Whitewater rapids are rated, based on relative difficulty, on a scale of one to six, usually written in Roman numerals. Class I is the smallest of rapids and waves, and Class VI the extremely difficult rapids.
Very small rapids with low waves, slow current, no obstructions.
Fairly frequent rapids of medium difficulty, few or no obstructions.
Difficult, large, irregular waves up to four feet, numerous rapids. The course requires some maneuvering of the raft.
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.
Long and violent rapids. Large waves that are unavoidable and irregular. Extremely difficult and complex course. Scouting is essential.
Maximum difficulty. Nearly impossible and extremely dangerous. Class V carried to the limit of navigability. Involves risk of life.