The basic format of the ACA Canyon Rating System includes two digits. The first digit is numeric and represents the values described below related to terrain and rope work. The second digit is an alpha character representing the values described below related to water volume and current. Additional values may be added to represent relative risk and time/commitment. Ratings are cumulative. For example: descending a Class 3 canyon will require the skills listed under Class 3, as well as those listed under Classes 1 and 2.
NOTE: Ratings refer to descents in normal conditions, during what is considered the normal season for the canyon. Adverse conditions, such as higher than normal water volume or colder temperatures, will increase the difficulty of the descent.
| TERRAIN / TECHNICAL ROPE WORK | |
| 1 | Canyon Hiking Non-technical; no rope required. May involve some easy scrambling requiring the occasional use of hands for balance and support. Travel is possible up or down canyon. See route description for more information. |
| 2 | Basic Canyoneering Scrambling, easy vertical or near vertical climbing and/or down-climbing requiring frequent use of hands. Rope recommended for hand lines, belays, lowering packs and possible emergency use. Travel is possible up or down canyon. See route description for more information. |
| 3 | Intermediate Canyoneering Exposed technical climbing. Down-climbing could be difficult and dangerous; most people will rappel. Rope required for belays and single-pitch rappels. Obvious natural or fixed anchors. Retreat up canyon will require ascending fixed ropes. Basic pothole escape techniques (i.e. partner assist, counterweights) may also be required. See route description for more information. |
| 4 | Advanced-Expert Canyoneering Route may involve any combination of the following: 1) difficult and exposed free climbing and/or down-climbing, 2) climbing using direct aid, 3) multi-pitch rappels, 4) complex rope work (i.e. guided rappels, deviations, rebelays), 5) obscure or indistinct natural anchors, 6) advanced problem-solving and anchor-building skills. See route description for more information. |
| WATER VOLUME / CURRENT | |
| 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. |
| RISK / SERIOUSNESS (OPTIONAL) | |
| G | General Audiences Should be straight-forward for those who possess appropriate skills. |
| PG | Parental Guidance Suggested Even with appropriate skills, beginners may sweat. |
| R | Risky One or more extraordinary risk factors exist that could complicate the descent. Solid technical skills and sound judgment critical. Not recommended for beginners. |
| X | Extreme Multiple risk factors exist that will complicate the descent. Errors in technique or judgment will likely result in serious injury or death. Descent should only be attempted by expert canyoneers. |
| XX | Double Extreme Definitely life-threatening. |
Risk factors include number and frequency of rappels, length of rappels (single- or double-rope) and exposure, anchor availability, anchor quality, route finding, obstacles, problem-solving, terrain encountered between technical sections, flash flood potential, availability of exits and high ground, water temperature, prolonged immersion, and difficulty of evacuation or rescue. Specific factors should be addressed in the route description.
| TIME / COMMITMENT (OPTIONAL) | |
| Half Day | |
| I | Short. Normally requires only a couple of hours. |
| II | Normally requires a half day. |
| Full Day | |
| III | Normally requires most of a day. |
| IV | Expected to take one long, full day. Get an early start. Bring a head lamp. Plan for possible bivy. |
| Multi Day | |
| V | Expected to take an average one and a half days. |
| VI | Expected to take two or more days. |
| S = SLOT DESIGNATION |
Tight slot canyons are in a class of their own. Slots can be so narrow that it is necessary to stem above the floor of the canyon to move horizontally. An "S" may be appended to the terrain rating to indicate some sections of the canyon are extremely narrow. A canyon rated S2 will serve as a warning to those with greater-than-average girth that they may have to stem more than their skinny partners. A canyon rated S6 will draw emphasize the need to execute difficult climbing/stemming moves that are likely to be high above the canyon floor.
In conversations, you will often hear canyoneers refer to portions of a canyon's complete rating. For example; they may simply refer to a canyon as Class 2 (referring to the terrain), as Class B (referring to the water), or as Grade IV (referring to the time required). In guidebooks and web sites, you should find complete ratings. A few examples:
3-B PG IV
Class 3 terrain. Water with no or very light current. Slighly more than average risk. Will require a long day for an average group.
S2-A III
Slot canyon that will require scrambling and climbing. Normally dry. Will require most of a day for an average group.
4-A V
Class 4 terrain. Normally dry. An overnight trip for an average group. Advanced canyoneers only due to terrain rating.
3-C3 II R
Class 3 terrain. Water with very strong current and dangerous hazards. One or more extraordinary risk factors exist. Solid technical skills and sound judgment critical. Expert canyoneers only due to water rating.
S4-A XX III
Slot canyon with very difficult and exposed climbing/stemming problems. Normally dry. Life threatening even for expert canyoneers. Will require most of a day for an average group. Expert canyoneers only due to terrain and risk/ seriousness ratings.
Information from canyoneering.net.