The River Thames, known alternatively in parts as the River Isis, is a river that flows through southern England, including London. At 215 mi (346 km), it is the longest river entirely in England and the second-longest in the United Kingdom, after the River Severn.
The river rises at Thames Head in Gloucestershire, and flows into the North Sea near Tilbury, Essex and Gravesend, Kent, via the Thames Estuary. From the west it flows through Oxford (where it is sometimes called the Isis), Reading, Henley-on-Thames and Windsor. The Thames also drains the whole of Greater London.
The lower reaches of the river are called the Tideway, derived from its long tidal reach up to Teddington Lock. Its tidal section includes most of its London stretch and has a rise and fall of 23 ft (7 m). From Oxford to the Estuary the Thames drops by 55 metres. Running through some of the drier 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 Thames' 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; the river is fed by at least 50 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 North Kent Marshes and covering 5,289 ha (20.4 sq mi).
Various species of birds feed off the river or nest on it, some being found both at sea and inland. These include cormorant, black-headed gull and herring gull. The mute swan is a familiar sight on the river but the escaped black swan is more rare. The annual ceremony of Swan Upping is an old tradition of counting stocks.
Non-native geese that can be seen include Canada geese, Egyptian geese and bar-headed geese, and ducks include the familiar native mallard, plus introduced Mandarin duck and wood duck. Other water birds to be found on the Thames include the great crested grebe, coot, moorhen, heron and kingfisher. Many types of British birds also live alongside the river, although they are not specific to the river habitat.
The Thames contains both sea water and fresh water, thus providing support for seawater and freshwater fish. However, many populations of fish are at risk and are being killed in tens of thousands because of pollutants leaking into the river from human activities. Salmon, which inhabit both environments, have been reintroduced and a succession of fish ladders have been built into weirs to enable them to travel upstream.
On 5 August 1993, the largest non-tidal salmon in recorded history was caught close to Boulters Lock in Maidenhead. The specimen weighed 14+1⁄2 lb (6.6 kg) and measured 22 in (56 cm) in length. The eel is particularly associated with the Thames and there were formerly many eel traps. Freshwater fish of the Thames and its tributaries include brown trout, chub, dace, roach, barbel, perch, pike, bleak and flounder. Colonies of short-snouted seahorses as well as tope and starry smooth-hound sharks have also recently been discovered in the river. The Thames is also host to some invasive crustaceans, including the signal crayfish and the Chinese mitten crab.
Aquatic mammals are also known to inhabit the Thames. The population of grey and harbour seals numbers up to 700 in the Thames Estuary. These animals have been sighted as far upriver as Richmond. Bottlenose dolphins and harbour porpoises are also sighted in the Thames.
On 20 January 2006, a 16–18 ft (4.9–5.5 m) northern bottle-nosed whale was seen in the Thames as far upstream as Chelsea. This was extremely unusual: this whale is generally found in deep sea waters. Crowds gathered along the riverbanks to witness the spectacle but there was soon concern, as the animal came within yards of the banks, almost beaching, and crashed into an empty boat causing slight bleeding. About 12 hours later, the whale is believed to have been seen again near Greenwich, possibly heading back to sea. A rescue attempt lasted several hours, but the whale died on a barge. See River Thames whale.