Hi, Tuesday,
>oh man, don't get me started on that stuff - I've been obsessed with London's lost waterworks for the last few years and it's not really a megalithic subject (it's pre-megalithic in fact) - but I think what you refer to is possibly a spring or tributary. The Fleet is a little further east and is still there deep underground - I visited it a couple of years ago. You can't get rid of a river.<
I don't know if you have read it, but as a keen Londonphile myself, I was very interested to read "Underground London" by Stephen Smith (Hardback ISBN 0316861340). He reflects on the liminal, and has a chapter devoted to London's "lost" waters......
Peace
Pilgrim