A fish ladder, also known as a fishway or fish pass, is a specialized hydraulic structure designed to provide migratory fish with a navigable detour around man-made or natural obstructions in a river system. This engineered channel effectively bypasses barriers such as dams and weirs that would otherwise block upstream movement. The design creates an alternate path where the overall change in elevation is broken down into a series of small, manageable steps. These structures allow fish to complete their life cycles by accessing necessary habitats, often by swimming or leaping through ascending pools until they reach the waterway on the other side of the obstacle.
Why Migration Routes Require Assistance
The life cycle of many fish species, particularly anadromous fish like salmon and steelhead, depends on unobstructed access to both marine and freshwater environments. Anadromous species are born in freshwater, migrate to the ocean to mature, and then return to their natal streams to spawn. Dams, locks, and hydroelectric facilities create physical barriers that fragment this habitat, often preventing adult fish from reaching the upstream spawning grounds required for reproduction.
A tall dam wall presents an insurmountable obstacle because fish cannot jump high enough to clear the crest. Furthermore, the velocity of the water spilling over the top is too powerful to swim against. This blockage can lead to the decline or local extinction of fish populations that rely on movement along the river. By impeding or delaying migration, these barriers impose significant energetic costs and increase the risk of predation, reducing the success of the migratory run.
The Principles of Fish Ladder Design
The fundamental engineering principle behind a successful fish ladder is the controlled dissipation of energy from the flowing water. A river’s elevation drop at a dam is too abrupt for fish to navigate. The ladder’s function is to distribute that total height difference across a long, gentle slope using a series of pools and baffles. This design effectively mimics a less turbulent, natural stream or rapid.
Each segment of the ladder is designed to reduce the velocity and turbulence of the water flow to a speed the target fish species can comfortably swim against. For strong-swimming species like salmonids, the maximum acceptable water velocity within the passage is typically engineered not to exceed 2.0 meters per second. The pools between the steps serve as resting areas. This allows fish to recover the energy expended during the short bursts of swimming required to pass from one pool to the next.
The concept of attraction flow is a consideration for the ladder’s efficacy. Since the dam spans the entire width of the river, fish must be guided to the relatively narrow ladder entrance. Engineers ensure the water exiting the fishway is a strong, recognizable flow that matches the sound and turbulence fish are programmed to follow upstream. This attraction flow must be greater than the flow coming from the dam’s turbines or spillways. This successfully draws the fish away from the main barrier and toward the bypass entrance.
Major Types of Fish Passage Structures
The general principles of flow control are implemented using several distinct structural designs, each suited to different site conditions and target species. The most traditional form is the Pool and Weir ladder, which uses a sequence of small dams, or weirs, to create pools of water. Fish ascend this type by either leaping over the top of the low weirs or swimming through a submerged opening, known as an orifice, between the pools.
A highly effective alternative, particularly where river flows fluctuate, is the Vertical Slot fishway. This design features a narrow, full-height slot in the partition wall between each pool, allowing water to flow continuously from top to bottom. The consistent flow profile allows fish to swim through the slot without leaping. This design also accommodates a wider range of water levels than a traditional weir design.
For lower-head barriers, Denil or Baffle fishways are often employed. These utilize a series of angled baffles within a steep channel to slow the water flow dramatically. The baffles create internal currents that push the water back upon itself, reducing the overall velocity to a manageable level for smaller or weaker-swimming fish.
Nature-like bypass channels are also constructed. These are artificial side-channels made with natural materials like boulders and logs to mimic the complex hydraulics of a natural stream or rapid.