Rivers are classified on the International Scale of River Difficulty. We've modified the official descriptions, which were originally intended for canoes and kayaks, to be more applicable to commercial whitewater rafting.
Rapids are classified as to their difficulty, not how much fun they are. For example, one of the biggest, best and most fun rapids on the New River is Surprise...and it's a Class III. It's a lot more entertaining than, say, Lower Railroad, which is a Class IV. The difference is that Surprise is not a particularly dangerous rapid. It's huge, it flips boats, it creates lots of swimmers, but it doesn't have any characteristics that make it particularly hazardous. Lower Railroad, on the other hand, is one of the most dangerous spots on the New at lower water levels. This is because of "Rocketship Rock," which can easily pin a boat and subject any swimmers to significant danger. But, unless you hit Rocketship, the rapid is pretty benign and nowhere near as much fun as Surprise.
So don't equate a higher classification with a bigger ride. The classification has to do primarily with how potentially hazardous the rapid can be.
Class I
Moving water with few riffles, small waves, few or no obstructions. Class I rapids can be thought of as "slowly moving ponds."
Class II
Easy rapids with small waves and wide, clear channels that are obvious. Some maneuvering required.
Class III
Waves may be high and irregular. Passages may be narrow and may require complex maneuvering. Obstructions are not life-threatening under normal circumstances. The route is clearly visible from the top of the rapid. Many of the New and Gauley's most entertaining rapids are Class III's.
Class IV
Very difficult water. Long, constricted, difficult passages that require complex maneuvering. Conditions make rescue difficult. Waves may be large, irregular, and breaking...capable of capsizing a raft. Life-threatening obstructions, such as undercut rocks, may be present. Routes are often not readily apparently from the top of the rapid. Any Class IV rapid is serious business and requires the attention of everyone in theboat.
Class V
Extremely difficult and violent rapids with significant hazard to life in the event of a mishap. Rescue is very difficult and there is significant hazard to life. Waves may be huge, numerous, and breaking. Life-threatening obstructions, such as undercut rocks and "keeper" hydraulics may be numerous. Many Class V rapids are relatively long stretches of whitewater that require constant maneuvering. This puts a premium on teamwork between the crew and the Guide.
Class V+
A commercial designation, this represents the absolute limit of commercially run whitewater. Huge waves and hydraulics and many obstructions which can be life-threatening. Rescue conditions are very difficult. For teams of experts only. The Upper Gauley has five Class V+ rapids...Insignificant, Pillow Rock, Lost Paddle, Iron Ring, and Sweets Falls.
Class VI
Nearly impossible and very dangerous. Can be run only by teams of experts with significant danger to life. There are no commercially-run Class VI rivers. Many of the steep creeks in West Virginia, such as Mann's Creek (site of our New River lunch spot) and the Lower Meadow river are genuine Class VI runs.
It's important to know that the class of any rapid will change as water conditions vary. In general, rapids get more difficult as the water level rises. A good example of this would be the Keeney Creek rapids on the New. At most levels, all three Keeneys would rate no higher than Class III-IV. But at higher water levels (usually in the Springtime), the three rapids combine to create one huge, massive monster that becomes a genuine Class V+.
Interestingly, a significant number of rapids on both the New and Gauley rivers actually get tougher as the water level drops. As mentioned, Lower Railroad is a good example of this phenomenon.
Rapid classifications must be thought of as guidelines, not descriptions. In the fluid world of hydrodynamics, things change quickly. Well-informed paddlers always consider the "big picture," not just the class of the rapids.
Notes
1. A river is given the rating of its most difficult rapid. Thus, a river with 20 Class III's and only one Class V would be rated a Class V river.
2. During cold weather (under 50 degrees), the rating is raised by one full class.
3. Western rivers are rated on a scale of 1 to 10...known as the Communist Rating System. Divide a western river's class in half to find its real class as compared to eastern rivers.
