The McMillan Reservoir, near Howard University and Washington Hospital, was an addition to the District's water system that went into service in 1905. Smith Spring, one of the sources of drinking water before the aqueduct was constructed, formed a natural reservoir that engineers selected for augmentation. On the grounds of the reservoir stand large concrete storage tanks, where water was pumped after filtering in the extensive sand filter structure hidden below the landscaped grounds.
The reservoir is fed by the Washington City Tunnel, which extends from the Georgetown Reservoir under Rock Creek to its terminus in the McMillan Reservoir. Begun in 1884, the workers experienced difficulty while building the tunnel, running through unstable gneiss and requiring expensive stabilizing structure.