London is the largest urban area and capital city of the United Kingdom. It is located in the southeast of Great Britain. The London region covers an area of , and had a population of 7,172,036 in 2001 and a population density of 4,542 people per square kilometre. A larger area, referred to as the London Metropolitan Region or the London Metropolitan Agglomeration covers an area of , and had a population of 12,653,500 and a population density of 1,510 people per square kilometre. London is a port on the Thames (see main article Port of London), a navigable river. The river has had a major influence on the development of the city. London began on the Thames' north bank and for a long time the main focus of the city remained on the north side of the Thames. For many centuries London Bridge was the only bridge in or close to the city. When more bridges were built in the 18th century, the city expanded in all directions as the mostly flat or gently rolling countryside presented no obstacle to growth. 360 degree panorama Rivers and canals River Thames The River Thames is the main river of London, flowing west to east across the London Basin. The river cuts into the London basin through the Goring Gap, draining parts of the Cotswolds and Vale of Aylesbury to the west. Similarly tributaries such as the Mole cut through the North Downs into the basin from the south. Further downstream the flow of the Thames is boosted by springs which open onto the chalky riverbed. The Thames was once a much broader, shallower river than it is today. It has been extensively embanked. The Thames is tidal (the Tideway) up to Teddington Lock, and London is vulnerable to flooding by storm surges. The threat has increased over time due to a slow but continuous rise in high water level, caused by both the slow 'tilting' of Britain (up in the north and down in the south) caused by post-glacial rebound and the gradual rise in sea levels due to climate change. The Thames Barrier was constructed across the Thames at Woolwich in the 1970s to deal with this threat, but in early 2005 it was suggested that a barrier further downstream might be required to deal with the flood risk in the future. Within London a considerable number of rivers and