The difficulty of a river is classified on a international scale of I to VI, with Class VI indicating an unrunnable stretch of river.

The difficulty of a river may vary significantly with fluctuations in water level. For example, the river is normally a Class II, but in times of high water, the difficulty may increase to Class III+. The ratings usually listed refer to the rivers' difficulties at normal, summer levels. Don't judge a river by it's put-in pool and take-out, know what's downstream. Know the rating class...

• Class I: EASY. Waves are small; passages clear. No serious obstacles.
• Class II: MEDIUM. Rapids of moderate difficulty with passages clear.
• Class III: DIFFICULT. Waves are numerous, high, irregular; rocks; eddies; rapids with passages clear though narrow, requiring expertise in maneuvering.
• Class IV: VERY DIFFICULT. Long rapids; waves powerful and irregular; dangerous rocks, boiling eddies; powerful and precise maneuvering required.
• Class V: EXTREMELY DIFFICULT. Long and violent rapids following each other almost without interruption; riverbed extremely obstructed; big drops; violent current; very steep gradient.