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The most widely used grading system is the International Grading System, where white water (either an individual rapid, or the entire river) is classed in six categories from Class I (the easiest and safest) to Class VI (the most difficult and most dangerous). The grade reflects both the technical difficulty and the danger associated with a rapid, with grade I referring to flat or slow-moving water with few hazards, and grade VI referring to the hardest rapids which are very dangerous even for expert paddlers, and are rarely run. Grade-VI rapids are sometimes downgraded to grade-V or V+ if they have been run successfully. Harder rapids (for example a grade-V rapid on a mainly grade-III river) are often portaged, a French term for carrying. A portaged rapid is where the boater lands and carries the boat around the hazard.
A rapid's grade is not fixed, since it may vary greatly depending on the depth and the current of the river. Although some rapids may be easier at high flows because features are covered or "washed-out," high water usually makes rapids more difficult and dangerous. At flood stage, even rapids which are usually easy can contain lethal and unpredictable hazards. (Briefly adapted from the American version [1] of the International Scale of River Difficulty [2].)
Very small rough areas, requires no maneuvering. (Skill Level: None)
Some rough water, maybe some rocks, small drops, might require maneuvering. (Skill Level: Basic Paddling Skill)
White water, medium waves, maybe a 3-5 ft drop, but not much considerable danger. May require significant maneuvering. (Skill Level: Experienced paddling skills)
White water, large waves, rocks, maybe a considerable drop, sharp maneuvers may be needed. (Skill Level: Whitewater Experience)
Whitewater, large waves, large rocks and hazards, maybe a large drop, precise maneuvering (Skill Level: Advanced Whitewater Experience)
White water, typically with huge waves, huge rocks and hazards, huge drops, but sometimes labeled thus due to largely invisible dangers (i.e., a smooth slide that creates a near-perfect, almost inescapable, hydraulic, as at ‘Tooth Fairy’ on the Brahmaputra). Class 6 rapids are considered hazardous even for expert paddlers using state-of-the-art equipment, and come with the warning "danger to life or limb." (Skill Level: Expert).
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