International Scale of River Difficulty

The International Scale of River Difficulty is an American system used to rate the difficulty of a stretch of river, or a single (sometimes whitewater) rapid. The scale was created by the American Whitewater Association for the purpose of evaluating rivers throughout the world, hence International in the title. It should not be confused with the internationally used whitewater scale, which is published and adapted by a committee of the International Canoe Federation ICF. The grade reflects the technical difficulty and skill level required associated with the section of river. The scale is of use to various water sports and activities, such as rafting, riverboarding, whitewater canoeing, stand up paddle surfing, and whitewater kayaking.

There are six categories, each referred to as "Grade" or "Class" followed by a number. The scale is not linear, nor is it fixed. For instance, there can be difficult grade twos, easy grade threes, and so on. The grade of a river may (and usually does) change with the level of flow. Often a river or rapid will be given a numerical grade, and then a plus (+) or minus (-) to indicate if it is in the higher or lower end of the difficulty level. While a river section may be given an overall grading, it may contain sections above that grade, often noted as features, or conversely, it may contain sections of lower graded water as well. Details of portages may be given if these pose specific challenges. A summary of river classifications as presented by the American Whitewater Association:

Because of the large range of difficulty that exists beyond Class IV, Class V is an open-ended, multiple-level scale designated by class 5.0, 5.1, 5.2, etc. Each of these levels is an order of magnitude more difficult than the last. That is, going from Class 5.0 to Class 5.1 is a similar order of magnitude as increasing from Class IV to Class 5.0.

Classifications can vary enormously, depending on the skill level and experience of the paddlers who rated the river. For example, at the 1999 International Conference on Outdoor Recreation and Education, an author of a paddling guide pointed out that there is too much variation in what is covered by the Class I designation, and proposed making further distinctions within the Class I flat water designations and Class I+ moving water designations, with the goal of providing better information for canoeists, instructors leading trips, and families with young children.

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