Canoeing Physical Hazards

Learn what dangerous terrain to look out for and how to prepare for outdoor emergencies whenever you're planning a canoeing route.

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Learn what dangerous terrain to look out for and how to prepare for outdoor emergencies whenever you’re planning a canoeing route.

To plan for potential rescue, and determine the equipment that should be carried, the canoeing group leader must find out as much as possible about the physical hazards of the route.

The individual nature of the canoe route, and particularly its own specific physical hazards, will significantly affect your rescue planning. A steep, rocky, technically difficult river will demand a completely different rescue plan than a week-long, remote lake canoeing trip. Unfortunately, it may be difficult to find out what the physical hazards are. You may need to be prepared for a number of possibilities. The important point is to ensure that all the equipment that is available to your group is suitable for the expected conditions and that it is in proper working order before you leave home. I remember a day when one of my friends, Cathy, capsized in a difficult rapid and there was only one rescuer downstream of her. The rescuer, Steve, jumped out of his canoe, onto the shore to toss his throwbag. Steve was confident in his ability to use a throwbag, and was caught totally by surprise when he swung the bag for a powerful throw and nothing happened. This was the first time this season that Steve had used this throwbag, and whoever was using it last had not stored it properly, leaving it stuffed in a tangled, useless mass. Cathy floated by with a forlorn look on her face, and a longer, unpleasant rescue ensued.

How Moving Water Affects Safety

River mishaps are different from lake mishaps in that the river does not stop flowing just because you have an accident. The situation is, therefore, more dynamic and likely to be more dangerous. Everything, including your rescue, must happen faster because the situation is continuously changing.

The organization, the planning for rescue, and the establishment of priorities are exactly the same as for any other emergency situation. However, you are now dealing with moving water, and moving water has an awesome force. The faster the current, the greater the force.

Because the amount of force varies directly with the square of the velocity of the water, a very small increase in the velocity produces a large increase in the force of the water.

Most of the time, the power of the river goes unnoticed by canoeists. While you are floating on the river, or even swimming in the water, you rarely appreciate the energy around you. It is only when you try to stop or interrupt the flow of the water that the incredible power of the river becomes immediately apparent. The objective of safe river paddling is to never try to work against the river. The river is indefatigable, it always wins in the end.


In the rivers, strainers present one of the greatest dangers to canoeists. Strainers are anything that allows free passage of water, but will catch and hold a solid object such as a canoe or a swimmer. The strainers most often encountered are trees. A strainer is usually first created on the outside of a curve in the bend of a river.

This is where the speed of the current is strongest, and the river banks are subject to the maximum amount of erosion. As the erosion undercuts the banks, it also undercuts the trees which then fall over until they are hanging straight out over the surface of the water. At this point, these trees are called sweepers, since they effortlessly sweep you out of your canoe if you happen to hit one. While they may cause a capsize which could become a problem, sweepers are not usually dangerous in themselves. If you are knocked over by one, you simply float underneath.

I was quite surprised to see this repeated three times one day on a small mountain river. My group was having a lunch break when we saw three, obviously novice canoes, paddling along the side of the river. Just downstream of them were two sweepers, about 20 metres apart. The paddlers in the lead canoe managed to duck down and crash under the first sweeper. However, the second sweeper knocked them over and they were spilled into the cold, glacier water. The interesting aspect of this accident was that the second and third canoe promptly did exactly the same thing. In moments we had 6 swimmers, 3 capsized canoes, and a group of very busy rescuers.

Once the branches of the tree penetrate the surface of the water, the sweeper then becomes a strainer. Now if you are knocked over, or: if you are already swimming from a previous capsize, you may not make it through the strainer. You may be caught by the branches, and be held firmly in place by the force of the current. Once this happens, it is extremely difficult or even impossible to get out by yourself. Quick and effective action on the part of your companions may be required in order to save your life.

As undercutting of the bank progresses, and the current continues to press on the branches, the tree may be ripped completely free of the bank. It then floats downstream until it gets grounded on a less deep area. Usually the roots of the stump catch first, and the tree swings parallel to the current, with the stump facing upstream. This is known as a “deadhead”, and can also be a dangerous strainer.


