Posted by: pblanc on Apr-11-12 7:23 AM (EST)
Those videos are probably as good a visual demonstration of Class I, II, and III water as you will find.
Erik's first video shows typical Class I water. There is swift current and some waves, but none of the waves are close to being large enough to swamp an open boat and the flow is completely unobstructed. The boaters can pretty much follow whatever path they wish and as long as they keep the boat aligned with the flow and don't accidentally cross an eddy line with the nose of their boat, they really don't have to do much of anything.
Erik's second video shows a Class III rapid (Zoar Gap). The boaters are doing a good job following a nice clean line through this rapid but you can see some sizable holes that could quickly separate a paddler from their boat if they can't spot the line and control their boat in heavy current.
Brian's video of the Slip Clinic shows some typical modestly technical Class II water. The current is not terribly pushy and there are no big drops but there are plenty of obstacles to be avoided and some degree of boat control is required to maneuver through them.