On small gravel, my FS MTB is faster than me, as they say. I rode down the local stretch of the C&O last summer as fast as I could stand to pedal, and the bike just shrugged it off. Another rider on a CX bike was doing well, too, but by the time we got to Georgetown, he was pretty beat. I don't have a speedometer on my bike, but he said his was reading over 20 mph most of the time. No doubt the slope helped our speed, too (we were going in the downstream direction of the canal).