The river's name appears always to have been pronounced with a simple "t" at the beginning; the Middle English spelling was typically Temese and Latin Tamesis. The "th" lends an air of Greek to the name and was added during the Renaissance, possibly to reflect or support a belief that the name was derived from River Thyamis in the Epirus region of Greece, whence early Celtic tribes are thought to have migrated. However, most scholars now believe Temese and Tamesis come from Celtic (Brythonic) Tamesa, possibly meaning 'the dark one'.
Indirect evidence for the antiquity of the name 'Thames' is provided by a Roman pot-sherd found at Oxford, bearing the inscription Tamesubugus fecit (Tamesubugus made this). It is believed that Tamesubugus's name was derived from that of the river.
The name Isis, given to the part of the river running through Oxford, may have come from the Egyptian goddess of that name but is believed to be a contraction of Tamesis, the Latin (or pre-Roman Celtic) name. Richard Coates has recently suggested that the river was called the Thames upriver, where it was narrower and Plowonida down river, where it was too wide to ford. This gave the name to a settlement on its banks, which became known as Londinium, from the original root Plowonida (derived from pre-celtic Old European 'plew' and 'nejd,' meaning something like the flowing river or the wide flowing unfordable river). For merchant seamen, it has long been just 'The London River'.
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. However, Seven Springs near Cheltenham, where the river Churn rises, is also sometimes quoted as the Thames' source, as this location is furthest from the mouth both in distance along its course and as the crow flies. The springs at Seven Springs also flow throughout the year, while those at Thames Head are only seasonal.
The Thames flows through Ashton Keynes, Lechlade, Oxford, Abingdon, Wallingford, Reading, Henley-on-Thames, Marlow, Maidenhead, Windsor, Eton, Staines and Weybridge, before entering the Greater London area.
The river itself rises in Gloucestershire, traditionally forming the county boundary, firstly between Gloucestershire and Wiltshire, between Berkshire on the south bank and Oxfordshire on the north, between Berkshire and Buckinghamshire, between Berkshire and Surrey, between Surrey and Middlesex and between Essex and Kent. Before the 1974 boundary changes, the current boundary between Berkshire and Surrey was between Buckinghamshire and Surrey. The Oxfordshire - Berkshire boundary was also moved at that time.
The area to the west of London is normally called the Thames Valley, whilst east is called Thames Gateway.
The whole of the river drains a catchment area of some 12,935 square km (4,994 square miles) or 15,343 square km (5,924 square miles) if the River Medway is included as a tributary.
Innumerable brooks, canals and rivers, within an area of 9,948 square km (3,841 square miles), combine to form 38 main tributaries feeding the Thames between its source and Teddington. These include the rivers Churn, Leach, Cole, Coln, Windrush, Evenlode, Cherwell, Ock, Thame, Pang, Kennet, Loddon, Colne, Wey and Mole.
Between Maidenhead and Windsor, the Thames supports an artificial secondary channel, known as the Jubilee River, for flood relief purposes.
More than half the rain that falls on this catchment is lost to evaporation and plant growth. The remainder provides the water resource that has to be shared between river flows, to support the natural environment and the community needs for water supplies to homes, industry and agriculture. During heavy rainfall events the Thames occasionally receives raw sewage discharge due to sanitary sewer overflow.