Different Classes Of Rapids Explained – The International Scale Of River Difficulty

Navigating rapids can be an exhilarating experience for kayakers and rafters, but understanding the different classes of rapids is crucial for safety and preparation. The International Scale of River Difficulty ranks rapids from Class I to Class VI based on factors like water speed, obstacles, and rescue challenges. We break down the characteristics of each rapid class and what skills and equipment are necessary to tackle them safely.
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Sam OBrien

Founder, Kayaking & Paddle Boarding Expert

Sam is the founder and editor of WaterSportsWhiz. With over 20 years of experience across various water sports, he provides trusted reviews and expert advice to help others pursue their passion for getting out on the water. When not working, you can find him kayaking, paddle boarding, or planning his next water-based adventure with family and friends.

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Nessa Hopkins

ACA-Certified Kayaking Instructor

Vanessa is a certified kayaking instructor, has taught over 500 people how to kayak, and is a senior member of the American Canoe Association. By combining her deep understanding of the sport and a background in journalism, she offers a wealth of experience and expertise to our growing water sports community, promising to educate and inspire paddlers of all levels.

Whether you’re a seasoned whitewater kayaker or a complete beginner, understanding the different classes of rapids is essential for safe and enjoyable river navigation. The river doesn’t discriminate based on experience level, so it’s crucial to follow proper precautions and assess whether a particular stretch of water is within your skill range.

The International Scale of River Difficulty (ISRD), also known as the American version of the rapid classification system, categorizes whitewater rapids into six distinct classes based on their difficulty and the techniques required to navigate them successfully.

The six rapid classes according to the ISRD are:

  • Class 1 Rapids (Easy)
  • Class 2 Rapids (Novice)
  • Class 3 Rapids (Intermediate)
  • Class 4 Rapids (Advanced)
  • Class 5 Rapids (Expert)
  • Class 6 Rapids (Extreme and Exploratory)

In this guide, we’ll dive deep into each of these rapid classes, exploring their characteristics, challenges, and the skills needed to tackle them. We’ll also discuss:

  • The four basic elements that make up a rapid and how to identify them
  • The importance of scouting and assessing rapids before attempting to navigate them
  • Safety considerations and precautions for each class of rapids
  • Tips for progressing through the rapid classes as your skills improve

By the end of this article, you’ll have a thorough understanding of the different classes of rapids and be better equipped to make informed decisions when planning your whitewater adventures.

What Are The Parts Of A Rapid?

kayaker in irregular waves

Even the nastiest, most aggressive whitewater rapids can be broken down to the same staple river features and the four basic elements.

When you look at a rapid and break it down in your head, you’ll find the same four parts – either separate or in a combination – because they are what creates these rapids in the first place.

Reading Water (Basic Level) | Ep. 151

So, when scouting a section of a river, use the following four elements as a basic framework for what to watch out for as you evaluate potential hazards:

  • Flow, measured in cubic feet per second, indicates the volume of water heading downstream. A significant increase in a river’s flow rate could up the difficulty level of previously navigable passages.
  • Gradient – the rate at which any given river changes its elevation along its course – measures the river’s slope and, in turn, the rate of flow. Steeper gradients are associated with raging rivers; shallow ones produce gentle, “lazy” rivers.
  • Obstructions can take on many forms, such as boulders, trees, strainers, anything that obstructs the river’s flow. Eddies tend to form behind such obstructions, but be wary of hydraulics and holes that might hide near these obstacles, too.
  • Constrictions will form rapids due to the river’s flow being forced into a narrower channel. The increased pressure will cause the river to flow more rapidly and react to surrounding riverbed features.

The Troubling Truth About Whitewater: Are Rapids Dangerous?

Kayakers navigating through the White Water Rapids

Yes, rapids can be dangerous and whitewater kayaking is never something that should be taken lightly. That said, the classification of rapids exists for a reason – to reflect the different intensity levels and categorize whitewater rapids according to their technical difficulty and the potential dangers associated with them.

A Class I rapid, for example, won’t pose nearly the same risk to the paddler as a Class III would – let alone the intense whitewater of a Class IV or Class VI rapid.

