A fish ladder, also known as a fishway, is a specialized structure built to help aquatic animals bypass man-made obstructions in a river or stream. These structures provide a detour route, allowing migratory fish to swim or leap past obstacles like dams, weirs, and culverts. The fundamental engineering goal is to dissipate the immense energy of the falling water while maintaining a continuous flow that attracts fish to the ladder’s entrance. The result is a gentler, stepped channel that enables fish to continue their upstream or downstream movement.
The Biological Necessity of Fish Passage
The construction of dams and other river blockages fragments aquatic habitats, severing the connectivity necessary for the life cycles of many fish species. Many of these species are diadromous, meaning they must migrate between freshwater and saltwater environments to complete their existence. This movement is essential for both reproduction and feeding.
Anadromous species, such as salmon and striped bass, spend their adult lives in the ocean but must return to freshwater streams and rivers to spawn. Conversely, catadromous species, such as the American eel, live primarily in freshwater as adults and migrate downstream to the ocean to reproduce. When dams impede these journeys, fish cannot reach their ancestral spawning grounds, leading directly to population decline and the loss of entire fish runs.
Engineering and Design Types
Fish ladders function by creating a channel with a low slope, spreading the vertical drop of a dam over a long horizontal distance. The engineering challenge is to reduce the water velocity to a manageable speed that fish can swim against. This requires a strong enough flow, called the attraction flow, to guide them toward the entrance, mimicking the natural current.
One of the oldest and most common designs is the Pool and Weir fishway, which consists of a series of small, interconnected concrete pools separated by low walls called weirs. Fish swim into a pool, rest briefly, and then either leap over the weir or pass through a submerged opening, repeating the process until they reach the reservoir level. The energy of the plunging water is dissipated by turbulence within each pool, creating a manageable flow for the ascending fish.
A variation is the Vertical Slot fishway, which replaces the weirs with a narrow, continuous opening or slot in the dividing wall between pools. This design allows fish to swim through the entire structure without leaping and better accommodates natural fluctuations in water levels. The slot maintains a relatively constant flow velocity regardless of the water height, improving upon the pool-and-weir style.
The Denil fishway employs a sloped channel fitted with a series of closely spaced internal baffles. These baffles are angled to redirect the current, creating a complex flow pattern that significantly reduces water velocity and turbulence. This system allows fish to swim through quickly, often without resting pools, and is generally more compact than a pool-and-weir system. For very high dams, mechanical solutions like Fish Lifts or Elevators collect fish in a tank at the base and lift them over the barrier.
Assessing Performance and Limitations
The effectiveness of a fish ladder depends on the interaction between the design, the environment, and the specific fish species it serves. Performance is measured by passage efficiency—the percentage of migratory fish that successfully enter and exit the ladder. While a well-designed ladder may achieve high passage rates, overall effectiveness is often limited by poor entrance location.
Fish are naturally attracted to the strongest flow of water, often near the dam’s turbines or spillways, rather than the ladder entrance. If the attraction flow is too weak or improperly positioned, fish may never locate the passage, wasting valuable energy. Most designs were historically optimized for strong swimmers (primarily adult salmon), making them less effective for smaller species or those with weaker swimming abilities, such as American shad or sturgeon.
Environmental factors also impose limitations, including water temperature (which can stress fish) and variable river flows (which affect the ladder’s hydraulics). The cumulative effect of multiple dams is challenging; if a fish encounters a 90% passage rate at each of five dams, the overall survival rate to the spawning grounds drops significantly. Concentrated fish waiting to use the ladder can also attract predators, reducing the number of successful migrants.