How Is Fish Passage Engineered Around Dams and Barriers?

The target species list is the design basis. Anadromous salmon and steelhead, shad, sturgeon, eels — each brings different body sizes, swimming speeds, leaping ability, and behavioral quirks, and the passage must serve the weakest swimmer among the targets, not the strongest. Design flows bracket the migration season's hydrology: the passage must operate across the range of river flows the fish actually migrate in, not just at a single design discharge. Behavior shapes the details as much as physiology. Fish follow flow, avoid sudden darkness and turbulence, and stage in tailrace eddies before attempting ascent — so entrance placement relative to the powerhouse and spillway discharge, lighting in enclosed sections, and resting pools along the route all come from understanding how the species moves. I bring fisheries biologists into the design team at the concept stage, because hydraulic engineers designing fishways without biologists is how you get structures optimized for water instead of fish.

Technical fishways — pool-and-weir, vertical-slot, and Denil ladders — suit moderate dam heights where a structured staircase fits the site. Vertical slots handle fluctuating tailwater better than weirs; Denils pack the climb into a shorter footprint. Each pool's drop, volume, and energy dissipation are sized to the species, and turning pools and resting areas break long ascents into manageable reaches. Taller dams and constrained sites call for other answers. Fish lifts collect migrants in a holding pool and mechanically raise them past the dam; navigation-like fish locks cycle them through; nature-like bypass channels recreate a stream around the barrier, serving weaker swimmers and non-salmonids beautifully where land allows. In every case the entrance hydraulics decide success: attraction flow of the right magnitude and velocity, positioned where migrating fish actually travel, competing successfully against powerhouse and spillway discharges that would otherwise lead them astray.

Juveniles heading downstream face turbines, spillways, and forebay delay. The design guides them with surface-oriented collectors — juveniles travel near the surface — paired with screens that exclude them from turbine intakes and bypass conduits that return them to the river below the dam. Spillway passage is evaluated for gas supersaturation from plunging flow, which can harm fish as surely as a turbine blade, and operations are tuned so passage routes are watered when the migrants are moving. Performance verification closes the loop. Counting facilities document passage numbers, telemetry reveals where fish hesitate or turn back, and the data feeds adaptive adjustments — tweaking attraction flows, modifying entrances, adjusting operations. I write performance monitoring into the project as a design phase, not a research luxury, with success criteria agreed before construction. The river's verdict is the only one that counts, and the design stays open until the fish deliver it.

• Set hydraulic criteria from the weakest target species' swimming ability and migration timing.
• Design entrance attraction flow to compete with powerhouse and spillway discharge flows.
• Provide resting pools and appropriate drops along the full ascent route length.
• Give juveniles a turbine-free downstream route with screened intakes and safe bypasses.
• Verify with fish counts and telemetry, and adjust the design until passage works.