From the Guardian:
The Environment Agency estimates that 70% of London's 600km river network is concreted, covered over, interrupted by weirs or otherwise modified. Some can only be glimpsed in odd places, such as a tunnel through Sloane Square tube station carrying the Westbourne, or guessed at from local road names such as Fleet Street or Spring Path.
Lots more here. I was particularly thrilled by the accompanying map (I used to live in a street called Roding Road.)
