The International Scale of River Rating Difficulty is a guide for assessing the difficulty of a stretch of water. Some rivers will not clearly fall into a neat system. Temperatures below 50 °F should change a rating to be one class more difficult than normal.

• Class I - Moving water with few riffles and small waves. Few or no obstructions.

• Class II - Easy rapids with waves up to three feet and wide clear channels that are obvious.

• Class III - Rapids with high, irregular waves often capable of swamping an open canoe. Narrow passages that often require complex maneuvering. May require some scouting from shore.

• Class IV - Long, difficult rapids and constricted passages that often require precise maneuvering in very turbulent waters. Scout from shore often necessary and conditions make rescue difficult. Canoeists and kayakers should have the ability to roll.

• Class V - Extremely difficult. Long very violent rapids with highly congested routes that nearly always must be scouted. Rescue conditions are difficult and there is a significant hazard to life in the event of a mishap. Ability to execute a roll is essential for all boaters in kayaks and closed canoes.

• Class VI - Difficulties in Class V carried to the extreme of navigability. Nearly impossible and very dangerous. For experts only.

Source: American Whitewater