There are many natural features in swift water that require special techniques to navigate safely. When it comes to hydraulics and other water hazards, whitewater rescue techniques are the only way to tackle them.
Vertical recirculating hydraulics
Vertical recirculating hydraulics are usually produced by crashing water at the bottom of vertical waterfalls. When engulfed by one of these hydraulics while still on rope, most of the time you will rely on your partners to notice your condition and promptly cut the rope, but there are techniques that you can try to do yourself too. For one, you may want to pull out your knife and cut the rope yourself, but in strong recirculating current this will be very difficult. It might be easier to take a big breath at the first opportunity you get, close your eyes, and work to free yourself from the descender. If you are trapped in a recirculating hydraulic in a deep pool, try to dive down farther to escape the recirculating current. If your pack is still attached to you, it will be much harder to escape the hydraulic. Use the quick release buckles (canyon packs only) to release the pack without having to take it off if you can't try to take it off the normal way. While you do so, try to kick or push on it to help you get away from the recirculating current. Your pack will survive getting stuck in a hydraulic, but you may not.
Horizontal recirculating hydraulics
Horizontal hydraulics are usually produced by water crashing down at an angle. These may not be strong enough to trap you underwater but they may be strong enough to trap you in a pool or filled up pothole and prevent you to escape them. In these cases, taking off your pack and bracing to it (assuming it floats) will provide a good resting position while you await assistance or think of a possible way out. With the pack on, leaning back and using your arms to swim backwards will prove less tiring and more powerful than front facing; you will be able to swim against the current for a longer time duration, but progress will be slower and with a poor control on the trajectory. Wearing your pack in the front, rather than on the back, and swimming forward will provide more flotation and more powerful strides; this is best for short, directed, peak performance. Sometimes, swimming behind a waterfall (in the space between the rock and the water flowing down on it) may allow you to escape the powerful hydraulic produced by the crashing water. Diving down deep underwater may also allow you to escape the power of the recirculating current, which is strongest at the surface. If you have several people trapped in a pool you may do a human train, hands to feet contact, to allow the front person to escape. Once one person escapes, that person can rescue the rest of the team by throwing them a rope.
This technique is basically an intentional belly flop on top of an hydraulic in order to overcome it. By exposing the maximum surface to the water you will produce a bounce effect that will keep you on the surface and quickly cross the hydraulic without getting trapped by it. If you can, climb on a rock and jump off from it. Wear your pack in the front and jump out completely flat and as far as you can. The splash will be gentle because there is no surface tension in whitewater, and make sure you start swimming while still in the air and keep swimming until out of danger.
Powerful water jets
Swift water can produce powerful jets of water. These will usually be vertical jets (water crashing down in free-fall), but they can also be at an angle and in some cases even horizontal. These jets may be strong enough to pin a rappeler against a rock or hold them in an indentation, unable to escape the heavy flow. These jets are very dangerous and should be avoided whenever possible by finding a new rappel route that bypasses them. If trapped by one of these jets, it is recommended to turn your torso so that the flow hits your side (shoulder and hip) rather than your chest or your back, as this may reduce the force of the water enough for you to overpower it and extricate yourself. Sometimes you may be able to use your torso as a "sail", using the powerful jet to push you away from the danger zone rather than towards it. If you are unable to extricate, you may still be able to position your body in a way that allows you to disconnect and try your luck sliding down the rest of the waterfall (a broken leg is always better than a body bag). In some cases the people at the top may be able to help by cutting the rope, but often the only way to extricate from a water jet is with a shock load. This is done by attaching a heavy weight to the rope and letting it slide down in free-fall with the hope that the force of the impact will extricate the victim. It's a last resort technique, one that offers no guarantees and no second chance. The load is usually several heavy packs clipped together sliding as a unit, but sometimes it has been done with actual people. Matteo Rivadossi, in Ogliana di Quarata in 2010, clipped himself onto the rope and jumped off to extricate his friend Giorgio that got pinned in an indentation by a powerful horizontal jet. It was a courageous act that saved the day, but it could also have been disastrous for both.
