The River Severn (Welsh: Afon Hafren, Latin: Sabrina) is the longest river in the United Kingdom, at about 220 miles (354 km).It rises at an altitude of 2,001 feet (610 m) on Plynlimon, Ceredigion near Llanidloes, Powys, in the Cambrian Mountains of mid Wales.ith an average discharge of 107 m³/s at Apperley, Gloucestershire, the Severn is the greatest river in terms of water flow in England and Wales. The river is usually considered to become the Severn Estuary after the Second Severn Crossing between Severn Beach, South Gloucestershire and Sudbrook, Monmouthshire.