The International Scale of River Difficulty defines how many different grades for white water rafting?

Whitewater rafting is a globally popular outdoor activity requiring the use of an inflatable raft to navigate a river and turbulent waters. The International Rafting Federation is the worldwide body that has established the International Scale of River Difficulty.

Class 1: Very small rough areas, might require slight maneuvering.

Class 2: Some rough water, maybe some rocks, might require some maneuvering.

Class 3: Small waves, maybe a small drop, but no considerable danger. May require significant maneuvering.

Class 4: Whitewater, large waves, large volume, possibility of large rocks and hazards, possibility of a large drop, requires precise maneuvering.

Class 5: Whitewater, large waves, large volume, possibility of large rocks and hazards, possibility of a large drop, requires precise maneuvering.

Class 6: 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 impact ratings beyond the structural capacities of almost all rafting equipment.

In the U.S, whitewater rafting is traced to 1811 when the first excursion is recorded on the Snake River in the state of Wyoming.