Class 1: Very easy. Waves small, regular. Riffles.
Class 2: Easy. Rapids with passages clear and wide and little to no consequences.
Class 3: Medium. Rapids with passages that are clear, though require expertise in maneuvering. Consequences with mistakes, though not likely grave.
Class 4: Difficult. Long rapids with powerful and irregular hydrolics. Dangerous rocks and boiling eddies. Precise maneuvering required. Consequences can be grave with mistakes.
Class 5: Very difficult. Continuous, long and violent rapids, following each other with short and few calm stretches for recovery. River bed extremely obstructed. Big drops, strong current and steep gradient. Scouting of rapids essential but difficult. Grave consequences with mistakes.
Class 6: Unrunnable or thought to be unrunnable.