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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.

Contents 1 Classification 1.1 Class/Grade I 1.2 Class/Grade II 1.3 Class/Grade III 1.4 Class/Grade IV 1.5 Class/Grade V 1.6 Class/Grade VI 2 Caution in application 3 Other classifications 4 See also 5 References

There are six levels, each commonly referred to as "Grade" or "Class" and then a number. The scale is not linear, nor is it fixed. There can be hard grade twos, easy grade threes, and so on. Also, 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. Even though a section of river may be given an overall grading, it may contain sections above that grade (these are often noted as features and details of portages may be given). Conversely, a higher grade will likely contain sections of lower graded water.

A Grade I (One) section will have long sections of flat, slow moving water, with minor ripples or waves and a course that is easily navigable. There is little danger to swimmers (other than the usual hazards of water) and self-rescue should be easy.

A Grade II (Two) section may have sections of straightforward rapids, some small waves, weirs, small drops or ledges and eddies. There will be a clear route through all features without a need for inspection.

A Grade III (Three) section will have numerous rapids, irregular waves and moderate drops, harder eddies that may recirculate and stoppers (also known as hydraulics or holes) may form below drops and in waves. The river may have a broken flow that might not always present a clear course. Often these sections have a series of drops creating a steep overall gradient. On the whole, from-the-water inspection should be sufficient, although some harder parts may need inspection from the river bank.

A Grade IV (Four) section will feature long, difficult rapids with highly irregular waves, a steep gradient, a stepped profile with drops up to 3 m in height, difficult eddies and whirlpools. The course of the river may be hard to recognise and powerful but predictable flows require precise handling, with a high risk to swimmers. Off-river inspection is highly advised, as is bank support for some features.

A Grade V (Five) section will be similar to a Grade IV, with larger, more violent features and less predictable flows. Often, there will be large, unavoidable dangers such as holes and boiling/recirculating eddies. Courses are difficult to find and will definitely incur a risk to both paddler and equipment. A pre-run inspection from the river bank is VITAL. Rescue is often difficult, and bank support with throw lines is always recommended.

Due to advances in what is runnable in recent years, many rapids are being revised from class 6 to class 5. Because of this, some boaters have began describing certain rapids as class 5.0, 5.1, 5.2, etc., each being equivalent to a higher class level. This provides additional information about the difficulty of a rapid, while preserving the special function of the class 6 category.

A Grade VI (Six) section is at the pinnacle of technicality and difficulty. Only to be attempted by teams of highly skilled experts, there is a definite risk to a paddler's life, as many of these sections have either never been (successfully) paddled before, or they have led to deaths. Often a Grade VI will be a single feature within a Grade IV or V section, such as a water fall. Bank support with rescue lines is always required, as is inspection from all possible angles, and luck is often considered an important part of a successful run.

The grade of a river or rapid is likely to change along with the level of the water. High water usually makes rapids more difficult and dangerous, although some rapids may be easier at high flows, because features are covered or "washed-out". At flood stage (spate), even rapids which are usually easy can contain lethal and unpredictable hazards. Conversely, some rapids may be easier with lower water levels when dangerous hydraulics become easier to manage.

Also, some rivers with high volumes of fast moving water may require little maneuvering, but will pose serious risk of injury or death in the event of a capsize.

Additionally, the application of this classification can vary enormously, depending on the skill level, experience, bravery or foolhardiness of the paddlers who rated the river. Ratings can also differ somewhat from country to country.

The grading system in Australia and New Zealand is as follows:

1. Small regular waves. Easy passage, but care may be needed with obstacles.
2. Regular medium sized waves and generally unobstructed passage.
3. Fairly high waves, and the passage may be difficult to recognise from on the river.
4. High, powerful, irregular waves, with the passage often difficult to recognise.
5. Very difficult rapids; the extreme for commercial operations.
6. Very dangerous, and at the limit of practicality.

• Whitewater
• Whitewater Kayaking
• Riverboarding

• American Whitewater: International Scale of River Difficulty
• Welsh Canoeing Association's Grading page