The River Severn (, ) is the longest river in Great Britain, with a length of . It is also the river with the most voluminous flow of water by far in all of England and Wales, with an average flow rate of at Apperley, Gloucestershire. It rises in the Cambrian Mountains in mid Wales, at an altitude of , on the Plynlimon massif, which lies close to the Ceredigion/Powys border near Llanidloes. The river then flows through Shropshire, Worcestershire and Gloucestershire. The 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 Prince of Wales Bridge, between Severn Beach in South Gloucestershire and Sudbrook, Monmouthshire. The total area of the estuary's drainage basin is . 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.
Etymology
Celtic Root
An etymology has been proposed, which signifies that 'Severn' is an anglicised version of an ancient Celtic phrase signifying 'a gap (in the coastline)', referring to the estuary.
Romano-British name
The name Severn is thought to derive from a British word sabrinā, possibly from an older form *samarosina, meaning "land of summertime fallow".
During the Roman occupation the Severn was known by the Romano-British Latin name Sabrina.