section_path: Geography/Topography
section_name: Topography
paragraph_text: Topography
  
  Greater London encompasses a total area of 611 square miles (1,583 km) an area which had a population of 7,172,036 in 2001 and a population density of 11,760 inhabitants per square mile (4,542/km). The extended area known as the London Metropolitan Region or the London Metropolitan Agglomeration, comprises a total area of 3,236 square miles (8,382 km) has a population of 13,709,000 and a population density of 3,900 inhabitants per square mile (1,510/km).
  
  Modern London stands on 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 flood plain surrounded by gently rolling hills including Parliament Hill, Addington Hills, and Primrose Hill. Historically London grew up at the lowest bridging point on the Thames. 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 because of a slow but continuous rise in high water level caused by climate change and by the slow 'tilting' of the British Isles as a result of post-glacial rebound.