Rapid Grading

The International Scale of River Difficulty is a standardized scale used to rate the safety of a stretch of river, or a single rapid. The grade reflects the technical difficulty and skill level required associated with the section of river.

Classification

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.

Rapid Class IClass I - Easy
sit back, relax and go with the flow
Waves small; passages clear; no serious obstacles.
Class II - Medium
Practise for the big ones
Rapids of moderate difficulty with passages clear.
Class III - Difficult
Malalu Rapid
Waves numerous, high, irregular; rocks; eddies; rapids with passages clear though narrow.
Class IV - Very Difficult
Bujagali Falls
Long rapids; waves high, irregular; dangerous rocks; boiling eddies; powerful and precise manoeuvring required. Demands expert boatman and excellent boat and good quality equipment.
Class V - Extreme Difficult
Long and violent rapids, following each other almost without interruption; riverbed extremely obstructed; big drops; violent current; very steep gradient.
Class VI - Unraftable
Murchison Falls

Source Wikipedia