According to the American Whitewater Association, it’s possible to identify the four common killers in whitewater rapids as:

  • Cold-water immersion, which leads to cold shock and, as body temperature continues to drop rapidly, potentially deadly hypothermia
  • High water levels, generally caused by spring run-off, dam releases, and heavy storms, that increases the flow and, in turn, the difficulty of most rapids and is accompanied by floating debris and strainers
  • Strainers, formed by underwater obstacles, such as undercut rocks, bridge pilings, and fallen trees, that allow the current to sweep through but pinning you against the obstacle
  • Dams, and low-head dams, in particular, are dangerous hazards nicknamed “killer in our river,” accompanied by weirs and destructive hydraulics, that can be almost impossible to escape 

How Are Rapids Classified: International Scale Of River Difficulty

The rapids on a Northern river

American Whitewater (AW), a non-profit organization and leading advocate for protecting and restoring America’s whitewater rivers, promotes paddling safety as a core issue, as well. They put together the internationally recognized American Whitewater Safety Code, among other safety resources for paddlers.

That Safety Code also includes the International Scale of River Difficulty, the codification of the rating system for whitewater rapids first developed in the 1950s.

Today, this rapid classification system is adopted and used by paddlers and safety personnel worldwide.

And how many classes of rapids are there based on the ISRD, exactly?

According to the International Scale of River Difficulty (ISRD) – the American version of a rating system used to assess river difficulty – there are six categories or “classes,” each followed by a number.

The six rapid classes are as follows:

  • Class 1 Rapids (Easy)
  • Class 2 Rapids (Novice)
  • Class 3 Rapids (Intermediate)
  • Class 4 Rapids (Advanced)
  • Class 5 Rapids (Expert)
  • Class 6 Rapids (Extreme and Exploratory) 

What Are The Classes Of Rapids According To ISRD?

Paddler in red whitewater kayak battling extreme rapids

As explained previously, there are six identifiable classes of rapids covered by the International Scale of River Difficulty (ISRD). From Class I to Class VI, all whitewater rapids are categorized based on how difficult they are to paddle in and navigate.

What do the different rapid classes indicate, though? What are their characteristics and the risks associated with each level? 

Class I Rapids (Easy)

  • Recommended Skill Level: Beginners
  • Risk to Swimmers: Slight
  • Self-Rescue Difficulty: Easy

Class I, or “Easy” rapids, is a category reserved for fast-moving sections of rivers broken up by small waves – no more than one to two feet high – and riffles. You’ll recognize these as minor, white disturbances on the water’s surface.

The currents in Class I rapids are gentle and relatively easy to enter and escape as needed. As for obstructions, there are some – but they are easy to spot from a safe distance and avoid with minimal technical maneuvers.

That’s why Class I rapids are generally considered a great choice for beginners, first-timers – and even young children. 

Class II Rapids (Novice)

  • Recommended Skill Level: Novices
  • Risk to Swimmers: Slight
  • Self-Rescue Difficulty: Slight; group assistance is seldom needed

Class II rapids, also referred to as “Novice,” feature straightforward rapids, wide, clear channels, and slightly more disturbances than Class I. They might still contain rocks and similar obstacles, but they all remain visible at the surface level and can be avoided with minimal maneuvering.

The waves are mostly medium-sized, one to two feet high, with an occasional higher wave that requires navigating around it.

For the most part, rapids in this category don’t require scouting, and group assistance is seldom needed. As such, they’re suitable for beginners and those experimenting with running rapids.

Class II rapids feature a Class II+ subcategory, reserved for rapids on the more complex end of this category that don’t qualify as Class III rapids.

Class III Rapids (Intermediate)

  • Recommended Skill Level: Intermediate
  • Risk to Swimmers: Slight
  • Self-Rescue Difficulty: May require group assistance

Class III rapids, known as “Intermediate,” are characterized by moderate to medium sized waves – typically two to three feet high and spaced irregularly – and can easily swamp a kayak. Expect to encounter powerful currents and strong eddies – especially on large-volume rivers – as well as occasional strainers that, for the most part, should be easy to avoid.

