A popular canal location is at Great Falls. There are actually two parks here: Great Falls NP (http://www.nps.gov/grfa/index.htm) is on the Virginia side of the Potomac River, and Great Falls Tavern and Locks 17-20 on the Maryland side.
Lots of hiking trails are available at these parks, and I explored a less crowded one called Ford Mine Trail. As you see in the next photo, the canal is not very deep, and has a generally U-shaped channel with shallow sides.
This portion of the C&O Canal NHP is located near Potomac MD, and the old Great Falls Tavern located adjacent Locks 20 and 19 now serves as a visitor center. You can even borrow a bike for free if you want to ride along the canal paths.
A towpath is located on one side of the canal. The towpath is where the mules travel as they pulled canal boats up and down the waterway to their next destination. Mules were preferred because there were considered smarter and ate less than a horse. That said, mules still required one or more people to guide them along the towpath — they are still mules, after all ! The following photo shows one of the canal boats visitors can ride.