Below are definitions describing the various levels or grades of white water difficulty. WSC typically holds to nothing more challenging than class II or II+. That said, it’s important to note that the water depth at any given time is a major factor in determining the difficulty level. What’s listed as class II could easily become class III following a heavy rainstorm.
There are six levels each referred to as "Grade" or "Class" followed by a number. The scale is not linear, nor is it fixed. For instance, there can be hard grade two's, easy grade three's, 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 (where to carry the boat around) may be given if these pose specific challenges.
Important: Some outfitters classify the degree of difficulty as it applies to running the river in an inflatable raft. What might be perfectly suitable for beginner or intermediate rafters could well be a serious headache for a kayaker.
Class I: Easy. Waves small; passages clear; no serious obstacles.
Class II: Medium. Rapids of moderate difficulty with passages clear. Requires some 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. Some consider rafting on a class VI river suicidal, and only extreme luck or skill will allow you through.