The easiest pub quiz question in the world: name a river that flows through London. Answer: the Thames. A somewhat more difficult question: name another river that flows through London. A few might know of the river Lee (or Lea) that springs near Leagrave in Bedfordshire and joins the Thames at Leamouth in the London borough of Tower Hamlets. But how about: name a third river that flows through London? And a fourth, a fifth, a sixth?