At this point, scouting is advisable – especially for less experienced kayaking groups. 

Class III rapids require precise maneuvering and better boat control and should only be attempted by paddlers who have mastered beginner rapids and want to challenge themselves further. 

Learning How to Kayak - First Class III Kayak Trip

These rapids are further split into two subcategories – Class III- and Class III+ – to include the lower or upper end of this Class. 

Class IV Rapids (Advanced)

  • Recommended Skill Level: Advanced
  • Risk to Swimmers: Moderate to high
  • Self-Rescue Difficulty: Difficult; group assistance is essential

Class IV (4) – or “Advanced” – rapids are characterized by intense, powerful, turbulent water and, depending on the river, can contain various obstacles, too. You might encounter holes, constricted passages, and waves and drops that can be up to four feet high – or more, in some cases. 

Although many of these features can generally be predicted by advanced paddlers, scouting is still necessary – and so is precise boat handling. The risk of capsizing is much higher; Eskimo roll is an essential skill and, should you take a swim, group assistance will be necessary. 

Connor's First Class IV Rapids! - Arkansas River - Royal Gorge Section

You’ll find two subcategories at the lower and upper ends of Class IV, designated as Class IV- and Class IV+ rapids. 

Class V Rapids (Expert)

  • Recommended Skill Level: Experts
  • Risk to Swimmers: High
  • Rescue Difficulty: Difficult

Class V rapids, also known as the “Expert” class of rapids, will be extremely long, violent, and feature much more obstructions and demanding routes. Any drops you encounter may contain significant and unavoidable waves and whirlpools. Eddies are few and far between, small, turbulent – and hard to reach.

Fast, unpredictable currents, tricky obstacles, and rapids that often continue for long distances pose a substantially high risk to paddlers. That’s why Class V rapids are generally considered the highest “runnable” rapids reserved for experts with proper rescue training – and only after advanced scouting. 

Rafting The Zambezi River *EPIC Class 5 rapids*

However, Class V rapids are a pretty broad category with multiple levels – 5.0, 5.1, 5.2, 5.3, etc. The increase in difficulty between each subcategory is the same as between two classes of rapids. 

Class VI Rapids (Extreme and Exploratory)

  • Recommended Skill Level: Teams of experts
  • Risk to Swimmers: Severe
  • Rescue Difficulty: Severe to impossible

Class VI rapids, labeled as “Extreme and Exploratory Rapids,” exemplify the extreme levels of difficulty, danger, unpredictability, and intensity of whitewater. The main characteristic of Class VI rapids is their very violent rapids and unpredictability. 

Niagara Great Gorge ~ White Water Walk

Scouting, although crucial, is complicated and can’t be done with the same degree of accuracy.

Due to the exceptionally high risk, nearly impossible rescue, and severe consequences of Class VI rapids, many are considered unrunnable. That means there are no safe routes, even with expert-level paddling skills and maneuvering. The risk of severe injuries and death is as high as it gets.

If Class VI rapids have been successfully run several times, the rating might be changed to a more suitable, five-point-something rating.

What Are The Classes Of Rapids In The Grand Canyon?

Hance Rapids in Grand Canyon National Park

Whitewater rapids are commonly rated based on the International Scale of River Difficulty – the Class I-VI system is, indeed, the “standard” scale, universally adopted worldwide.

But not all rivers are rated using this system.

The Grand Canyon stretch of the Colorado River is one of those exceptions – along with a few other big-water Western rivers – where the rapids are classified on the 1-10 rating scale

If Class VI rapids are labeled as “extreme” and “unrunnable,” though, what does that mean for Grand Canyon rapids that can have a rating as high as 8 to 10?

This section of the Colorado River is, in fact, home to passable rapids – class IV to V rapids, at most. They’re just rated on a slightly different scale; that’s it.

