Class I:
Fast moving water with riffles and small waves. Few obstructions, obstacles are obvious and easily missed. Risk to swimmers is slight; self-rescue is easy. You practically float this stuff blindfolded.
Class II:
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. Swimmers are seldom injured and group assistance, while helpful, is seldom needed. Fun for the whole family.
Class III:
Rapids with moderate, irregular waves which may be difficult to avoid. Complex maneuvers in fast current, and good boat control in tight passages, or around ledges are often required. Large waves or strainers may be present but are easily avoided. Strong eddies and powerful current can be found. Scouting is advisable. Injuries while swimming are rare; self-rescue is usually easy but group assistance may be required to avoid long swims. Things are getting interesting and your face is getting wetter.
Class IV:
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. Scouting may be necessary the first time down. Risk of injury to swimmers is moderate to high, and water conditions may make self-rescue difficult. Group assistance for rescue is often essential but requires practiced skills. You’re breaking into the adrenaline zone and your knuckles are getting whiter.
Class V:
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. All possible precautions must be taken, requires best person, boat, and outfit suited to the situation, the pinnacle of rafting.
Class VI:
These runs have almost never been attempted and often exemplify the extremes of difficulty, unpredictability and danger. You may know someone who knows someone who ran this rapid. Not recommended for swimming.