The longest river in Great Britain is the River Severn, which is 6 letters long. The River Severn is a major river in the United Kingdom, flowing through the counties of Shropshire, Worcestershire, and Gloucestershire before entering the Bristol Channel. It is the longest river in the UK, with a length of 220 miles (354 km). The River Severn is also the second longest river in the British Isles, after the River Shannon in Ireland. The source of the River Severn is located in the Welsh mountains, and it flows through several towns and cities including Shrewsbury, Worcester, and Gloucester before emptying into the Bristol Channel.