Class I. Easy.
Waves small, passages clear; no serious obstacles.
Class II. Medium.
Rapids of moderate difficulty with passages clear.
Class III. Difficult.
Waves numerous, high, irregular; rocks; eddies; rapids with passages clear though narrow, requiring expertise in maneuvering.
Class IV. Very Difficult.
Long rapids; waves powerful, irregular; dangerous rocks; boiling eddies; powerful and precise maneuvering required.
Class V. Extremely Difficult.
Exceedingly difficult, long and violent rapids, following each other almost without interruption; riverbed extremely obstructed; big drops; violent currents; very steep gradient.
Class VI. Unrunnable.
