The International Scale of River Difficulty is a rating system designed to provide a benchmark for paddlers to rate rivers. It is important to understand that this scale must be applied with caution. A guidebook rating generally assumes optimal conditions (optimal flow, warm weather, warm water, etc.). If the weather is colder than usual, the water is cold, or the river is at a higher than normal level, the river should be classed higher than it's guidebook rating. Also, if rapids are closer together and more continuous, it should be classed harder. In general, it's best to ask someone with experience on a river, whom you trust, to rate it for you.
Moving water with a few riffles and small waves. Few or no obstructions.
Easy rapids with waves up to 3 feet, and wide, clear channels that are obvious without scouting. A little maneuvering is required.
Rapids with high, irregular waves often capable of swamping a canoe. Narrow passages that often require complex maneuvering. May require scouting from the shore.
Long, difficulty rapids with constricted passages that often require precise maneuvering in very turbulent waters. Scouting from the shore is often necessary, and conditions may make a rescue difficult. Decked boaters should have ability to Eskimo roll beef re attempting class IV.
Extremely difficult, long, and very violent rapids with highly congested routes which nearly always must be scouted from the shore. Rescue conditions are difficult and there is a significant hazard to life in the event of a mishap.
Difficulties of class V carried to the extreme of navigability. Nearly impossible and very dangerous. For teams of experts only, after close study and with all precautions taken. Mistakes will lead to serious injury on this type of water.
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