River Severn

The River Severn, at 220 miles (354 km) long, is the longest river in Great Britain. It is also the river with the most voluminous flow of water by far in all of England and Wales, discharging an average of 107 m3/s (3,800 cu ft/s) into the Bristol Channel at Apperley, Gloucestershire. It rises in the Cambrian Mountains in mid Wales, at an altitude of 2,001 feet (610 m), on the Plynlimon massif, which lies close to the Ceredigion/Powys border near Llanidloes. The river then flows through Shropshire, Worcestershire and Gloucestershire. The three county towns of Shrewsbury, Worcester and Gloucester lie on its course.
The Severn's major tributaries are the Vyrnwy, the Tern, the Teme, the Warwickshire Avon, and the Worcestershire Stour.
By convention, the River Severn is usually considered to end, and the Severn Estuary to begin, after the Second Severn Crossing, between Severn Beach in South Gloucestershire and Sudbrook, Monmouthshire. The total area of the Estuary’s drainage basin is 4,409 square miles (11,419 km2). That figure excludes the area of the River Wye and the Bristol Avon, both of which flow into the Severn Estuary. The Estuary discharges into the Bristol Channel, which opens into the Celtic Sea and from there into the Atlantic Ocean.