Whitewater rapids are rated, based on relative difficulty from Class I (the smallest of rapids and waves) to Class VI (extremely difficult rapids). The classification or rating of a river varies considerably with fluctuating water levels. Although higher water levels normally increase the difficulties of rapids, higher water occasionally covers rocks sufficiently to wash out rapids. Conversely, some rapids become more difficult at lower water levels because more rocks are exposed.

Class I: Very small rapids with low waves, slow current, no obstructions.

Class II: Fairly frequent rapids of medium difficulty, few or no obstructions.

Class III: Difficult, large, irregular waves up to four feet, numerous rapids. The course requires some maneuvering of the raft.

Class IV: 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: Long and violent rapids. Large waves that are unavoidable and irregular. Extremely difficult and complex course. Scouting is essential.

Class VI: Maximum difficulty. Nearly impossible and extremely dangerous. Class V carried to the limit of navigability. Involves risk of life.