Its tidal section, reaching up to Teddington Lock, includes most of its London stretch and has a rise and fall of 7 metres (23 ft). Running through some of the driest parts of mainland Britain and heavily abstracted for drinking water, the Thames' discharge is low considering its length and breadth: the Severn has a discharge almost twice as large on average despite having a smaller drainage basin. In Scotland, the Tay achieves more than double the average discharge from a drainage basin that is 60% smaller.
Along its course are 45 navigation locks with accompanying weirs. Its catchment area covers a large part of South Eastern and a small part of Western England and the river is fed by 38 named tributaries. The river contains over 80 islands. With its waters varying from freshwater to almost seawater, the Thames supports a variety of wildlife and has a number of adjoining Sites of Special Scientific Interest, with the largest being in the remaining parts of the North Kent Marshes and covering 5,449 hectares (13,460 acres).
The Thames, from Middle English Temese, is derived from the Brittonic Celtic name for the river, Tamesas (from *tamēssa), recorded in Latin as Tamesis and yielding modern Welsh Tafwys "Thames". The name may have meant "dark" and can be compared to other cognates such as Russian темно (Proto-Slavic *tĭmĭnŭ), Latvian tumsa "darkness", Sanskrit tamas and Welsh tywyll "darkness" (Proto-Celtic *temeslos) and Middle Irish teimen "dark grey". The same origin is shared by countless other river names, spread across Britain, such as the River Tamar at the border of Devon and Cornwall, several rivers named Tame in the Midlands and North Yorkshire, the Tavy on Dartmoor, the Team of the North East, the Teifi and Teme of Wales, the Teviot in the Scottish Borders, as well as one of the Thames' tributaries called the Thame.
Kenneth H. Jackson has proposed that the name of the Thames is not Indo-European (and of unknown meaning), while Peter Kitson suggested that it is Indo-European but originated before the Celts and has a name indicating "muddiness" from a root *tā-, 'melt'.