Thames River from top, London. The Thames is only 346 km (215 miles) long. It is England’s longest river and the second longest river in the United Kingdom. Lon…g ago, before Britain was separated from continental Europe, the Thames was a tributary of the Rhine. The English Channel was formed about 7,000 years ago at the end of the last Ice Age. The Route of the River Thames The River Thames rises in the Cotswold Hills in Gloucestershire, and flows roughly eastwards passed Oxford (where it is known as the River Isis), until the Chilterns and on through London to the North Sea.