The London Marathon, Dedicated To The Olympic Games In 2012

Greater London covers an area of 1,579 square kilometres (610 sq mi), an area which 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 8,382 square kilometres (3,236 sq mi) has a population of 12,653,500 and a population density of 1,510 people per square kilometre. Modern London stands athwart the Thames, its primary geographical feature, a navigable river which crosses the city from the south-west to the east. The Thames Valley is a floodplain surrounded by gently rolling hills including Parliament Hill, Addington Hills, and Primrose Hill. The Thames was once a much broader, shallower river with extensive marshlands; at high tide, its shores reached five times their present width.
Since the Victorian era the Thames has been extensively embanked, and many of its London tributaries now flow underground. The Thames is a tidal river, and London is vulnerable to flooding. The threat has increased over time due to a slow but continuous rise in high water level by the slow 'tilting' of Britain (up in the north and down in the south) caused by post-glacial rebound.
In 1974, a decade of work began on the construction of the Thames Barrier across the Thames at Woolwich to deal with this threat. While the barrier is expected to function as designed until roughly 2070, concepts for its future enlargement or redesign are already being discussed.