The Westbourne once flowed from Hampstead through a fishpond at Kilburn to Hyde Park, where it created the valley now occupied by the Serpentine. It eventually reached the Thames near Chelsea hospital, and the water was once even diverted to a reservoir at Chelsea Waterworks.
Nowadays, it doesn't even get to see the light of day, and has become a river of sewage which only gets close to the surface in a metal prison inside a tube station. It's all very sad really.
For more, see http://www.guardian.co.uk/environment/2005/mar/30/pollution.g2