shropshire it receives a number of tributaries, the chief of which is the Tern. In Worcestershire it passes Stourport, where it receives the Stour (left), and Worcester, shortly after which it receives the Teme (right). It enters Gloucestershire close to Tewkesbury, where it receives the Upper Avon (left), after which, bending south-west, it passes Gloucester, below which it becomes estuarine and tidal. A high bore, for which the Severn is notorious, may reverse the flow as high up as Tewkesbury Lock (13-i m. above Gloucester), and has sometimes caused great destruction. The estuary merges into the Bristol Channel at the point where it receives the Lower or Bristol Avon (left) and the Wye (right).
Like the Yorkshire Ouse, the river Severn collects on its right bank a number of streams which have remarkably parallel courses. Of these the Upper Severn, Teme, Wye and Usk flow in a direction a little south of east, whilst the Ebbw, Rhymney and Taff flow generally southward. The whole system drains over a half of Wales. It is assumed by Dr. A. Strahan (Geology in the Field, p. 856) that the system originated on a land surface of Upper Cretaceous and Tertiary strata, of which however nothing remains in the area. The eastward flowing streams, which may probably at one time have formed part of the Thames system, were captured one by one and diverted southward into the Bris tol channel. The river had reached its base level when, in Plio cene times, the district, along with a much more extensive area, was affected by an uplift of from 400 to 600 feet. The effect
of this uplift is seen in the incised meanders of the Wye and the occurrence of rock-steps in most of the valleys themselves. Above these steps the streams present all the characteristics of mature rivers. One such step occurs at the Ironbridge gorge which was considered by Prof. Lapworth (op. cit., p. 768) to be due to river capture during the glacial period. The present Upper Severn and the Vyrnwy he considered were affluents of a river which, flowing northwards, was dammed by ice coming down from north Wales so that the water was ponded back and overflowed at Ironbridge into the Severn. This overflow channel was deepened sufficiently to bring about the permanent diversion of the streams when the ice retreated.
The upper part of Severn estuary is difficult of navigation, and therefore the Gloucester and Berkeley ship canal (i6i m. long) was constructed, admitting vessels of 35o tons to Gloucester from the docks at Sharpness on the estuary. The navigation ex tends up to Arley, above Bewdley, 47 m. from Gloucester, and Stourport (43 m.), where the Staffordshire and Worcestershire canal gives access to the Wolverhampton and the Trent and Mersey navigation. There is connection with the Thames by the Stroudwater canal from Framilode, joining the Thames and Sev ern canal near Stroud. The Wye is in part navigable; the Bristol Avon gives access to the great port of Bristol, and the Upper Avon is in part navigable. The Severn is a good salmon river, and is famous for its lampreys, while many of the tributaries afford fine trout-fishing, such as the Teme and the Vyrnwy.
The Severn tunnel, carrying the G.W. railway under the estuary, forms the direct route between the south of England and south Wales. In order to relieve the pressure of traffic, the G.W. rail way sought powers in 1924 to construct a bridge across the estuary. The scheme has been postponed until fuller enquiry can be made into the effects of such a construction upon tidal currents, etc. Another important project which has been postponed indefinitely is the "Severn barrage scheme" to utilize the high tides of the river to generate electrical power.