In 1865, Col. Egbert L. Viele, a West Point graduate who served as the chief engineer for Central and Prospect Parks, charted every existing waterway in Manhattan, and laid the map across the street grid. It records a city that had not yet been topographically reshaped to make room for its explosive growth in the second half of the 19th century; today, very little of those waterways can be seen without intrepid exploration in sewer lines.

But the map is digitally rendered online in the collection of David Rumsey at davidrumsey.com. It is still used by developers in Manhattan.

In pursuit of one stream, Ms. Levy is running a sidewalk workshop at 4:30 that begins at 68th Street and Lexington Avenue, followed by a lecture under the sponsorship of NYC H2O, a nonprofit that provides education on city water and ecology.

“We are going to meet at Hunter College and walk the path of a tributary,” Ms. Levy said. “Actually, roughly the path. It would be winding through lobbies and entryways of buildings, but we will be on the sidewalk. We are going to be painting with blue chalk paint, iterating its passages.”

Image A detail of Egbert L. Viele ‘s map of New York City’s waterways shows De Voor’s Mill Stream, in dark green, running into the East River.

The stream being traced made its way along the east side of Manhattan, with some of it captured in a pond in the southeast corner of Central Park, and other branches eventually flowing into the East River at 47th Street. Historical texts show that a “kissing bridge” crossed it around 50th Street, and the name of the waterway appears to have been De Voor’s Mill Stream, said Steve Duncan, who has explored and documented the city’s watercourses, past and present.