Reference in C&O Canal Companion: Mile 1

"Aqueduct of Potomac, Georgetown"
The Aqueduct with the original "queen-post" truss design.
Note causeway and buildings on Mason's Island, beyond the aqueduct,
and the Washington Monument and the Long Bridge in the distance.
(cropped from digital image, Library of Congress, Prints and Photographs)

To be added

Mile 1:

The government commandeered the aqueduct during the Civil War, draining it and using the bottom of the trough as a road. To protect the aqueduct from Confederate attack, three forts were built on the hill at Rosslyn: Fort Corcoran, Fort Haggerty, and Fort Bennett. Along with several blockhouses, these defenses formed what Union generals termed a tete-a-pont (a fancy French phrase that translates literally to "bridge-head"). After the war, the bridge was turned back over to private ownership, and in 1868 the owners added a toll road on the top.

In 1886, the trough of the aqueduct begain leaking and the bridge was closed to traffic. The government eventually persuaded the owners of the Aqueduct Bridge to sell it for $125,000, and the Corps of Engineers replaced the superstructure with an iron-truss road bridge that opened in 1888. (Congress had also authorized an alternative solution -- the construction of a new bridge across Three Sisters Islands, just upstream.) The canal route to Alexandria was no longer necessary, as steam tugboats were being used to propel the canal boats up and down the river from the Tidewater Lock (mile 0).

Ironically, Key's house on M Street was later demolished for a ramp of the Whitehurst Freeway that connects to the bridge that bears his name. As a measure of recompense, a small park honoring Key has been opened on M Street, adjacent to the bridge and above the canal.

Add to photo caption: Hahn dates this view as 1868, just before the addition of the roadway.

Additional Notes

The Aqueduct Bridge:

The original aqueduct was begun in 1833, but not completed until ten years later. Major William Turnbull of the Army Topographical Engineers was in charge of the project. The piers were made of gneiss boated down from quarries upstream. The original design was a timber "queen-post" truss construction. The design changed significantly in 1868, when arching Howe trusses were installed.

Congress authorized the construction of a replacement bridge in 1916, and work began in 1917. When the Francis Scott Key Bridge was completed in 1923, the Aqueduct Bridge was abandoned, and its superstructure was removed in 1933. The old piers were not removed until 1962, though a portion of the pier nearest the Virginia shore was left as a memento.

The defensive works in Rosslyn:

None of the blockhouses or the three forts in Rosslyn remains, though there are historical markers for Fort Bennett (outside the Fort Bennett Apartments on 22nd Street) and Fort Haggerty (intersection of Arlington Ridge Road and Wilson Road).

Illustration: One of the blockhouse at Rosslyn, as drawn by Benson Lossing, from the Pictorial Field-book of the Civil War.



Detail of a Civil War era photograph of the Aqueduct, with original truss design,
taken by William Morris Smith from the Virginia side, between 1860 and 1865.
(Civil War Photographs, Prints and Photographs, Library of Congress)

Detail of the Aqueduct as it appeared with arch-reinforced trusses
circa 1869-1886. Water pouring out of the aqueduct may be one
of the leaks that led to its closing in 1886. (
Washingtoniana Collection,
Martin Luther King Public Library, District of Columbia.)

The former Aqueduct in its final incarnation,
converted to a highway bridge, circa early 1900's.
The C&O Canal is in the foreground;
Mason's Island appears at upper left.
Commission of Fine Arts.)

A restored version of the outlet lock in Alexandria.
(Photo by M. High, 2002)


  • The Alexandria Canal: Its History & Preservation, Thomas Swiftwater Hahn & Emory L. Kemp, Institute for the History of Technology and Industrial Archeology, West Virginia University Press, 1993
  • Building the Potomac Aqueduct, Donald Beekman Myer, AIA, Preservation Press, National Trust for Historic Preservation, Washington DC, copyright 1975 by Donald Beekman Myer
  • Civil Engineering Landmarks of the Nation's Capital, edited by Gary A. Borch and Steven M. Pennington for The Committee on History and Heritage of the National Capital Section of the American Society of Civil Engineers, Washington, DC, 1982.
  • Mr. Lincoln's Forts: A Guide to the Civil War Defenses of Washington, Benjamin Franklin Cooling III and Walton H. Owen II, White Mane Publishing Company, 1988. [Detailed and profusely illustrated.]
  • Pictorial Field-book of the Civil War, Volume 1, Benson J. Lossing, originally published in 1877, republished in 1997 by The Johns Hopkins University Press, with new introduction by Reid Mitchell. [See text and drawing of the "Aqueduct at Georgetown," page 481.]
  • Illustration of the Aqueduct at Georgetown (top of this page) is by Frederick Dielman (1847-1935), who is best known for his mosaic panels in the Library of Congress, Law and History. But he also served as a topographer with the United States Engineers from 1866 to 1872 and participated in a survey of the canals in the Allegheny mountains of Virginia (perhaps the James River & Kanawha). The color lithograph is attributed to E. Sachse & Co. in Baltimore, dated 1865, but it has the pastoral feel of the antebellum canal. Library of Congress number: LC-USZC4-1967

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