Class I – Easy. Fast moving water with riffles and small waves. Few obstructions, all obvious and easily missed with little training.
Class II – Novice. Straightforward rapids with wide, clear channels which are evident without scouting. Occasional maneuvering may be required, but rocks and medium-sized waves are easily missed.
Class III – Intermediate. Rapids with moderate, irregular waves. Complex maneuvers in fast current and good boat control in tight passages or around ledges are often required; large waves or strainers may be present but are easily avoided. Strong eddies and powerful current effects can be found, particularly on large-volume rivers. scouting is advisable for inexperienced parties. Self-rescue is usually easy but group assistance may be required to avoid long swims.
Class IV – Advanced. Very difficult, long, extended rapids that require careful maneuvering of the raft. Powerful irregular waves and dangerous rocks are common. The course is hard to determine and scouting is necessary.
Class V – Expert. Long and violent rapids. Large waves that are unavoidable and irregular. Extremely difficult and complex course. Scouting is essential.
Class VI – Extreme and Exploratory Rapids. Maximum difficulty. Nearly impossible and extremely dangerous. Class V carried to the limit of navigability. Involves risk of life.