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Dam 4

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1832–1835, Joseph Hollman; 1855–1860 rebuilt, Chesapeake and Ohio Canal Company. Milepost 84.40 of Chesapeake and Ohio Canal.
  • (Photograph by Lisa P. Davidson)
  • (Photograph by Lisa P. Davidson)
  • (Photograph by Lisa P. Davidson)

Dam 4 is one of the seven dams built along the Chesapeake and Ohio Canal to impound water and regulate fluctuating water levels. Described as the best dam on the river, the original rubble-filled crib version was built by Joseph Hollman between September 1832 and June 1835. After ongoing problems with flood damage, Dam 4 was rebuilt in masonry in the late 1850s. Dam 4 spans the Potomac River and impounds water in a 13-mile-long upriver pool known as “Big Slackwater.” It provided water for the canal between here and Dam 3 at Milepost 62.2 near Harper’s Ferry. The site of fighting during the Civil War, the structure sustained severe damage from floods in 1879 and 1937.

Writing Credits

Justine Christianson
Lisa P. Davidson
Catherine C. Lavoie



  • 1832

    Initial construction
  • 1855

    Replacement dam built

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Justine Christianson, "Dam 4", [Sharpsburg, Maryland], SAH Archipedia, eds. Gabrielle Esperdy and Karen Kingsley, Charlottesville: UVaP, 2012—,

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