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Short answer
Fish passage is the biological and engineering practice of enabling migratory and resident aquatic species to navigate artificial barriers such as hydroelectric dams, weirs, and road culverts. Defined by hydraulic criteria from NOAA Fisheries and USFWS, passage facilities balance water velocity, resting pool volume, and bypass screening to restore river connectivity across all life stages.
Definition and scope
Fish passage encompasses physical structures, behavioral guidance systems, and operational protocols that allow fish to move past barriers between spawning grounds and rearing habitats. Artificial river barriers—including hydropower dams, water diversions, and road culverts—fragment river networks and disrupt migratory life cycles.
In the United States, fish passage authority at non-federal hydropower projects is codified under Section 18 of the Federal Power Act (16 U.S.C. § 811). This statute authorizes the Secretary of Commerce (through NOAA Fisheries / NMFS) and the Secretary of the Interior (through USFWS) to prescribe mandatory fishways during FERC licensing. Projects must also satisfy Endangered Species Act mandates and state water quality certifications under Clean Water Act Section 401. In research and engineering, fish passage links fluid dynamics, fish behavior, and hydrology research.
How it works or how it is used in practice
Fish passage engineering divides facility design into two distinct regimes: upstream passage for adult spawners and downstream passage for emigrating juveniles and spent adults. Authoritative standards, such as the NOAA Fisheries West Coast Region Anadromous Salmonid Passage Facility Design Manual and FAO Fisheries Technical Paper No. 419, define hydraulic criteria for each:
Upstream Passage Hydraulics: Upstream facilities dissipate hydraulic barrier head through stepped increments:
- Attraction Flow: Migrating fish orient against currents via positive rheotaxis. Entrances must provide attraction velocities of 4.0 to 8.0 feet per second (fps) located adjacent to turbine discharges or spillways where fish congregate.
- Hydraulic Head Drop: In pool-and-weir and vertical-slot fishways, water surface drops between pools are regulated. NOAA criteria set a maximum drop of 1.0 foot (0.3 m) per weir for adult salmonids, and 0.5 to 0.75 feet for weaker swimmers like shad or sturgeon.
- Energy Dissipation Factor (EDF): To prevent exhaustion in resting pools, turbulence must not exceed 4.0 ft-lb/s/ft³ (approx. 191 W/m³) for adult salmonids. Sizing coordinates pool geometry with discharge statistics modeled via USGS StreamStats.
Downstream Passage and Screening: Downstream migrants risk turbine entrainment hazards: mechanical blade strike, rapid decompression (barotrauma), and shear stress. Protection systems utilize two mechanisms:
- Physical Exclusion Screens: Wedge-wire or perforated plate screens physically block entry into penstocks. NOAA standards require approach velocities ≤ 0.33 to 0.40 fps, with sweep velocities exceeding approach velocity to direct migrants into bypass flumes.
- Surface Bypass Collectors: Because smolts migrate near the surface, surface collectors guide fish into low-velocity flumes that discharge safely into the tailrace.
Types or related concepts
Fish passage designs vary according to dam height, species swimming stamina, and river hydrology:
| Passage System | Direction | Operating Principle | Engineering Criteria | Source |
|---|---|---|---|---|
| Pool-and-Weir Fishway | Upstream | Stepped pools separated by weirs; fish burst-swim or leap across pools. | Head drop ≤ 1.0 ft/weir; EDF ≤ 4.0 ft-lb/s/ft³; pool depth ≥ 3.0 ft. | NOAA Design Manual § 4 |
| Vertical-Slot Fishway | Upstream | Narrow vertical slots in baffles maintaining velocity through water level shifts. | Slot velocity V = √(2gΔh); Δh = 0.8–1.0 ft; self-regulating with pool depth. | FAO Paper 419 § 4; NOAA Manual § 4 |
| Denil & Steeppass | Upstream | Steep chutes (10–20% slope) with angled baffles turning flow upon itself. | High velocity; for strong swimmers; resting pools required every 15–30 ft. | NOAA Manual § 4; USFWS Criteria |
| Fish Lifts & Elevators | Upstream | Mechanical hopper traps fish and hoists them vertically into the reservoir. | Suited for high dams (>60 ft) where ladders are unfeasible; requires aerated tanks. | FAO Paper 419 § 4; 16 U.S.C. § 811 |
| Exclusion Screens | Downstream | Physical wedge-wire or perforated screens blocking penstock entry. | Approach velocity ≤ 0.33 fps; sweep velocity > approach velocity. | NOAA Manual § 11; FAO Paper 419 |
| Behavioral Deterrents | Downstream | Sensory guidance arrays using strobe lights, sound, or bubble curtains. | Supplemental guidance; effectiveness depends on turbidity and species response. | FAO Paper 419 § 4; DOE WPTO R&D |
| Nature-Like Fishways | Both | Engineered rock ramps and bypass channels replicating natural stream beds. | Low slope (1–3%); passes multispecies assemblages and benthic invertebrates. | Interagency NLF Guidelines |
Where it matters for renewable energy and lab/field research
Fish passage is a vital regulatory and technical requirement for renewable hydropower facilities and aquatic ecological research.
