Otherwise known as the International Scale of River Difficulty, below are the six grades of difficulty in white water rafting. They range from simple to very dangerous and potential death or serious injuries.
Grade 1: Very small rough areas, might require slight maneuvering. (Skill level: very basic)
Grade 2: Some rough water, maybe some rocks, might require some maneuvering. (Skill level: basic paddling skill)
Grade 3: Whitewater, small waves, maybe a small drop, but no considerable danger. May require significant maneuvering.
Grade 4: Whitewater, medium waves, maybe rocks, maybe a considerable drop, sharp maneuvers may be needed.
Grade 5: Whitewater, large waves, large volume, possibility of large rocks and hazards, possibility of a large drop, requires precise maneuvering.
Grade 6: Class 6 rapids are considered to be so dangerous that they are effectively unnavigable on a reliably safe basis. Rafters can expect to encounter substantial whitewater, huge waves, huge rocks and hazards, and/or substantial drops that will impart severe impacts beyond the structural capacities and impact ratings of almost all rafting equipment. Traversing a Class 6 rapid has a dramatically increased likelihood of ending in serious injury or death compared to lesser classes.