Low head dams are engineered structures built across the width of a river or stream designed to allow water to continuously flow over the top of the barrier. These structures are typically less than 15 feet tall, distinguishing them from larger, reservoir-creating dams. Often referred to as weirs or run-of-river dams, their presence reflects the nation’s older industrial and agricultural history where water power was harnessed locally.
Defining Physical Features and Function
Low head dams are characterized by a short height relative to the waterway’s width, and are often constructed from concrete, rock, or masonry. During normal or high flow, the top edge, known as the crest, is completely submerged as water passes over it. This continuous overflow defines their function within the river system.
The primary purpose of these dams is to raise the upstream water level, creating a pool for various human needs. Historically, they powered mills and small industries, with tens of thousands built across the eastern United States. Today, functions include maintaining water levels for municipal intakes, diverting water for irrigation, or supporting small-scale hydroelectric power generation. By spanning the entire channel, the structure directs all river flow over the crest, making it a complete barrier.
The Unique Hydraulic Danger
The most hazardous characteristic of a low head dam is the formation of a recirculating hydraulic roller immediately downstream of the crest. As water drops over the dam, it plunges to the riverbed and is forced to reverse direction, creating an intense backflow toward the dam face. This phenomenon, known as a submerged hydraulic jump, traps objects and individuals in a continuous, washing-machine-like cycle.
The water in this turbulent zone becomes highly aerated, meaning air bubbles are mixed throughout the volume, which significantly reduces buoyancy. This loss of flotation capacity can be as much as one-third, making it exceedingly difficult for a person wearing a life jacket to stay afloat.
This danger is often underestimated because the dam’s low height makes it appear benign, especially from the upstream perspective where only a subtle horizon line may be visible. High flow conditions intensify the recirculating forces, increasing the speed and power of the backflow. Even strong swimmers cannot overcome the velocities and turbulent forces generated by this hydraulic condition.
Environmental and Sediment Disruption
Low head dams significantly alter the natural ecological balance of a river by impeding the movement of both aquatic life and geological materials. The structure acts as a complete barrier, preventing the upstream migration of fish and other aquatic species needed for spawning or feeding. This blockage, known as a fish passage issue, can fragment populations and reduce the overall biodiversity of the river system.
The dam also disrupts the river’s natural sediment transport process, which is essential for maintaining riverbed and bank stability. Sediment, such as gravel and sand, accumulates on the upstream side because the slower-moving water can no longer carry the material. This buildup can smother natural riverbed habitat and change the depth and temperature of the water behind the dam.
Conversely, the river section immediately downstream experiences “sediment starvation” because the dam blocks the natural supply of material. The water flowing over the crest is largely clear of sediment, leading to increased erosion of the downstream riverbed and banks. This erosion destabilizes stream banks and alters the downstream habitat.