How Fish Passages Help Trout Move Past Hydropower Dams

Fish passages—also called fishways or fish ladders—are engineered structures that let trout and other migratory fish swim upstream past hydropower dams. They work by creating a continuous, slower-flowing channel or series of pools that fish can navigate, bypassing the dam’s turbine and spillway. This guide covers the main types of fish passages, how they function, and the key steps involved in designing and operating one so trout can successfully migrate.

What You’ll Need

• Understanding of local fish species and their swimming abilities (e.g., trout jump height and burst speed)
• Hydrological data: river flow rates, water levels, and seasonal variations
• Site survey: dam dimensions, tailwater depth, and head differential (height difference between upstream and downstream water)
• Engineering design software or consultation with a fish biologist and hydraulic engineer
• Construction materials: concrete, rock, or prefabricated metal sections
• Maintenance equipment for debris removal and flow control

Step-by-Step

1. Identify the target species and their migration needs. Trout typically migrate upstream to spawn in spring or fall. Determine the local trout species (e.g., rainbow, brown, cutthroat) and their maximum swimming speed (usually 1–2 m/s for short bursts) and jumping ability (often 0.5–1 m). This informs the design of the fishway’s slope and pool dimensions.
2. Measure the head differential and flow. The head is the vertical drop between the upstream reservoir and downstream tailwater. For a fish ladder, the total head is divided into smaller steps (usually 15–30 cm per pool) so trout can ascend gradually. Also measure the minimum and maximum river flows to ensure the fishway operates year-round.
3. Choose the fish passage type. The most common for trout are:
   – Pool-and-weir: a series of pools separated by low weirs with notches; trout leap from pool to pool.
   – Vertical-slot: a channel with vertical slots that maintain a consistent flow; trout swim through without jumping, good for lower energy.
   – Denil: a steep, baffled chute that slows water; compact but tiring for fish.
   – Nature-like: a bypass channel mimicking a natural stream with rocks and gravel; best for habitat but needs more space.
4. Design the fishway dimensions. For pool-and-weir, each pool should be at least 1.5–2 m long and 1–1.5 m wide, with a water depth of 0.3–0.6 m. The weir notch should be wide enough (0.3–0.5 m) to pass the expected flow. For vertical-slot, the slot width is typically 0.3–0.5 m, and the slope is 1:10 to 1:20 (vertical to horizontal).
5. Calculate the flow through the fishway. Use the formula for flow over a weir: Q = C × L × H^1.5, where Q is flow (m³/s), C is a discharge coefficient (≈1.8 for sharp-crested weirs), L is the weir length (m), and H is the head over the weir crest (m). Ensure the total fishway flow is between 1% and 5% of the river’s average flow—enough to attract trout but not so much that it’s costly or turbulent.
6. Position the entrance correctly. The fishway entrance must be near the dam’s tailwater, where trout naturally gather. It should discharge a strong, distinct current (attraction flow) that is easy for fish to detect. For trout, the entrance velocity should be 1–2 m/s, slightly higher than the surrounding river flow.
7. Construct the fishway. Build the structure using concrete, rock, or prefabricated sections. Ensure the bottom is rough (e.g., with gravel or baffles) to reduce water velocity and provide resting areas. For nature-like channels, use natural materials like boulders and logs to create pools and riffles.
8. Install flow control and monitoring. Add a gate or valve at the upstream end to regulate water flow. Install a fish counter (e.g., a camera or electronic counter) at the exit to monitor passage success. This data helps adjust flows seasonally.
9. Test and adjust. During the first migration season, observe fish behavior. If trout are not using the fishway, increase the attraction flow, reduce turbulence, or modify the entrance location. Ensure the pools are not too deep or too shallow—trout need to rest between leaps.
10. Maintain regularly. Clear debris from the weirs and slots, check for erosion, and adjust flows after high-water events. A clogged fishway is useless. Annual maintenance is essential.

Common Mistakes

• Designing for the wrong species. A fishway built for salmon (which jump high) may be too steep for trout. Always use trout-specific swimming and jumping data.
• Incorrect flow rates. Too little flow won’t attract fish; too much creates velocities trout cannot overcome. The attraction flow must be just right.
• Entrance placed too far from the dam. Trout may not find the entrance if it’s not near the main flow or if the tailwater is too turbulent.
• Ignoring seasonal water levels. Fishways must function at both low and high flows. If the entrance is exposed during low water, fish cannot enter.
• No maintenance plan. Debris accumulation and sediment can block the fishway within weeks. Regular upkeep is not optional.

Tips

• Use a nature-like bypass when possible. Trout often prefer a natural channel over a concrete ladder, and it provides additional habitat.
• Monitor with a camera. A simple underwater camera at the exit can confirm whether trout are actually passing, not just entering.
• Coordinate with dam operations. If the dam peaking (releasing large pulses of water), the fishway flow may need to be increased during those times to maintain a stable attraction flow.
• Consider a fish screen at the turbine intake. This prevents trout from being drawn into turbines, complementing the fishway.
• Consult local fish biologists. They can provide region-specific data on trout behavior and migration timing.

Troubleshooting

• Trout are not entering the fishway. Check the entrance flow: it should be a strong, visible current. Increase the attraction flow by opening the gate wider. Also, ensure the entrance is not blocked by debris or silt.
• Trout enter but cannot ascend. The pools may be too deep (making it hard to leap) or the water velocity too high. Reduce the slope by adding more pools, or widen the slots to slow the water.
• Fishway is dry during low flow. The upstream water level may be below the fishway intake. Install a low-flow notch or a small pump to maintain minimum flow.
• Fishway is overwhelmed during floods. High flows can create dangerous turbulence. Install a flow-limiting weir at the entrance to cap the maximum flow through the fishway.
• Trout pass but then get injured. Check for sharp edges or concrete spalling. Smooth all surfaces and add a resting pool at the exit.

Frequently Asked Questions

How long does it take for trout to learn to use a fishway?

Most trout will find and use a properly designed fishway within the first migration season, often within days. However, if the flow or entrance is not attractive, it may take longer. Monitoring helps identify issues early.

Can trout jump over a fishway weir?

Yes, trout can jump up to about 1 meter vertically, but they prefer to swim through slots or over low weirs (15–30 cm). Design the weirs low enough that jumping is not required for every pool.

Do fishways work for all trout species?

Most trout species can use fishways if the design matches their swimming ability. However, some trout like lake trout (which are not strong jumpers) may need a vertical-slot or nature-like channel instead of a pool-and-weir.

How much water does a fishway use?

Typically 1–5% of the river’s average flow. For a small river with 10 m³/s average flow, that’s 0.1–0.5 m³/s. It’s a small trade-off for fish passage.

Can a fishway be added to an existing dam?

Yes, many older dams are retrofitted with fishways. The challenge is fitting the structure into the existing site, but it’s often feasible with proper engineering.

Do fishways help downstream migration?

Fishways are primarily for upstream migration. Downstream migrants (like juvenile trout) need separate measures, such as spillway passage or turbine screens, to avoid injury.

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