In Yonkers, the fourth largest city in New York State, officials are a third done with a "daylighting" project—a term for the opening up of underground streams (see "11 Rivers Forced Underground"). In addition to exposing a waterway that had long been covered, the effort has already sparked plans for a new minor-league ballpark and new housing.

"I credit the city and the people who ... figured that having a nice river in a downtown was something that was, economically, really good," said Ann-Marie Mitroff, director of river programs for Groundwork Hudson Valley, an environmental justice nonprofit.

But why are all these streams covered at all? Flash back more than a hundred years. In many urban areas around the world,