FERC Relicensing and Mandatory Prescriptions: Hydropower projects in the U.S. renew operating licenses every 30 to 50 years. Under Section 18 of the Federal Power Act (16 U.S.C. § 811), NOAA Fisheries and USFWS exercise statutory authority to mandate upstream and downstream fishways. Licensees must construct, operate, and maintain these facilities at their own expense. Compliance connects directly with state water quality certifications and broader environmental impact assessments.
Fish-Friendly Turbine Engineering: Through the Hydropower Testing Network (HyTN), the DOE Water Power Technologies Office (WPTO) finances development of advanced turbines, such as the Alden turbine. These systems employ thick leading blade edges, lower rotation speeds, and tight blade-to-housing tolerances to reduce shear and blade strike, achieving direct survival rates exceeding 98%.
Biological Telemetry and Field Protocols: Field researchers evaluate passage efficiency through advanced monitoring technologies:
- PIT Telemetry Arrays: Implanted RFID tags detected by antenna loops inside fishways record individual travel times, attraction delay, and passage success.
- Acoustic Telemetry (JSATS): 3D tracking arrays monitor juvenile movements in forebays, verifying whether hydraulic conditions guide fish into bypass portals.
- Imaging Sonar (DIDSON/ARIS): High-frequency sound beams provide near-video imagery in turbid water, enabling non-invasive observation of fish behavior and predator avoidance. Researchers deposit and share these ecological and hydrologic datasets through platforms like CUAHSI HydroShare.
Common misunderstandings
Key operational misunderstandings surround fish passage systems:
- Misunderstanding 1: An upstream fish ladder provides complete river passage. Ladders only address upstream migration. If downstream migrating smolts lack bypass conduits and pass through turbines, cumulative downstream mortality can collapse the population.
- Misunderstanding 2: Salmon passage criteria work for all aquatic species. Salmonids are exceptional leapers and burst swimmers. Species such as American shad, sturgeon, and lamprey cannot jump weirs and require continuous submerged slots or lower velocities.
- Misunderstanding 3: Behavioral deterrents substitute for physical screening. Sound and light arrays are supplemental aids whose effectiveness drops with turbidity and habituation. Regulators require physical exclusion screens for water intake protection.
- Misunderstanding 4: Hatcheries eliminate the necessity of fish passage. Hatcheries do not replace the genetic diversity of wild fish, nor do they restore the longitudinal nutrient transport that anadromous fish provide to upstream river ecosystems.
Frequently asked questions
What is the difference between upstream and downstream fish passage?
Upstream passage provides ladders, vertical slots, or elevators enabling adult spawners to ascend past dam heights. Downstream passage uses exclusion screens and surface flumes to route juvenile smolts safely around turbines and spillways.
What authority prescribes mandatory fishways at U.S. hydropower dams?
Section 18 of the Federal Power Act (16 U.S.C. § 811) authorizes the Secretary of Commerce (via NOAA Fisheries) and the Secretary of the Interior (via USFWS) to prescribe mandatory fishways for FERC-licensed dams.
What hydraulic limits govern resting pools in fish ladders?
NOAA Fisheries criteria specify a maximum head drop of 1.0 foot per weir for adult salmonids and an Energy Dissipation Factor (EDF) not exceeding 4.0 ft-lb/s/ft³ to avoid muscular exhaustion.
What is an Alden turbine in fish passage engineering?
An Alden turbine is a fish-friendly hydro turbine designed with DOE WPTO support that utilizes thick, curved blades and lower rotational speeds to achieve over 98% fish survival during downstream passage.
Sources
- NOAA Fisheries, West Coast Regional Office. Anadromous Salmonid Passage Facility Design Manual. Published June 2022; Addendum #1 February 22, 2023; updated February 24, 2023. Accessed November 11, 2026. https://www.fisheries.noaa.gov/resource/document/anadromous-salmonid-passage-facility-design-manual
- Food and Agriculture Organization of the United Nations (FAO). Dams, fish and fisheries: Opportunities, challenges and conflict resolution. FAO Fisheries Technical Paper No. 419. Edited by Gerd Marmulla. Section 4: ‘Downstream Fish Passage Facilities’ by Michel Larinier. Rome, 2001. Accessed November 11, 2026. https://www.fao.org/4/y2785e/y2785e00.htm
- U.S. Department of Energy (DOE), Water Power Technologies Office (WPTO). Hydropower Environmental Mitigation and Fish Passage Research Initiatives. Accessed November 11, 2026. https://www.energy.gov/eere/water/water-power-technologies-office
- Legal Information Institute (LII), Cornell Law School. 16 U.S. Code § 811 – Operation of navigation facilities; rules and regulations; penalties (Federal Power Act Section 18: Fishways). Accessed November 11, 2026. https://www.law.cornell.edu/uscode/text/16/811
