Thames River, one of the principal rivers of Great Britain. From its source in the Cotswold Hills of Gloucestershire, the Thames flows generally eastward for 210 miles (340 km) to the North Sea. Its mouth is a tidal estuary, about 10 miles (16 km) wide. Among the tributaries of the Thames are the Cherwell, Kennet, Wey, and Lea rivers. London lies on both sides of the river, about 50 miles (80 km) from the mouth.

The tidal portion of the river—the section from London to the mouth—is controlled by the Port of London Authority. This board supervises docks and wharves and guards against pollution. A deep channel allows passage of most vessels as far upstream as London. The principal port for oceangoing ships, however, is Tilbury, near the mouth. In 1982 a flood barrier of 10 pivoting gates was completed near London to protect the city from surging tides, which occasionally occur during winter.

The upper Thames is controlled by the Thames Conservancy and is navigable for small boats to Lechlade, about 100 miles (160 km) upstream from London. Along this section of the river are Oxford University, Windsor Castle, and Eton.