Rope tangle caused by turbulent water
Turbulent water and hydraulics can cause your rope to tangle or knot even after the rope length has been set and even after the first person has gone down. Before entering any dangerous section of a waterfall (e.g. powerful or turbuent water jet going through a crack), every rappeller should lock off before the dangerous section, pull up the rope, and examine that there are no tangles or knot in the rope.
A strainer is an obstruction in swift water where a person may get stuck. Underwater fallen trees are usually the biggest problem. They go unseen when covered by whitewater and can easily tangle a foot or a leg, with the powerful current pushing the victim forward and underwater. Holes and gaps in a waterfall are also dangerous, the white water will make them go unseen and can get someone's foot stuck, with gravity or current inverting the victim upside-down. The best way to avoid strainers it to always be wary of what may lurk under whitewater. When swimming in whitewater it is best to adopt the defensive position: feet first in a half seating position looking forward and paddling with your arms to control the trajectory. If someone gets trapped in a strainer, it is imperative for the rest of the team to notice and act quickly because often there is not much the victim can do to extricate himself. Taking off the pack and bracing it as a flotation device may help keep the head out of the water and endure the danger while waiting for assistance. In some cases, letting go of the pack and adopting a sideways position may reduce the force of the water just enough to reach back and extricate the extremity. In deep water, diving down underwater rather than above it might also work, since current may be less powerful below the surface.
Swift water can produce powerful underwater currents which are extremely dangerous, as they can go completely undetected until it is too late. Siphons are usually invisible from the surface; the only hint of their presence is a calm pool that receives a lot of water but does not show any substantial surface water outlet. The pool is usually safe to swim except in the vicinity of the siphon, where the suctioning power may be so strong as to trap a foot or a leg or even swallow a person. If a siphon is unobstructed, a person may get suctioned in, go through it completely and surface downstream unharmed. But if packed with branches and debris, it may entrap the swimmer underwater. In general, it is best to stay as far away as possible from siphons, but in some canyons, siphons need to be traversed as they are the only way out. In these cases it is best to tie a rope to a pack and send this down the siphon while keeping the other end anchored above the siphon. If the pack makes it through without obstruction, it will put the rope in tension. With the rope deployed, swimmers will have a route to follow and can use the rope to pull themselves forward or backward while in the siphon. The last person down should untie the rope and follow it down the siphon, knowing that there will be no backtrack option.
In some cases, you will have to cross a raging river. The best way to cross it is to find a wide shallow area upstream or downstream and cross there. If the current is still strong, you may have the strongest swimmer go first with a rope tied to the releasable ring on their rescue PFD. If the current overpowers the swimmer, pull the person out. If the swimmer succeeds, the rope can then be anchored at a 45 degree angle going downstream; the other people can clip onto the rope and let the current push them along to the other side, following the rope. Make sure people position themselves downstream of the rope, or the current may push them against the rope and trap them. If the current is monstrous, you may consider using a Tyrolean traverse, where people clip on the rope and hang from it without touching the water. If you do so, make sure the rope is tensioned enough to support a person weight. Otherwise, you run the risk of the rope sagging into a V shape, trapping people in the center, often in the middle of the water flow they were trying to avoid.
Flash floods are a common problem in the dry canyons of the United States desert southwest, but they can happen in Class C canyons too. Because the water is always running, a flash flood is not as common. Most Class C canyons will flood with a gradual rise of the water level, which can be quick and substantial but not as sudden as a flash flood. In many cases, there is enough forewarning to stop the descent and exit the canyon, or to find safe ground whereby to wait for the water level to recede. Safe ground is usually high ground away from the watercourse, or if none is available, wide areas of the canyon where water will not have a strong current. In general, wherever there is big vegetation there is safe ground. A big, old tree is proof that current has never been too strong in that location.
Adequate thermal protection is required to prevent non-freezing cold injury in very cold water.