The most important river of Great Britain, formed by the junction at Lechdale of four head-streams—the Isis, Churn, Coln, and Leach—which spring from the SE. slope of the Cotswold Hills; winds across the southern midlands eastwards till in a wide estuary it enters the North Sea; forms the boundary-line between several counties, and passes Oxford, Windsor, Eton, Richmond, London, Woolwich, and Gravesend; navigable for barges to Lechdale, and for ocean steamers to Tilbury Docks; tide is felt as far as Teddington, 80 m.; length estimated at 250 m.