See this map of Manhattan here? I just learned from Scouting New York that even to this day city engineers and planners consult this map called the Viele map (circa 1874) to figure out where the original waterways, rivers, and canals are underneath NYC. If you’re intrigued, there’s a post over at Scouting New York about an instance where a flowing river has literally carved its own path in the floor of a building. Check it out.