Thames, Or Isis (anc. Tamesis or Tamesa), the largest and most important river of England. Its source, called Thames Head, is in the Cotswold hills, about 3 m. S. W. of Cirencester, 376 ft. above the sea level. In the first 30 m. of its course it receives the Churnet, the Coln, and the Lech, and below Lechlade becomes navigable for barges; from Lechlade its course is first E. and then N. N. E. and S. S. E. to Oxford, through a level country, the river receiving on its way the Windrush and the Cherwell. Flowing generally S. S. E. from Oxford to Reading, it receives the Thame and the Kennet; thence making a considerable circuit to the north by Henley, Great Marlow, and Maidenhead, it turns eastward to Windsor, then makes a detour southeastward by Staines and Chertsey to Kingston, where it turns N., and, passing Richmond, reaches Brentford, whence its course is nearly due E. to its mouth. From Brentford it passes by Putney, Hammersmith, and Chelsea to London, receiving in its course the Loddon, Colne, Mole, Cran, Brent, and Wandle, all small streams. From London to its mouth, nearly 60 m., the Thames is navigable for vessels of TOO or 800 tons, and for vessels of any burden to Dept-ford, 3 m. S. E. of London bridge.
It is about 300 yards wide at London bridge; at Woolwich, 9 m. below, 500 yards; at Coalhouse point, 20 m. further down, 1,300 yards; at the Nore, 6 m.; and at its mouth, 18 m. Below London it receives the Ravensbourne, Roding, Darent, and Medway. Its tide is perceptible as far as Teddington, 72 m. above its mouth. The Thames and Severn canal connects it with the Severn; the Oxford canal with the grand canal system of the central counties; the Wilts and Berks and the Kennet and Avon canals with the Avon and the Severn; the Wey and Arun and the Basingstoke canals with the Sussex coast; the Grand Junction, the Regent's, and the Paddington canals connect the Brent with the Oxford canal, and encircle the N. and E. sides of the metropolis. The whole course of the Thames is about 220 m. Its commerce is surpassed probably by that of no river in the world. Its docks are described in the article Dock. It is crossed at and above London by numerous bridges, and several tunnels pass under it. For a description of the bridges, the tunnels, and the new Thames embankments, see London, vol. x., pp. 592 and 617.