The River Fleet is now largely hidden underground after it became an open sewer for many centuries. Under Blackfriars Bridge, you can see the confluence of the Fleet and the Thames.

References

• https://www.youtube.com/watch?v=8TiE4ZHMddA (Chris Spargo: Why did London turn a river into a sewer?)
• https://londonist.com/2016/09/where-to-see-and-hear-the-hidden-river-fleet
• https://www.londonslostrivers.com/river-fleet.html
• https://livinglondonhistory.com/the-lost-river-fleet-a-self-guided-walk-and-map/