structural:scholar_edition
Course

The usually quoted source of the Thames is at Thames Head (at grid reference ST980994). This is about 2.4 km (1.5 mi) north of the village of Kemble in southern Gloucestershire, near the town of Cirencester, in the Cotswolds. However, Seven Springs near Cheltenham, where the Churn (which feeds into the Thames near Cricklade) rises, is also sometimes quoted as the Thames' source, as this location is farthest from the mouth and adds some 23 km (14 mi) to the river's length. At Seven Springs above the source is a stone with the Latin hexameter inscription "Hic tuus o Tamesine pater septemgeminus fons", which means "Here, O Father Thames, your sevenfold source".

The springs at Seven Springs flow throughout the year, while those at Thames Head are seasonal (a winterbourne). With a length of 346 km (215 mi), the Thames is the longest river entirely in England. (The longest river in the United Kingdom, the Severn, flows partly in Wales.) However, as the River Churn, sourced at Seven Springs, is 23 km (14 mi) longer than the section of the Thames from its traditional source at Thames Head to the confluence, the overall length of the Thames measured from Seven Springs, at 369 km (229 mi), is greater than the Severn's length of 350 km (220 mi). Thus, the "Churn/Thames" river may be regarded as the longest natural river in the United Kingdom. The stream from Seven Springs is joined at Coberley by a longer tributary which could further increase the length of the Thames, with its source in the grounds of the National Star College at Ullenwood.

The Thames flows through or alongside Ashton Keynes, Cricklade, Lechlade, Oxford, Abingdon-on-Thames, Wallingford, Goring-on-Thames and Streatley (at the Goring Gap), Pangbourne and Whitchurch-on-Thames, Reading, Wargrave, Henley-on-Thames, Marlow, Maidenhead, Windsor and Eton, Staines-upon-Thames and Egham, Chertsey, Shepperton, Weybridge, Sunbury-on-Thames, Walton-on-Thames, Molesey and Thames Ditton. The river was subject to minor redefining and widening of the main channel around Oxford, Abingdon and Marlow before 1850, when further cuts to ease navigation reduced distances further.

Molesey faces Hampton, and in Greater London the Thames passes Hampton Court Palace, Surbiton, Kingston upon Thames, Teddington, Twickenham, Richmond (with a famous view of the Thames from Richmond Hill), Syon House, Kew, Brentford, Chiswick, Barnes, Hammersmith, Fulham, Putney, Wandsworth, Battersea and Chelsea. In central London, the river passes Pimlico and Vauxhall, and then forms one of the principal axes of the city, from the Palace of Westminster to the Tower of London. At this point, it historically formed the southern boundary of the medieval city, with Southwark, on the opposite bank, then being part of Surrey.

Beyond central London, the river passes Bermondsey, Wapping, Shadwell, Limehouse, Rotherhithe, Millwall, Deptford, Greenwich, Cubitt Town, Blackwall, New Charlton and Silvertown, before flowing through the Thames Barrier, which protects central London from flooding by storm surges. Below the barrier, the river passes Woolwich, Thamesmead, Dagenham, Erith, Purfleet, Dartford, West Thurrock, Northfleet, Tilbury and Gravesend before entering the Thames Estuary near Southend-on-Sea.