|The McMillan Park Reservoir Historic District consists of the McMillan Reservoir (built 1883-1888), which remains an integral part of the city's water supply system; the Sand Filtration Plant (1905), designed and built as the city's first water treatment facility; and McMillan Park, built 1908-1913. The park, designed by Frederick Law Olmsted, Jr. as a public park and memorial to the late Senator James McMillan whose McMillan Park Plan of 1901-1902 was instrumental in the establishment of the park at McMillan Reservoir. The reservoir was built as an extension to the Washington Aqueduct first designed and built by Civil War Quartermaster General Montgomery Meigs in 1852 to supply water from the Potomac River via a gravity-fed aqueduct from Great Falls to Georgetown. Beginning in 1882, the U.S. Army Corps of Engineers, under whose jurisdiction the Washington Aqueduct fell, began construction of a tunnel from the Georgetown Reservoir through Rock Creek to a new reservoir (McMillan Reservoir) in order to extend the water supply to the growing population centers in the eastern part of the city. From this new reservoir, initially called Washington City Reservoir, but named McMillan Reservoir in 1906, water was fed by gravity to the city's mains. Shortly after construction of the reservoir, Congress approved the establishment of a water filtration system to filter and purify the city's water prior to distribution. The McMillan Reservoir site was selected for the new plant, and between 1902 and 1905, the Slow Sand Filtration Plant with its vast array of filter beds and sand bins, was constructed at the reservoir, and on a 22-acre site immediately adjacent to and east of the reservoir. Upon its completion in 1905, water was pumped from the reservoir to the twenty-nine slow filtration beds-vaulted and sand-filled structures built of unreinforced concrete-where the water was cleansed and piped to an underground clear reservoir before being distributed. Ln. 1986, a chemical treatment facility on the reservoir side of the site replaced the Slow Sand Filtration Plant, and the former filtration complex ceased operation. At the time it was closed, the sand filtration plant was one of the last working examples of the slow sand filtration method in the United States.