Low head dams are artificial barriers constructed across a river, characterized by a small drop in water elevation that is often inconspicuous from upstream. These structures are typically built from concrete, rock, or timber and are designed to allow water to continuously flow over the top. Because they create only a minimal change in the river’s surface profile, they can be difficult to spot from a distance. This deceptive appearance of a gentle cascade hides the powerful and hazardous currents that form immediately downstream, making them a serious safety concern for anyone using the waterway.
Structure and Historical Purpose
A low head dam is a river-wide barrier that stands less than 15 feet tall, which distinguishes it from the massive scale of large hydroelectric dams. These structures are sometimes referred to as weirs or run-of-river dams because the water flows freely over the entire crest.
Historically, a vast network of these dams was constructed across U.S. rivers and streams during the 1800s to support industrial and agricultural growth. Their primary functions were to regulate water flow and elevate the upstream water level for specific local needs. Common purposes included diverting water for irrigation or municipal supplies, powering gristmills, and deepening waterways for navigation or recreation. While many of these structures have outlived their original utility, they still remain in place across the country.
The Mechanism of Hydraulic Danger
The primary danger associated with low head dams is the intense recirculating current that forms at the base, often earning the dam the nickname “drowning machine.” As the river water accelerates over the dam crest and plunges down, it creates a submerged hydraulic jump or “roller.” This roller is a powerful, localized current that traps objects and people, continuously pushing them toward the face of the dam and pulling them under the water.
The current is so forceful that it prevents a person from escaping the cycle by swimming, even with a flotation device. Water in the roller becomes highly aerated as air bubbles mix into the turbulence, which significantly reduces the buoyancy of objects caught within it. The violent, churning action can repeatedly slam a victim against the dam structure or against trapped debris, causing trauma and making survival nearly impossible. The “boil line” marks the point where the submerged jet of water surfaces downstream, and only if a person can pass this line are they likely to escape the recirculating flow.
Environmental Impact and Decommissioning
Low head dams have significant long-term consequences for the river ecosystem. These structures act as physical barriers that impede the natural migration of fish and other aquatic species, blocking their movement to upstream spawning grounds. The dams also alter the natural flow dynamics of the river, transforming a free-flowing environment into a pond-like reservoir upstream.
This pooling effect can lead to impaired water quality, as the slower water allows sediment and nutrients to accumulate. The resulting conditions promote excessive algal growth, which depletes the dissolved oxygen levels in the water, sometimes causing fish kills.
As many low head dams are now obsolete, dam removal, or decommissioning, has become a common strategy to mitigate these ecological and safety issues. Alternative solutions, such as constructing rock ramps or bypass channels, are also used to restore the river’s natural gradient and allow for fish passage while maintaining some water elevation control.