This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Most of what a visitor pictures when they hear the name Thames, the tidal river through central London, the working docks, the bridges, is only the last third of the river's course. Above Teddington Lock the Thames is a different river entirely: non-tidal, narrower, lock-managed rather than tide-managed, and it starts a long way from London, at Thames Head in the Cotswold hills of Gloucestershire, 110 metres above sea level and about 346 kilometres from the sea by the river's own winding path.
The upper river carries an older name for part of its length. Through Oxford and the countryside around it, the Thames has traditionally been called the Isis, a name still used locally and still printed on some maps for that stretch, distinct from the river's general English name, which traces to a Proto-Celtic root, tamēssa, thought to mean dark. Downstream of Oxford, at Dorchester-on-Thames, the River Thame joins from the north, and the main river carries the name Thames the rest of the way to the sea.
The non-tidal upper Thames is a working river in its own right, just not the same kind of work as the tidal river below it. A series of locks, rather than the tide, manages boat traffic and water levels along this stretch, supporting leisure boating and, further downstream toward Reading and the Thames Valley, a chain of towns that grew up along the river without ever being seaports. It is easy, from London, to think of the Thames as a harbor with a very long approach channel. Upstream of Teddington, the river was never built for that role, and the Isis, the locks and the source at Thames Head are the part of the story that never had London in mind.