The River Severn is the longest river in Great Britain, at about 354 kilometres (220 mi), and the second longest in the British Isles, behind the River Shannon. It rises at an altitude of 610 metres (2,001 ft) on Plynlimon, Ceredigion near Llanidloes, Powys, in the Cambrian Mountains of mid Wales. It then flows through Shropshire, Worcestershire and Gloucestershire, with the county towns of Shrewsbury, Worcester, and Gloucester on its banks.