Below is a summary of the international classification system of rafting river difficulty.
Class 1 Easy. Moving water with occasional small rapids. Few or no obstacles.
Class 2 Moderate. Small rapids with regular waves. Some manoeuvring required, but easy to navigate.
Class 3 Difficult. Rapids with irregular waves and hazards that need avoiding. More difficult manoeuvring required but routes are normally obvious. Scouting from the shore is occasionally necessary.
Class 4 Very difficult. Large rapids that require careful manoeuvring. Dangerous hazards. Scouting from the shore is often necessary and rescue is usually difficult. Kayakers should be able to roll. Turbulent water and large irregular waves may flip rafts. In the event of a mishap, there is significant risk of loss, damage and/or injury.
Class 5 Extremely difficult. Long and very violent rapids with severe hazards. Continuous, powerful, confused water makes route-finding difficult, and scouting from the shore is essential. Precise manoeuvring is critical and for kayakers rolling ability needs to be 100 percent. Rescue is very difficult or impossible, and in the event of a mishap there is a significant hazard to life.
Class 6 Nearly impossible. Might possibly (but not probably) be run by a team of experts at the right water level, in the right conditions, with all possible safety precautions, but still with considerable hazard to life.