THAMES, the most important river of Great Britain; usually said to rise about 3 miles S. W. of Cirencester in Glouces tershire, near a bridge over the Thames and Severn canal, called Thameshead Bridge, but is more properly formed by the Isis, Churn, Colne, and Leach, which have their sources on the E. side of the Cotswold Hills, and unite near Lechlade, where the counties of Gloucester, Wilts, Berks, and Oxford border on each other. Proceeding from Lechlade, where it be comes navigable for barges, it flows first E. N. E., then S. S. E., past Oxford and Abingdon to Reading, then N. W. past Great Marlow, and S. E. past Windsor to Staines. From Staines it pursues a circuitous course E., passing the towns of Chertsey, Kingston, Richmond and Brentford, separating the counties of Middlesex and Surrey, and passing through London. Below London its course E. to the Nore, between Kent and Essex, is 47 miles. Its total course is estimated
at 250 miles. Its tributaries include the Windrush, Cherwell, Thame, Colne, Brent, Lea, and Roding, on the left; the Kennet, Loddon, Wey, and Mole, on the right. Thameshead Bridge is 376 feet above sea-level; the junction of the Colne above Lechlade is 243 feet. At London Bridge the width of the river is 266 yards, at Woolwich 490 yards, at Graves end 800 yards, and 3 miles below, 1,290 yards. The basin of the Thames has an area of 5,400 square miles, and belongs entirely to the upper part of the Second ary and to the Tertiary formations. The depth of the river in the fair way above Greenwich to London Bridge is 12 to 13 feet, while its tides have a mean range of 17 feet and an extreme rise of 22 feet. By means of numerous canals immediate access is given from its basin to those of all the great rivers of England.