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Carrying water over 15 miles through tunnels and over bridges from Great Falls, Maryland to the nation’s capital, the gravity-based Washington Aqueduct was completed in 1864. The Army Corps of Engineers began construction, under the supervision of Montgomery Meigs, in 1853 and the aqueduct still provides drinking water to over one million residents of Washington D.C. and Arlington and Fairfax counties in Virginia. It was one of the country’s largest and earliest public water works, after those built in New York City and Boston, and began the Army Corps of Engineers involvement in public works projects throughout the country. The aqueduct’s Cabin John (or Union Arch) Bridge is listed as a Historic Civil Engineering Landmark and the aqueduct itself was designated a National Historic Landmark in 1973.

  • This map shows the path the aqueduct follows from Great Falls to D.C.
  • The Cabin John Bridge carries the aqueduct and MacArthur Blvd over Cabin John Creek and Parkway.
  • The Dalecarlia Reservoir and Water Treatment Plant.
  • The diversion dam that collects water from the Potomac and directs into the aqueduct.
  • The picturesque Castle Gatehouse at the Georgetown Resivoir.
  • The Pennsylvania Avenue Bridge which is supported by the arched aqueduct below it.

In 1800, D.C.’s population stood at just over 3,000 people, by 1850 it exceeded 58,000 and those people had to rely on unreliable wells, springs and cisterns which brought them water through wooden and cast-iron pipes. In 1851, a fire destroyed most of the holdings within the Library of Congress, then located within the Capitol Building, due partly to exceedingly low water pressure. The growing population’s desire for a reliable source of water and the safety issues presented by inadequate water pressure provoked Congress to action. In 1852, Congress commissioned the U.S. Army Corps of Engineers to plan for and create a public water system for Washington D.C.

The Corps assigned West Point graduate, Montgomery Meigs, to design and supervise the construction of the aqueduct. Meigs had previously helped build Forts Mifflin, Delaware, Wayne and Montgomery and served under Robert E. Lee while making navigational improvements on the Mississippi River. While building the Washington Aqueduct, Meigs also supervised the construction of the wings and dome of the U.S. Capitol and the expansion of the General Post Office Building. His supervision of all three projects would end as the Civil War approached and he was appointed Quartermaster General of the U.S. Army by President Lincoln. After the war, Meigs helped establish Arlington National Cemetery, where he was buried in 1892.

Since the Potomac and Anacostia Rivers are tidal rivers at D.C. with a saline content too high for drinking water, Meigs decided to pull water from Great Falls, Maryland, ten miles up the Potomac River. The Corps then built a diversion dam and gatehouse within the Potomac and began construction on a nine-foot diameter conduit to carry the diverted water southwest to Washington. An access road was also required to transport construction materials and to allow for maintenance, known as Conduit Road, but later renamed MacArthur Boulevard. 

This gravity-fed aqueduct traveled mostly below the surface but emerged to cross culverts and creeks in the form of bridges. Two of the better-known aqueduct bridges are the Cabin John (or Union Arch) and Pennsylvania Avenue (or Meigs) Bridges. Built of granite and sandstone, the Cabin John Bridge was the longest, single-arch masonry bridge in the world for 40 years. The Meigs Bridge is still one of the longest, unsupported metal pipe arches in the world and its cast-iron conduits, which support the road above, were exposed until 1916 when they were partially concealed by granite facing and soffit. With the completion of the Cabin John Bridge, the entire aqueduct came online in 1864. 

Along with the bridges and tunnels, the Georgetown and Dalecarlia Reservoirs were constructed to allow the silt to settle out of the water prior to arriving at its final destination. In 1901, the City Tunnel expansion opened after years of delay due to cost overruns, fraud and mismanagement. The McMillan Reservoir was added near Howard University in 1902 and a slow-sand filtration system, to filter out particulates, was added in 1905. This 1905 filtration system was replaced by a modern, fast sand system in 1985. Chlorine began to be added to the water in 1923 and a fast-sand filtration system with chemical treatment tower was added at Dalecarlia in 1927. Since then, both the Dalecarlia and McMillan Reservoirs have become water treatment plants.

The Washington Aqueduct has been in continuous use since it became operational in 1864. It is one of the few federally owned and operated public water supply agencies in the country and provides over 135 million gallons of water every day to its two water treatment plants. Its original gatehouse, while no longer operational, still stands near the Great Falls Tavern and its first 1.5 miles is now contained within the C&O Canal National Park.        

Rowlands, D.W. "The fascinating story of DC's aqueducts and reservoirs." Greater Greater Washington. August 20, 2018. Accessed March 23, 2020.

"Washington Aqueduct." National Park Service. December 13, 2017. Accessed March 23, 2020.

Carter, Elliot. "The Washington Aqueduct." War Department, United States Engineer Office. June, 1939. Washington Tunnels. Accessed March 23, 2020.