The American Canyoneering Association created a rating system for canyons that has become the standard in North America.
A standard ACA rating, such as "4B IV", consists of three pieces of information regarding the technical requirements, water challenges, and time commitment of the canyon. Some variations add additional information such as extraordinary risks and rock climbing rating. Many of these ratings are subjective and different beta authors may select different ratings. There's been much controversy on the ACA water rating classification.
The first number in an ACA rating denotes the degree of technical skill (especially rope work) required to complete the canyon successfully.
- 1: A hike on a path with no special physical obstacles (though navigation may be difficult)
- 2: Scrambling and/or easy climbing required, but no ropes are necessary. Many groups may want a rope or webbing length for convenience, however
- 3: Technical canyon requiring ropes and rappelling and/or ascending
- 4: Extra challenge(s) beyond the obstacles encountered in most class 3 canyons. These may include very long drops, hanging rebelays, unusual exposure, or other difficulties
The letter following the technical ACA rating denotes the type of challenge presented by water in the canyon.
- A: Normally dry or very little water. Dry falls. Water, if present, can be avoided and/or is very shallow. Shoes may get wet, but no wetsuit or drysuit required.
- B: Normally has water with no current or very light current. Still pools. Falls normally dry or running at a trickle. Expect to do some deep wading and/or swimming. Wetsuit or drysuit may be required depending on water and air temperatures.
- C: Normally has water with current. Waterfalls. Expect to do some deep wading and/or swimming in current. Wetsuit or drysuit may be required depending on water and air temperatures. Class C canyons may be rated more precisely using the following system:
- C1: Normally has water with light to moderate current. Easy water hazards.
- C2: Normally has water with strong current. Water hazards like hydraulics and siphons require advanced skills and special care.
- C3: Normally has water with very strong current. Dangerous water hazards. Experts only.
- C4: Extreme problems and hazards will be difficult to overcome, even for experienced experts with strong swimming skills.
Water level in any canyon can fluctuate greatly from year-to-year, season-to-season, even day-to-day. If, upon arrival at a canyon, you discover the water volume/current is greater than indicated by the rating, descent will be more difficult than suggested by the route description. It will be necessary to reevaluate your decision to attempt the descent.
The water ratings above were extracted word for word from page 2 of the ACA RATINGS PUBLICATION). They have been subject to much controversy because of their ambiguity in the case of a 'Low flow' canyon ('a3' in the French rating) and lack of a clear definition of 'current'. To resolve this ambiguity, canyoneers should use the idea of 'consequence' as described by Rich Carlson (original lead developer of the ACA rating system):
In my courses I describe Class A as a canyon that has no water or no water of any consequence. Class B as a canyon with water, but no current or no current of any consequence. Class C as a canyon with current that has potential consequences. (link)
I've been in Class A and got my feet wet. That was not water of any consequence. If I have to swim, the canyon can't be rated A because there are consequences if the water is cold and I didn't bring a wetsuit or I am not able to swim. Same kind of logic should apply when going from B to C.(link)
If the flowing water creates any kind of additional challenge or risk, it can't be rated B. (link)
Rating something higher than C1 should suggest that a person needs at least a little swiftwater skill or at least the ability to recognize hydraulic hazards. (link)
That same flow running over a different rock formation could create a syphon, or an undercut, etc. It's not the amount of flow that makes it C1. It is the absence of hydraulic problems. (link)
I'm sure you all understand even the term "consequences" can be subjective. I had no heartburn with Chris rating canyons B that had flow when the water was ankle-shin deep during the rappel and shin-thigh deep in the pool at the bottom. But watch some of your partners as they rappel in waterfalls that hit them in the chest. They struggle to see where they are placing their feet and struggle harder to remain upright. So even in the absence of significant current, force or hydraulic problems, those flows can have consequences beyond what is encountered in Class B. (link)
5 CFS in one canyon can be incredibly difficult while 50 CFS in another might be quite easy. C1, C2, C3 and C4 were meant to be a little vague because they were never intended to be tied to a CFS level for the reason already mentioned. (link)
CFS is comprised of 3 variables -- width, depth and velocity. Would you rather be in a stream running at 50 CFS that is 20 feet wide or 2 feet wide? (link)
There have been many discussions regarding the interpretation and application of the ACA water rating. A few are listed below.
- Water reference level, as compared to ACA rating
- Whether a rating of B/C should be added to RopeWiki
- Poll for how canyoneers would rate hypothetical low-flow canyons
- Water condition options
- Rating of San Antonio Falls
The roman numerals following the water ACA rating indicate approximately how long the entire canyon trip generally takes a typical group to complete. Actual time required can vary substantially in either direction, but this value is a guideline for distinguishing long canyons from short canyons.
- I: A few hours [<2h]
- II: Around half a day [2h-4h]
- III: Most of a day [4h-8h]
- IV: A full day with a prompt start [>8h]
- V: More than one day
- VI: Many days; an expedition
Some beta authors will append a movie-style R or X to the ACA rating indicating that extraordinary risks of different degrees exist in the canyon. Also, some ratings will also include a rating for the difficulty of climbing required by the canyon in the Yosemite Decimal form (5.11a, for instance)
The allowed values for this property are:
- (blank): No extraordinary risk
- PG (or R-): Some extraordinary risks, may be hard for beginners
- R: Risky, not recommended for beginners
- X (or R+): Extreme, only for experts
- XX: Double extreme, life-threatening even for experts