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)