Rapids on rivers can vary dramatically in difficulty. They can range from a few riffles on otherwise smooth easy flowing water to raging torrents of water piling into a maze of boulders. As you might imagine, the safety concerns with this range of rapids vary as well. To assist boaters in determining the difficulty of a particular rapid (and hence the difficulty of the section of river), the American Whitewater Association (AWA) has developed a scale for rating the difficulty of a rapid.

To the uninformed, the system numbers mean virtually nothing - though in fact the rating system is straight forward. Any rapid or shoal on a river is given a difficulty rating ranging from 1 to 6. To make things look more official, roman numbering is used and the numbers are called ‘Classes’. So the classes are (from easiest to hardest) Class I, Class II, Class III, Class IV, Class V, and Class VI. Just like in school with grades, the Classes can be given ‘+’ or ‘-‘ designations as well. A rapid may be a ‘Class II+’ - this means it is more difficult than most Class II rapids, but not quite difficult to be classified as ‘Class III’. Similarly, a ‘Class IV-‘ rapid would be considered easier than most Class IV’s, but not quite easy enough to be classified as ‘Class III’

Now that you understand the range of the ratings, it would be nice to know what each of these class ratings mean on the river. The AWA uses the following criteria to rate a rapid:

CLASS I--Moving water with a few ripples and small waves. Few or no obstructions.
CLASS II--Easy rapids with waves up to 3 feet and wide, clear channels that are obvious without scouting. Some maneuvering is required.
CLASS III--Rapids with high, irregular waves often capable of swamping an open canoe. Narrow passages that often require complex maneuvering. May require scouting from shore.
CLASS IV--Long difficult rapids with constricted passages that often require precise maneuvering in very turbulent waters. Scouting from shore is necessary, and conditions make rescue difficult. Generally not possible for open canoes. Boaters in covered canoes and kayaks should have the ability to Eskimo roll.
CLASS V--Extremely difficult, long, and very violent rapids with highly congested routes, which should always be scouted from shore. Rescue conditions are difficult and there is significant hazard to life in the event of a mishap. Ability to Eskimo roll is essential for boaters in kayaks and decked canoes.
CLASS VI--Difficulties of Class V carried to the extreme of navigability. Nearly impossible and very dangerous. For teams of experts only, after close study has been made and all precautions have been taken.

Remember that the ratings of rapids are generally given for a specific water level. Knowing that water level and what that number means is very important when judging a river against your skills/desires. This web-site has a page that discusses How to Understand Water Levels.