International Scale of River Difficulty
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, skill level required and danger 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.

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 manoeuvring; 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 manoeuvring 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 (or U) Unrunnable.


Specific location/national classifications
International Scale of River Difficulty India

Grade I: Small, easy waves; mainly flat water.
Grade II: Mainly clear passages; some areas of difficulty.
Grade III: Difficult passages; narrow in places and with high waves.
Grade IV: Very difficult, narrow and requiring precise maneuvering.
Grade V: Extremely difficult. Very fast-flowing waters which can be maneuvered only by experts
Grade VI: For all practical purposes, unmanageable- even suicidal.