Low-Head Dam Hydraulics and Safety
Drowning Machines Powered by Strong Recirculating Currents
Low-head dams are a particularly dangerous river feature that padders need to avoid. Hydraulic currents caused by these structures are perilous to boaters and swimmers.
The only obstacle or feature on a river more dangerous to a paddler than a strainer is a hydraulic. Strainers are typically tree branches or trunks lying across a river that allow water to flow through them, but trap solid objects like kayaks. Hydraulics form when water flows over a flat object or shelf into a pool or flat riverbed. Water flows down towards the bottom and recircuates, curving back towards the obstruction.
Low-Head Dams - The most perilous hydraulic is manmade, caused by the construction of low-head dams. These dams are common features in many rivers, often built to divert water for irrigation purposes. Water flowing over a low-head dam forms a backwash that can trap paddlers against the face of the dam. Water pouring over the top of the dam then forces them under again. The resulting continuous cycle has deadly consequences for river runners. The strength of the hazard depends on the height of the obstruction, distance the water falls, shape of the river bed, velocity of the river and volume of water passing over the dam. Many of these hydraulics require outside assistance for escape.
Hazards - Low-head dams offer many hazards for paddlers. Often they trap debris like tree branches, logs and old tires. Under periods of increased water flow, the circulation cell extends further downstream, making the hazard that much more dangerous. Faces of a low-head dam are nearly vertical and their concrete surfaces are smooth. A person trapped against the dam face is unlikely to climb it. Water pouring over the dam will be highly aerated. The presence of these bubbles reduces buoyancy by a third and reduces the effectiveness of Personal Flotation Devices. Pools formed by low-head dams are attractive to swimmers and present the same danger of drowning.
Prevention and Avoidance - Low-head dams are difficult to see while approaching them from upstream. Their small size can make them appear harmless. A paddler may not realize the danger present until it is too late. River runners should be alert for warning buoys or signs that may warn of a dam ahead and watch for any smooth lines across the water on the horizon. Concrete retaining walls along a river bank can give the presence of a dam away. Pre-trip planning by consulting published topographic maps of your river route can identify the presence of these hazards. The ability to read the river and recognize features like hydraulic takes time and experience. The safest option is pull up ahead of a known low-head dam and portage around. Reenter the water well downstream from the dam.
Education - In May of 2007, four people drown in Minnesota after being trapped by a low-head dam. Tim Smalley of the Minnesota Department of Natural Resources (MDNR) warns, "While the water around dams can look quite peaceful, at some times of the year, they can become extremely dangerous." In response to the hazard, MDNR has researched methods of rescue that can be used by first responders and published a number of brochures on the subject including one titled The Drowning Machine.
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