Table 1

International scale of river difficulty

River classExplanation
Class IEasy: waves small; passages clear; no serious obstacles.
Class IIMedium: rapids of moderate difficulty with passages clear. Most open canoeists should never tackle anything tougher than class II.
Class IIIDifficult: rapids are longer and rougher than class II. Waves numerous, high, irregular; rocks; eddies; rapids with passages clear though narrow, requiring expertise in manoeuvring; scouting usually needed. Requires good operator and boating equipment.
Class IVVery difficult: rapids are generally longer, steeper and more heavily obstructed than class III rapids. Waves powerful, irregular; dangerous rocks; boiling eddies; passages difficult to scout; scouting mandatory first time; powerful and precise manoeuvring required. Demands expert boatman and excellent boat and outfit.
Class VExtremely difficult: exceedingly difficult, long and violent rapids, following each other almost without interruption; riverbed extremely obstructed; big drops; violent current; very steep gradient; close study essential, but often difficult. Requires best man, boat, and outfit suited to the situation. All possible precautions must be taken.
Class VIExtreme and expedition: rapids which have rarely been run. Once such a rapid has been repeatedly run, it is usually reclassified as a class 5x. Risk to swimmers and boaters is extremely high.