I have a love affair with London and its history. I like to imagine what the Fleet River used to look like (but not what it smelled like). It's one of the rivers that used to run into the Thames but the city subsumed it as it grew over the years. It exists now as the Fleet Sewer. I found this map that shows the rivers that used to feed into the Thames. A lot of these are now gone. There's a great history and modern visit of the Fleet here.
The Fleet, the Tyburn, the Walbrook, the Westbourne, gone. The Westbourne is pretty cool because it still sort of exists.
See that large grey conduit running above the Tube station? That's what remains of the Westbourne River. It still opens up into the Thames and runs underground for most of its length.
Manhattan has its share of buried streams (Canal Street actually is a filled in Canal) but London's hydrology is way more interesting. There is some nattering about restoring some sections of the Fleet but I'd be amazaed if it amounted to more than just talk.