Low head dams are structures built across a river channel that span the full width of the waterway, creating a change in water elevation. These dams are typically small, often standing less than 15 feet tall. Unlike larger dams, these structures are designed for water to flow continuously over the crest, preventing them from controlling or storing large volumes of water. This constant overflow generates a powerful and potentially deadly hydraulic phenomenon downstream, earning them the nickname “drowning machines.”
Defining Physical Features and Function
Low head dams are characterized by their simple, uniform structure, typically made of concrete or rock, built from bank to bank across a stream or river. The top edge, known as the crest, is usually submerged or barely visible, allowing water to pass freely over it. This design creates a small, tranquil pool of water immediately upstream and an abrupt drop on the downstream side.
The primary historical purpose for constructing these structures was to raise the water level upstream for various local needs. These included diverting water for irrigation, supplying municipal and industrial water, or providing flow to power mills and factories. They were also used for grade control to stabilize the riverbed and maintain navigation depths. Thousands of these structures remain across the country, even though many have outlived their original function.
The Dangerous Recirculating Current
The most dangerous characteristic of a low head dam is the recirculating current formed immediately downstream, often called a “hydraulic jump” or “backroller.” As water flows over the crest, it picks up speed and plunges to the riverbed below, then the momentum forces the water back upstream toward the dam face. This continuous, powerful cycle creates a pocket of churning water that can trap objects and people against the dam, similar to a washing machine.
The force of the back-flow is powerful enough to trap and hold even strong swimmers. Compounding the danger, the violent turbulence mixes a significant amount of air into the water. This aeration drastically reduces the water’s density, which in turn reduces the buoyancy of anyone caught in the current by up to one-third. This loss of flotation makes it nearly impossible to stay above the surface, even when wearing a life jacket.
Impact on River Ecosystems
A defining characteristic of low head dams is their impact on the surrounding river ecosystem. The structures act as a physical barrier that restricts the movement of fish and other aquatic organisms. This fragmentation prevents species from migrating upstream to reach necessary spawning or feeding grounds, effectively isolating populations and reducing genetic diversity. The effect is particularly damaging to migratory species that require long stretches of free-flowing water.
Furthermore, the dams significantly alter the river’s natural sediment transport characteristics. The impoundment area upstream causes water velocity to slow down dramatically, which leads to the deposition of fine sediments. This accumulation smothers the natural stream bottom, degrading habitats for aquatic insects and mussels, while also altering water quality. Downstream of the dam, the sediment-starved water can cause increased erosion of the riverbed and banks.