The Grand Canyon 1-10 rapid classification roughly translates as follows:

  • Grand Canyon Rating 1-2 – Class I Rapids
  • Grand Canyon Rating 3-4 – Class II Rapids 
  • Grand Canyon Rating 5-6 – Class III Rapids 
  • Grand Canyon Rating 7-8 – Class IV Rapids 
  • Grand Canyon Rating 9-10 – Class V Rapids 

What Can Affect A River’s Rapid Classification Or Difficulty Rating?

Class Five Whitewater Extreme Kayaking

Rating systems – like the International Scale of River Difficulty – are pretty helpful for assessing the difficulty of a particular river or rapid.

However, they’re not without limitations – and they’re certainly not set in stone.

Rivers are anything but static, and the river’s difficulty rating should never be taken as fixed and final. 

Many factors could affect these ratings, such as fluctuations in water levels, bad weather, recent floods, and even regional interpretations. So, when assessing a rapid, use these as guidelines – rather than an exact science. 

Area (Geographical Region)

The classification of rapids is a somewhat subjective scale. Depending on the region, you might find some slight discrepancies regarding the river’s difficulty. 

It’s more than just a matter of different regions using different rating systems, though. It’s often a matter of different local interpretations.

When in doubt, check with local resources. 

Location & Remoteness: How Easy It Is To Access 

Another factor that will significantly impact the river’s difficulty rating is the actual location, or, to be more specific, its remoteness. Ease of access is a pretty big deal – in more ways than one.

Let’s say you have Class III rapids on a river that stretches along a road.

If you were to move that same river in the backcountry, with limited access, and poor rescue conditions, it might qualify as a Class IV – or even Class IV+, depending on how remote the area is. 

Weather & Water Temperatures

Lousy weather conditions and lower water temperature will up the hazard level of any river, regardless of its general difficulty rating. Getting wet and taking a swim is pretty much a given in whitewater – and the colder the water, the higher your risk of cold shock and hypothermia.

Never forget that cold-water immersion is one of the most common killers in whitewater rapids! 

Changing Water Conditions: Water Level & River Flow

The characteristics of a river – and its difficulty – can change significantly with the water levels, from season to season, and at times, even overnight. That’s why it’s best to treat given ratings as snapshots:

They are based on informed opinions – but they can’t accurately portray the rapid’s difficulty on any given day, year-round.

The ISRD does take water conditions and river flow into account, using the following letters and labels to identify and describe the varying water levels and rate of flow – the so-called river stages

  • Low (L) – Below-normal levels that interfere with paddling
  • Medium (M) – Normal water levels
  • Medium-High (MH) – Higher than average water level with slightly faster flow
  • High (H) – Well above normal water level
  • High-High (HH) – Very high water level, complex hydraulics, and frequent debris
  • Flood (F) – Abnormally high water level that overflows the banks and extremely violent currents

Quick Word About Your Abilities, Responsibilities & Personal Preparedness

Whitewater River Rafting Adventure Team

I cannot stress the importance of paddling in a group of at least three paddlers enough. Solo runs are generally discouraged.

That said, each paddler is responsible for their safety and needs to take sole responsibility for every decision made during the trip.

That brings me to my next point:

Certain elements must be evaluated before you can confidently decide that you’re competent to handle a particular river.

When you’re judging your abilities and trying to determine whether you should run those rapids or sit that one out, here are some requirements to consider:

Classification Of Rapids: Conclusion

Whitewater doesn’t care about how experienced you are. It will still hit you with all its might, and it will push you to your limits – and then some.

That’s why it’s vital – as in, potentially life-saving – to familiarize yourself with different classes of rapids before you ever attempt to run a particular river stretch. Even more so, it’s crucial to understand how they relate to your experience level and skills.

These numerical scores reflect the technical difficulty and the risks and dangers associated with the specific rapid. Remember to add any fluctuations in water level and your ability into the equation – and you should have no trouble deciding whether you’re ready to tackle that river! 

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Sam OBrien

Sam is the founder and editor of WaterSportsWhiz. With over 20 years of experience across various water sports, he provides trusted reviews and expert advice to help others pursue their passion for getting out on the water. When not working, you can find him kayaking, paddle boarding, or planning his next water-based adventure with family and friends.

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