Low head dams are human-made structures spanning rivers or streams, allowing water to flow continuously over their crest. Despite their simple appearance, these dams conceal significant dangers, posing a serious threat to anyone on or near the water. Their unassuming nature masks powerful, deceptive currents that can trap and injure individuals.
The Hydraulic Recirculation Phenomenon
The primary danger of low head dams stems from hydraulic recirculation, often called the “drowning machine.” As water flows over the dam’s crest, it plunges downward and curls back on itself, creating a powerful, continuous recirculating current at the base of the structure. This turbulent area, known as a hydraulic or backroller, can trap objects and individuals, pulling them repeatedly back towards the dam face.
A visible “boil line” marks where this recirculating current meets the downstream flow. Water in this area appears to bubble intensely, and anything caught upstream of this line is pulled back into the dam’s powerful grasp. Even strong swimmers find it nearly impossible to escape this continuous force, as the current can pin them against the dam, making forward movement or swimming against it ineffective.
The danger is further amplified when the dam is submerged or partially submerged due to higher water levels. A “submerged hydraulic jump” forms, creating a strong upstream-directed current that is less visible from the surface. Water caught in this hydraulic becomes highly aerated, significantly reducing a person’s buoyancy and making it harder to stay afloat. This combination of powerful, recirculating forces and reduced buoyancy makes escape difficult, even for those wearing personal flotation devices.
Additional Dangers of Low Head Dams
Beyond hydraulic recirculation, low head dams present several other hazards. The turbulent waters below these structures often trap large debris, such as trees, branches, and trash. This hidden debris can create unseen obstacles, posing entanglement risks and potentially battering anyone caught in the current.
The structures themselves, many of which are old and in disrepair, can also pose threats. Crumbling concrete or exposed rebar can injure or trap individuals who come into contact with the dam. The water around low head dams can be deceptively cold, especially during colder seasons. Cold water immersion can quickly lead to cold water shock and hypothermia, incapacitating a person and making escape even more challenging.
Swift currents exist not only within the hydraulic but also upstream and downstream of the dam. These currents can unexpectedly pull individuals or watercraft towards the dam from upstream or create challenges for those trying to navigate away from the structure downstream. Surfaces on and around the dam can also be slick with algae, posing a slip-and-fall risk for anyone attempting to walk on or near the structure.
Recognizing a Low Head Dam
Identifying a low head dam from a distance is a key step in preventing incidents, as their deceptive appearance often leads to underestimation of their danger. From upstream, these dams can appear as a smooth, uniform line stretching across the river, resembling a small, innocuous ripple or waterfall. This consistent horizon line, where the water appears to drop evenly, is a common visual cue.
Another indicator is a sudden change in the water’s surface, such as glassy or flat water just before the drop. Downstream, the presence of stationary whitewater or boiling water that does not dissipate can signal the turbulent recirculating current. While some dams may have warning signs or buoys, many do not, or signs may be obscured or damaged, making visual and auditory recognition important. A noticeable murmur or roar of rushing water can also serve as an auditory warning of an approaching dam.
Staying Safe Around Low Head Dams
Prioritizing safety around low head dams involves proactive measures. Before embarking on any recreational activity on a waterway, check maps and local information to identify dam locations. This allows individuals to plan their route and avoid hazardous areas.
A fundamental safety practice is to “portage” around all low head dams: exit the water well upstream, carry boats and gear around the dam, and re-enter safely downstream. Always wearing a properly fitted personal flotation device (PFD) is crucial, as it provides buoyancy, though its effectiveness can be reduced in the aerated water of a dam’s hydraulic. Educating friends and family about the dangers of low head dams and sharing safety knowledge helps foster a culture of awareness. If someone becomes trapped in a low head dam, the immediate action should be to call emergency services (911); attempting a rescue without proper training and equipment can result in additional victims.