The canal was 184.5 miles long, six feet deep, and sixty to eighty feet wide. It used seventy-four lift locks to climb 605 feet from sea level at Georgetown to the Cumberland terminus. It crossed eleven aqueducts and pulled water from seven dams. Its most ambitious engineering, the Paw Paw Tunnel near mile 156, runs 3,118 feet straight through a mountain that would otherwise have required six miles of meanders to skirt. The tunnel took fourteen years to build. The total cost of the canal came to about fourteen million dollars in 1850 money. Mules walked the towpath beside the canal, pulling barges loaded with coal, lumber, grain, and flour down to tidewater at Georgetown. The trip from Cumberland took about a week. The canal operated from 1850 to 1924, when a series of floods finally rendered it unrepairable. The federal government acquired the bed in 1938 and declared the National Historical Park in 1971.

The park is a thin band of green along the Maryland bank of the Potomac, never more than a few hundred yards wide, sometimes only the width of the canal bed itself. Twelve thousand acres in total. The towpath is wide, sandy, and almost completely level, which makes it one of the great long-distance trails in the East. Walk south from anywhere in Georgetown and you hit it. The C&O connects at Cumberland with the Great Allegheny Passage, which carries the trail another 150 miles into Pittsburgh, giving cyclists a continuous traffic-free route from the Potomac to the Monongahela. Most people, of course, do not bike the whole thing. They show up at the Great Falls Tavern Visitor Center for an afternoon, or walk a few miles from Swain's Lock on a Saturday morning.

The signature destination of the park is Great Falls, about fifteen miles upstream from Georgetown. Here the Potomac drops seventy-six feet through a complex of small cataracts and rapids in less than a mile, then funnels through Mather Gorge below. The Olmsted Island Bridges Trail crosses a sequence of boardwalks and footbridges to an overlook directly above the main falls, and the view there is one of the most dramatic natural spectacles within thirty minutes of downtown Washington. The Billy Goat Trail's Section A runs 1.7 miles across Bear Island along the river, requiring some scrambling over boulders and ledges, and rewards the effort with views back into the gorge. The river here is beautiful and deeply dangerous. People drown in the Potomac every year, usually after wading into what looks like calm water and being pulled under by currents the surface does not reveal. The Park Service warning signs are not decoration.

One of the canal park's less obvious distinctions is its biodiversity. The Park Service considers it among the most biologically diverse units in the entire National Park System. The Potomac cuts a continuous corridor across multiple physiographic provinces, from the tidewater Coastal Plain at Georgetown through the Piedmont, across the Blue Ridge, and into the Ridge and Valley of Appalachia at Cumberland. Northern and southern species overlap. Wetlands, forests, cliffs, river bars, and floodplain meadows pack into the narrow ribbon. Beavers, muskrats, river otters, and freshwater mussels live in the mud. White-tailed deer, gray and red fox, raccoons, and the occasional black bear or bobcat work the forest. Bald eagles nest on the cliffs above Mather Gorge and are now common along the river, a remarkable recovery from their near-extinction in the mid-twentieth century. Several federally listed rare plant species cling to the rocky bars.

Five drive-in campsites are scattered along the route at ten dollars a night, with basic tent pads, grills, chemical toilets, and potable water. Thirty free hiker-biker sites are spaced roughly every five miles between Swain's Lock at mile 16.6 and Evitts Creek at mile 180.1, each limited to one night per stay. At the Georgetown and Great Falls visitor centers, replica canal boats are pulled by live mules along restored sections of canal in season, with park rangers and reenactors explaining how lock tending and barge work actually happened. The Paw Paw Tunnel, near mile 156, is unlit; bring a flashlight if you want to walk through the 3,118-foot bore. Off the trail, two of the most important neighbors are Harpers Ferry, just across the Potomac in West Virginia, and Antietam National Battlefield, a few miles inland. The canal touches both.