The Thames has a length of 215 miles (346 km). Its usually quoted source is at Thames Head (at grid reference ST980994), about a mile north of the village of Kemble and near the town of Cirencester, in the Cotswolds. This makes it the longest river entirely in England, although the River Severn, which is partly in Wales, is a longer river in the United Kingdom. Seven Springs near Cheltenham, where the river Churn rises, is also sometimes quoted as the Thames' source,[1] as this location is furthest from the mouth adding some 14 miles (22 km) to the length. The springs at Seven Springs also flow throughout the year, while those at Thames Head are only seasonal.

The Thames flows through or alongside Ashton Keynes, Cricklade, Lechlade, Oxford, Abingdon, Wallingford, Goring-on-Thames, Reading, Henley-on-Thames, Marlow, Maidenhead, Windsor, Eton, Staines,Sunbury, Weybridge and Thames Ditton before entering the Greater London area. The present course is the result of several minor redirections of the main channel around Oxford, Abingdon and Maidenhead and more recently the creation of specific cuts to ease navigation.
From the outskirts of Greater London, the river passes Hampton Court, Kingston, Teddington, Twickenham, Richmond (with a famous view of the Thames from Richmond Hill), Syon House and Kew before flowing through central London. In central London, the river forms one of the principal axes of the city, from the Palace of Westminster to the Tower of London and was the southern boundary of the mediaeval city, with Southwark on the opposite bank.
Once past central London, the river passes between Greenwich and the Isle of Dogs, before flowing through the Thames Barrier, which protects central London from flooding in the event of storm surges. Below the barrier, the river passes Dartford, Tilbury and Gravesend before entering the Thames Estuary near Southend-on-Sea.

Vocabulary:
length - длина, расстояние
source - верховье, исток
entirely - вполне, всецело
boundary - помещение, местожительство
flood - наводнение, потоп
barrier - помеха, преграда