The River Thames is 215 miles (346km) long and flows from near the village of Kemble in Gloucestershire to Southend-on-Sea in Essex and drains a catchment area of 4,994 square miles (12,935 square km).  Until about 475,000 years ago the Thames used to flow from Wales to Clacton-on-Sea and crossed what is now the North Sea to become a tributary of the Rhine.  The river was moved to its present position by advancing and retreating ice.

The Thames is home to approximately 121 different species of fish including; Salmon, Trout, Carp, Dace, Anchovy's, Herring, Plaice and even Seahorses, 350 species of invertebrates and over 15,000 birds which winter on the Thames. Seals are now also becoming a common sight, they are frequently spotted swimming up the river to Teddington and Richmond!   Under normal conditions the river is freshwater up to London Bridge and fully salt water beyond Southend, the bit in between is brackish as the freshwater mixes with the water from the North Sea. However during dry summers the salt water moves further up the estuary than normal as the freshwater flow is reduced.  TheThames has 101 bridges crossing the river and 44 locks, all of which have a resident lock keeper.

The water level of the Thames has been slowly rising for the last 10,000 years, since the last ice age.

To help prevent flooding within the low-lying areas of London the Thames Barrier was opened in 1984 and is used several times a year.  To complement this in the late 1990's the 12km Jubilee River was built as a flood channel for the river around Maidenhead and Windsor.

During the summer, when the river flow is low, the level of oxygen in the water can fall near zero, which can result in the death of species found in the river.  To combat this the 'Thames bubbler' can inject up to 30 tonnes of oxygen per day directly into the river  .