We know D.C. Get our free newsletter to stay in the know.

The last time most Washingtonians thought about the fragility of their water supply, they were scrambling for the last Evian at the Safeway. Not so some 50-odd years ago, when no such reserves existed. The entry of the U.S. into World War II made the aqueduct at Great Falls (at the time D.C.’s only freshwater source) a critical resource—and a likely target for would-be saboteurs. Join a park ranger for “Where Were You in ’42?,” a four-mile hike and discussion of the time the U.S. Army was called in to protect the canal. At 1 p.m. at the Great Falls Tavern Visitor Center, 11710 MacArthur Blvd., Potomac. FREE. (301) 299-3613. (Andrew Webster)