The following river hazards are described briefly which forces the paddler to adapt an inquisitive attitude. The river sense of experienced boaters is based on this approach.
Don't let these descriptions intimidate you. Your purpose is to understand the hazards clearly, enabling you to know when they are a factor to your safety. If you would like further explanation, ask local instructors to point them out on nearby rivers.
A Foot entrapment is simply catching a foot in rocks on the bottom of the river. It is caused by trying to stand up while getting swept downstream in water usually in water mid-thigh to mid-torso deep. Prevention is easy: stay in the safe swimmer's position (on your back, feet up and pointed downstream) unless the water is less than knee deep. Practice swimming and maneuvering through rapids aggressively, on your back, looking between your feet at the side of the river you wish to avoid. In very deep water practice swimming freestyle, on your stomach. River swimming wisdom is to ball up when swimming over a sheer drop of more than 3-4 feet.
Strainers are trees or single branches in the current, with river water flowing through, causing a severe pinning hazard. Strainers are caused by erosion. Trees fall because of old age, floods, and storms. Look for them on wooded riverbanks, along small creeks after high water, often found on the outside of bend, and on less frequented rivers. Assume they are present unless you know otherwise. Use downstream vision to spot bobbing twigs or irregular flow patterns.
Man Made Entrapments Anything manmade in the river is dangerous and are a constant cause of alarm and are inherently more dangerous than most things natural. Keep an eye out for bridge pilings, low head dams, junked cars, any man made junk found commonly in urban riverways, under highway crossings, and at abandonned dam sites. Maintain a habit of visual downstream scanning. Avoid anything suspicious!
Broaches Getting pinned on a rock, either amidship or at the ends. Avoid sharp rocks that can potentially crease a boat or serve as point to be wrapped by your kayak! Develop the instinct to lean into the rock with your boat and body leaning together like a bell buoy. Reach your body out to "Love the rock". Practice this skill with an instructor on gentle, shallow water until it becomes instinct.
Undercut Rocks Undercuts are a water feature where a slab of rock, or rock shape, forces the current flow to go under the surface. Learn to spot them by the dark shadow on the upstream side of the rock, the lack of pillowing action by oncoming water, and by the lack of a predictable eddy on the downstream side. Most dangerous undercuts are well known by locals, and listed in guidebooks.
Entanglement Getting tangled exiting your boat is most likely to be caused by ropes, and loose lines, in your boat. Practice wet exits and critically evaluate your outfitting for entanglement potential. Treat throw ropes as a potential hazard. Keep them neatly bagged, and carry a knife for rescue.
Vertical Pins occur when the bow buries and gets pinned on the bottom after a steep drop. This is not a concern until you are paddling drops of over 3 or 4 feet. Advanced paddlers prevent them by checking the water depth first, and leaning back into a 'boof' move to keep the bow up. Paddling boats with a large volume bow reduces this risk substantially- Thats why creek boats have high volume!
Hydraulics The killer hydraulics have evenly formed backwash, water moving back upstream for four or more feet. Holes with more of a wave shape are intimidating, but typically less hazardous than water flowing smoothly upstream. Dams, and hydraulics that are very regular, and perpendicular to the current are far more dangerous than hydraulics angled with one end downstream.
Long Swims Many people unfamiliar with the sport might expect long swims to be a primary killer. Since most beginner/intermediate rivers have pools between the drops, this is rarely the case. Wearing a tight PFD, matching your ability to an appropriate river, and being dressed for a swim can be excellent defense against a long swim. Of course another great precaution is a competent group of friends with either a shore or boat based rescue plan.
Back to basics: wear a helmet in kayaks, and learn to tuck tight forward to the deck when you flip ...dress appropriately for the water and air temperatures. Drysuits and wetsuits are a must if the combined water and air temperature is under 100 degrees.