Class I: Easy. Fast moving water with riffles and small waves. Few obstructions, all obstacles are obvious and easily missed with little training.
Class II: Novice. Straightforward rapids with wide, clear channels which are evident without scouting. Occasional maneuvering may be required. Rocks and medium sized waves are easily missed.
Class III: Intermediate. Rapids with moderate, irregular waves that may be difficult. Complex maneuvers in fast current and good boat control in tight passages or around ledges are often required; large waves may be present but are easily avoided. Strong eddies and powerful current effects can be found, particularly on large-volume rivers.
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.
Class V: Expert. Extremely long, obstructed, or very violent rapids. 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 exist may be small, turbulent, or difficult to reach. At the high end of the scale, several of these factors may be combined.
Class VI: These runs often exemplify the extremes of difficulty, unpredictability and danger. The consequences of errors are very severe and rescue may be impossible. For teams of experts only, at favorable water levels, after close personal inspection and taking all precautions.