The Monocacy Aqueduct — or C&O Canal Aqueduct No. 2 — is the largest aqueduct on the Chesapeake and Ohio Canal, crossing the Monocacy River just before it empties into the Potomac River in Frederick County, Maryland, USA. The 438 foot aqueduct was built by three separate contractors between 1829 and 1833 at a cost of US$127,900.