The longest river entirely in England is the Thames, which is 6 letters long. The Thames is a major river that flows through southern England and is the second longest river in the UK. It rises near Kemble in Gloucestershire and flows through Oxford, Reading, Henley-on-Thames and Windsor before reaching the North Sea at the Thames Estuary. The Thames has been an important waterway since prehistoric times and is now used for leisure activities such as boating and fishing.