The River Severn, at 220 miles 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, with an average flow rate of 107 m3/s at Apperley, Gloucestershire. It rises in the Cambrian Mountains in mid Wales, at an altitude of 2,001 feet, on the Plynlimon massif, which lies close to the Ceredigion/Powys border near Llanidloes. The river then flows through Shropshire, Worcestershire and Gloucestershire.