There are six levels each referred to as “Grade” or “Class” and then a number. The scale is not linear, nor is it fixed. For instance, there can be hard grade twos, easy grade threes, and so on. The grade of a river may change with the level of flow. Often a river or rapid will be given a numerical grade, and then a plus (+) or minus (-) to indicate if it is in the higher or lower end of the difficulty level. Also note that while a river section may be given an overall grading, it may contain sections above that grade, often noted as features, or conversely, it may contain sections of lower graded water as well. Details of portages may be given if these pose specific challenges.
Class I : Easy
Waves small; passages clear; no serious obstacles.
Class II : Medium
Rapids of moderate difficulty with passages clear. Requires experience plus suitable outfit and boat.
Class III : Difficult
Waves numerous, high, irregular; rocks; eddies; rapids with passages clear though narrow, requiring expertise in maneuvering; scouting usually needed. Requires good operator and boat.
Class IV : Very difficult
Long rapids; waves high, irregular; dangerous rocks; boiling eddies; best passages difficult to scout; scouting mandatory first time; powerful and precise maneuvering required. Demands expert boatman and excellent boat and good quality equipment.
Class V : Extremely 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 person, boat, and outfit suited to the situation. All possible precautions must be taken.
Class VI : Class U
Formerly classified as unrunnable by any craft. This classification has now been redefined as “unraftable” due to people having recently kayaked multiple Class VI around the world.