Biking the Monocacy Aqueduct The Monocacy Aqueduct is the largest aqueduct on the Chesapeake and Ohio Canal, crossing the Monocacy River just before it empties into the Potomac River. The 438 foot aqueduct was built by three separate contractors between 1829 and 1833. Thousands of people visit the canal each year for recreational opportunities such as hiking and biking. Subject Matter: Parks and outdoor recreation, C&O Canal, hiking, biking, architecture, Frederick County tourism, Maryland