Free floating logs and trees will often get hung up at the same obstruction, or even on each other. When this happens a log-jam develops. Log-jams are the worst type of strainers because they can be very large. They can form almost anywhere there is some sort obstruction in the river for the first log to catch on. They are most dangerous when they form in areas where the current is strong, such as the outside of bends, upstream edges of gravel bars, rocks or bridge abutments, or in narrow, constricted areas. Log-jams can be anything from one or two trees, to huge masses of logs hundreds of feet across, and can become unavoidable river obstacles.


Next to strainers, recirculating hydraulics are probably the most dangerous obstacles to be found on rivers. They are also known as holes, souse holes, or keepers. A hydraulic is formed at a location where the water flows over a steep shelf or ledge, like a small waterfall, into a low gradient pool or relatively flat river bottom. The current then boils up to the surface and flows back upstream towards the face of the drop. This water flow pattern acts as a powerful recirculating system that can be impossible for a paddler to escape from. The amount of danger presented by any individual hydraulic is a function of a number of factors; the height and steepness of the drop, the depth and slope of the river bottom at the base of the drop, and the amount and speed of the water going over the drop. Of particular importance is the uniformity of all these things across the extent of the drop.

The most dangerous type of hydraulic is formed by artificially constructed, low head dams or weirs. Because of the very steep faces, and particularly the exact uniformity of these constructions, they can be virtually inescapable drowning machines. However, very dangerous hydraulics can also be formed by naturally occurring ledges and waterfalls in a river. The importance of hydraulics is that you must be able to recognize them and determine what is dangerous and what is not. The ability to read a river and judge what is a dangerous keeper hole and a safe hole requires experience. A ledge across a river may be very dangerous in some areas, but it might also have chutes and breaks in it which are quite runnable. As a general rule, any feature you encounter where the water drops steeply over a rock, shelf, or ledge, should be suspect and carefully assessed. Dangerous hydraulics should be avoided at all times.

Rocks and Boulders

The other major hazard on rivers for canoeists are rocks and boulders. Because rocks are solid, immovable objects which force the flow of current around them, individual rocks and boulders do not form strainers and do not cause the same problems as trees. If you are in the water and swimming, the current will most often carry you around individual rocks. The major danger that rocks represent is the possibility of being caught between the rock and a canoe full of water. A canoe full of water moving at any kind of speed can have several tons of force. This force is more than enough to cause serious injury or death if you are caught between the canoe and a rock, or other obstruction. Once you are caught, and possibly injured, you can remain pinned, as the current continuously presses on the canoe holding you in place. In addition to the possibility of personal injury, rocks also represent the potential for catching canoes so they become broached and pinned around the rock by the current.

However, a group of rocks can also act as a strainer. Small channels between rocks can allow free flow of water but still be small enough to prevent the passage of canoe or paddler. Rocks can be small strainers as well, catching your foot or ankle while the current prevents you from freeing yourself. If your foot becomes stuck and the water is deeper than your knees, the force of the water can easily push you over. The current can make it impossible for you to pull yourself upright, especially if your ankle was broken in the process of becoming trapped. You end up pinned in the water by the force of the current, and can very rapidly drown. It is recommended that you never attempt to stand up in fast water that is deeper than your knees. When the water is less than knee deep, there should be less chance of being pushed over even if you do get a foot stuck.

Entrapment is also possible in places where rocks and cliffs are eroded away and undercut by the current. Swimmers and canoes can be carried and pushed into undercuts, where they may be firmly pinned by the force of the current. Rock undercuts can be very dangerous as they often occur in sites that provide very little, or very difficult and hazardous access to the victim by the rescuers.

Assessing the potential dangers caused by rocks and trees is not easy. Any river with forested banks has the potential for sweepers and strainers to form. The more bends in the river, and the more obstacles present, the more likely it is to have log-jams and sweepers. Poor logging practices along a river can also provide a great deal of fodder for the formation of log-jams.

More from Canoeing Safety and Rescue:

Reprinted with permission from Canoeing Safety and Rescue by Doug McKown and published by Rocky Mountain Books, 2005

  • Updated on Jun 14, 2022
  • Originally Published on Dec 3, 2018
Tagged with: canoe, canoeing, emergency preparedness, rescue, rivers