classification

Rapid Classification:-

Class I - Small rapids with low waves and no obstructions
Class II - More frequent rapids with few or no obstructions
Class III - Waves up to four feet, with some maneuvering around obstructions
Class IV - Very difficult, extended rapids. Scouting is necessary.
Class V - Long and violent rapids. Large waves that are unavoidable. Complex course. Scouting is a must.
Class VI - Maximum difficulty. Involves serious risk to life.

taken from http://whitewater.safpar.com/