Two people stand on a grassy hillside in the King Range Wilderness, overlooking the Pacific Ocean.
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Mtn. Bike Rider on the Bizz Johnson Trail King Range National Conservation Area Poppy Three Pump Jacks, Midway-Sunset Oilfield
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BLM>California>What We Do>Recreation>River Classifications - International Scale
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River Classifications

International Scale of River Difficulty

CLASS I

EASY- Waves small; passages clear; no serious obstacles.

CLASS II

MEDIUM - Rapids of moderate difficulty with passages clear. 
Most open canoeists should never tackle anything tougher than class II.

CLASS III

DIFFICULT - Rapids are longer and rougher than class II. Waves numerous, high, irregular; rocks; eddies; rapids with passages clear though narrow, requiring expertise in maneuver; scouting usually needed. Requires good operator and boating equipment.

CLASS IV

VERY DIFFICULT - Rapids are generally longer, steeper and more heavily obstructed than class III rapids.Waves powerful, irregular; dangerous rock; boiling eddies; passages difficult to scout; scouting mandatory first time; powerful and precise maneuvering required. Demands expert boatman and excellent boat and outfit.

CLASS V

EXTREMELY DIFFICULT - Exceedingly difficult, long and violent rapids, following each other almost without interuption; riverbed extrememly 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 VI or U

UNRUNNABLE  ! ! !

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