Sadly in my home state of Indiana 10% of drowning deaths are people who are trapped in the deadly negative current of a “Low head dam”. Nationwide 340 people have fallen victim to the drowning machine since 1980. Low head dams are designed to maintain the water levels during the dry months so that fish and plants are not trapped in dry sections of a river. The construction of one of these dams is simply a large concrete barrier stretching from one bank to the other. The height of this wall of concrete is essential for the average annual water level. When the water level is low there may be absolutely no water flowing over the concrete. Or there may be a few inches of water flow. Once someone has seen the dam in this non threatening state, it’s easy to conclude that there is no danger.
On another occasion this same person without knowledge of the mechanism that is; “The drowning machine”, may fall victim to its awesome power. It’s important to know that most explanations given to the public, in the media, and on the warning signs posted near these dams, are wholly inadequate. There is mention of circular currents and a graphic of a swimmer being tumbled around in the swirling water in the “boil”. It almost looks like fun. It almost seems that the wrong mentality could dismiss these imprecise depictions as a challenge. It may even be a coming of age moment for some, a time to rise to the challenge and prove to all those overly cautious adults that this can be conquered!
Here’s how it works!
Water weighs about 8.6 pounds per gallon. That’s about 62.2 pounds per cubic ft. The water spilling over that concrete dam is falling as fast as the water at the surface above the dam. It’s falling almost as fast as gravity would take it toward earth. No human can oppose the weight, speed and force of that falling water. Objects at the surface of the boil can stay afloat only if they have enough buoyancy and if they are not submerged far enough for the force of the falling water to act on them. A boater may actually pass the bow of a boat over the spill and into the boil. There may be a short time where it seems like the boat will simply drift over the boil. The falling water creates a negative force on the surface downstream from the dam. This negative force draws a boat or a boater back to the boil to be forcefully swamped and or submerged. A boat trapped in the boil may stay on the surface for a short time. Eventually the boat’s long axis becomes parallel to the boil. The boat is unavoidably drawn sideways into the boil where the falling water can easily swamp the boat. The boaters then are at the mercy of the drowning machine. Even with a life preserver, it’s possible to be submerged and taken to the bottom of the river. From there, if you have your wits about you, you might find that the water has fallen as far as it will. As the water falls to the river bottom it then flows downstream. It may be possible to push downstream and maybe even surface. Then as you surface you realize that the boil is again pulling you back toward the spill. And this will repeat as long as you are in the water. Moving water possesses no heat energy so you would be cold and getting colder and more susceptible to hypothermia. Without a flotation device you would be circulated in the boil for longer periods of time and more likely to inhale water, causing drowning immediately. Drowning occurs if you inhale water. You may be conscious or unconscious it matters not. The water in your lungs goes rapidly into your bloodstream, engorging blood cells causing them to burst by the millions. Blood continues to circulate widening the range of damaged cells. The ruptured blood cells don’t carry oxygen anymore so your body and brain suffocate. And you will have spent the last moments of your life in unimaginable fear.
Of all the water craft I’ve seen in photographs caught at a low head dam. Pontoon boats seem to stay afloat in the boil. That doesn’t mean that the boaters weren’t thrown into the boil by the violent rocking near the spill. V-hull speed boats tend to be stuck or damaged above the boil. If they get in the boil they too can be swamped. Canoes and kayaks are the most common on the small rivers where low head dams would be. Portage around the drowning machine is always the best course of action.
The video below was chosen because it only shows a log in the perilous boil. See how the log even though buoyant gets circulated by the current as if it’s a toy.
Pretend for a moment that the log is a person trying to stay afloat on the surface of the water! It is possible to stay afloat but the current will pull a swimmer back to the boil. Once there the force of the water falling over the dam will submerge you. That can be scary, it should be! As the surface water pulls you back to the boil you need to plan your escape! Struggling against the current will not get you out of danger! As frightening as this can be, you must do the following! Let the surface water pull you toward the boil. Make sure you are prepared to hold your breath when the boil pulls you down to the bottom. You can still get away from this current but you must wait for the right time to escape. At the boil you will feel the currents pulling you down, you are two steps from escaping! As you descend keep you hands close to your face and you arms close to your chest to protect you from limbs and rocks. Once you hit bottom you are one step from escaping! The current will then pull you along the bottom and downstream! If you surface too early the boil will circulate you again! You must wait until the current is pulling you up and you must push downstream and toward the surface at a shallow angle. I have seen this work on film. If unsuccessful and you are are being pulled back to the boil, you need to prepare another attempt! It’s tempting to shoot for the surface too soon but you MUST wait until you can forcefully push downstream and up toward the surface at a shallow angle! Get out of the water as soon as you can! Hypothermia and exhaustion can set another trap for you!