London is situated in south-east England, 45 kilometres west of Thames and covers an area of 1 579 square kilometres . This places the city as one of the largest cities in the world. The altitude varies from sea level to 245 m at Biggin Hill.

The river, which crosses the city from west to east, had a major influence on the development of the city. London was originally founded on the north shore of the river and has had for several centuries, only one bridge, London Bridge. The main focus of the city was therefore confined to this side of the Thames, until the construction of a series of other bridges in the eighteenth century. The city was then extended in all directions, this expansion was hampered by many natural obstacle.

Thames was once broader and less deep than it is today. The banks of the river have been massively upgraded, most of the tributaries have been diverted and are now underground, sometimes turned into sewers . Thames is subject to the tide and London is widely flood. The threat of flooding also increased over time given the steady rise of water level at high tide and the slow tilt of Britain (raised in the north, lower in the south) caused by a isostatic rebound phenomenon. A dam, the Thames Barrier was built across the Thames at Woolwich in the 1970s, to overcome this threat. In 2005, however, it was proposed the construction of a dam of about fifteen miles long lower downstream to help avoid future risks of flooding.