184 miles: Chesapeake & Ohio Canal (Washington, DC to Cumberland, MD)

The 184.5 mile long Chesapeake & Ohio Canal is located along the north bank of the Potomac River, starting in Washington, DC and ending in Cumberland, MD. The canal was built between 1828 and 1850, and it operated sporadically between floods until 1924. In 1954, US Supreme Court Justice William O. Douglas organized an eight day hike up the canal's towpath in an effort to save it from being converted to a parkway. His efforts succeeded, and in 1971 the canal became a National Historic Park.

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