The whitewater river rating system classifies rivers from Class I to Class VI. While the system is often discussed and debated, it is imperfect.
To get an accurate idea of the difficulty of the run you need to get a full description. This will include information about the nature of the rapids. Are they drop pool, or continuous. What is the gradient? How many major rapids, and are they easily portaged. Is the river generally though to be safe, or dangerous? What is the water temperature, and how remote is the river?
This system is not exact; rivers do not easily fit one category, and regional or individual interpretations may cause misunderstandings. Allow an extra margin for safety when the water is cold or if the river is remote.
Class I EASY. Fast moving water with riffles and small waves. Few obstructions, all obvious and missed with little training. Risk to swimmers is slight, self rescue is easy.
ClassII Novice. Straightforward rapids with wide clear channels which are evident without scouting. Occasional maneuvering may be required, but rocks and medium sized waves are easily missed by trained paddlers. Swimmers are seldom injured and group assistance, while helpful is seldom needed.
Class III: Intermediate. Rapids with moderate, irregular waves which may be difficult to avoid and which can swamp an open canoe. Complex maneuvers in fast current, and good boat control in tight passages or around ledges is often required. Large waves or strainers may be present but can be easily avoided. Strong eddies and powerful current effects can be found, particularly on large volume rivers. Scouting is advisable for inexperienced parties. Injuries while swimming are rare; self rescue is usually easy, but group assistance may be required to avoid long swims.
Class IV: Advanced. Intense, powerful but predictable rapids requiring precise boat handling in turbulent water. Depending on the character of the river it may feature large, unavoidable waves and holes or constricted passages demanding fast maneuvers under pressure. A fast reliable eddy turn may be required to initiate maneuvers, scout rapids, or rest. Rapids may require “must” moves before dangerous hazards. Scouting is necessary the first time down. Risk of injury to swimmers is moderate to high, and water conditions make self rescue difficult. Group assistance for rescue is often essential but requires practiced skills. A strong Eskimo roll is highly recommended.
Class V: Expert. Extremely long, obstructed, or very violent rapids which expose a paddler to above average endangerment. Drops may contain large, unavoidable waves and holes or steep congested chutes with complex demanding routes. Rapids may continue for long distances between pools, demanding a high level of fitness. What eddies that may exist may be small, turbulent, and difficult to reach. Scouting is mandatory, but often difficult. Swims are dangerous, and rescue is difficult even for experts. A very reliable eskimo roll, proper equipment, extensive experience, and practiced rescue skills are essential for survival.
Class VI: Extreme. One grade more difficult than class V. These runs often exemplify the extremes of difficulty, unpredictability, and danger. The consequences of errors are severe and rescue may be impossible. For teams of experts only, at favorable water levels, after close inspection and taking all precautions. This class does not include drops thought to be unrunnable, but may include drops only occasionally run.