The International River Rating Scale
CLASS I (one) - Fast moving water with riffles and small waves. Few obstructions, all obvious and easily missed with little training. Risk to swimmers is slight; self-rescue is easy.
Class II (two) - Straightforward rapids with wide, clear channels which are evident without scouting. Occasional maneuvering may be required, but rocks and medium-sized waves are easily avoided by trained paddlers. Swimmers are seldom injured and group assistance, while helpful, is seldom needed. Rapids that are at the upper end of this difficulty range are designated Class II+.
Class III (three) - Rapids with moderate, irregular waves and complex maneuvers in fast current and good boat control in tight passages or around ledges are often required. Strong eddies and powerful current effects can be found, particularly on large-volume rivers. Injuries and swimmers are rare. On most class III river rafting trips prior experience and swimming abilities are not required for participants. Rapids that are at the lower or upper end of this difficulty range are designated Class III- or Class III+ respectively.
Class IV (four) - 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. Rapids may require “must make” moves above dangerous hazards. Risk of injury to swimmers is moderate and water conditions may make self-rescue difficult. Group assistance for rescue from trained guides is often essential. Little to no rafting experience is required for participants on many class IV rivers but swimming and good fitness is generally preferred. Rapids that are at the lower or upper end of this difficulty range are designated Class IV- or Class IV+ respectively.
Class V (five) - Extremely long, obstructed, or very violent rapids which expose a paddler to added risk. 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. Scouting is recommended but may be difficult. Swims are dangerous, and rescue is often difficult even for experts. Proper equipment, extensive experience, and practiced rescue skills are essential for your guides and prior experience along with good fitness and swimming skills is necessary for participants.