“The quality of the work is phenomenal,” said Ellen Macnow, a parks department official who has worked on the project, off and on, for 18 years. “We had inspections before the work began, and found mortar that had crumbled, but the stones hadn’t moved. It was truly overengineered. It is so strong.”

In the early 19th century, Manhattan was in a crisis that could have choked its growth. Its freshwater wells were overpumped, frequently contaminated by brackish water or sewage. A cholera outbreak in 1832 drove the affluent out of the city; left behind were immigrants and blacks, 3,500 of whom died, 2 percent of the city’s population.

John Jervis, an engineer, came to the rescue. He designed and built, with immigrant labor, a dam on the Croton River in Westchester County, and then sent the water south solely on gravity. It traveled 40 gently sloping miles through mains that dropped 13.25 inches every mile, pure, unfiltered water rolling stanza by stanza through a poem of civil engineering.

It crossed from the mainland over the Harlem River at the High Bridge and continued to two reservoirs, including one built on what is now the site of the main public library. Croton water reached a fountain in City Hall Park and rose 50 feet, unpumped: gravity’s geyser. The city grew tenfold in the 19th century and, following the Jervis plans, more water was brought from upstate into the city.

Image Twenty years ago, Maaret Klaber, then 10, asked the community board to reopen a path near the High Bridge as a bike way. Credit... Stephanie Diani for The New York Times

For mid-19th-century New Yorkers, the High Bridge was in the country and a weekend destination that was a ferry ride away. “They came by the thousands on Sundays,” said Sidney Hornstein, a geologist and historian of the city.