Dalecarlia Reservoir

Dalecarlia Reservoir is the primary storage basin for drinking water in Washington, D.C., fed by an underground aqueduct in turn fed by low dams which divert portions of the Potomac River near Great Falls and Little Falls. The reservoir is located between Spring Valley and the Palisades, two neighborhoods in Northwest Washington, and Brookmont, a neighborhood in Montgomery County, Maryland.

The 50-acre reservoir was completed in 1858 by the United States Army Corps of Engineers as part of the Washington Aqueduct project. It began providing water on January 3, 1859. Initially the reservoir provided water to the city from the adjacent Little Falls Branch until the aqueduct construction was completed.

Source: Wikipedia