WASHINGTON—A small stream gurgles under a historic stone bridge, once used by Revolutionary War patriots to transport supplies. For more than a century, the bridge was bricked up, the stream beneath it just a dry, eroded channel.

As in many places, this tributary of the Broad Branch stream had been forced underground around the turn of the century, through a program designed to rid Washington, D.C., of surface water. At the time, malaria was a major killer, and cities around the world were draining any kind of standing water or "swamp," out of both a fear of mosquito-borne disease and a desire to create more land for development.

Burying streams created other problems, though. When the little stream and its tributary