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Why fish passage is also a water-management decision
A dam, weir, intake, or flood gate can interrupt fish movement, but it may also serve a necessary purpose: delivering irrigation water, controlling flood levels, or maintaining navigation depths. A fish-passage project therefore cannot be judged only by whether fish can get around a structure under one set of conditions. It must consider whether fish can move when they need to, whether the water system still meets its obligations, and whether the river and its habitat function as intended.
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The relevant questions are connected. A screen can keep fish out of a canal but does not necessarily allow them to move upstream past the diversion. A bypass channel can provide a route around a barrier but may need a replacement weir to sustain irrigation deliveries. A flood gate can protect land while closing off a migration route at the wrong time. Even a successful passage structure can fail to deliver lasting benefits if flows, sediment, or surrounding habitat remain unsuitable, or if no one is responsible for maintenance.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallThere is no universal fishway or screen specification that resolves these trade-offs. Designs have to account for target species, body size and life stage, direction of movement, flow range, the structure’s purpose, and local regulation. Regional guidance is useful, but its criteria should not be transferred to another jurisdiction or species without evaluation.
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Start with the fish, the barrier, and the river
Define which fish need passage
Passage performance is species- and flow-dependent. A facility suited to strong-swimming adult salmonids may not work for weaker swimmers, resident fish, juveniles, or fish moving downstream. Identify the species and life stages that need access, when they move, and whether the project must support both upstream and downstream movement. The design objective should describe those needs rather than simply call for a “fish pass.”
Flow matters as much as the structure. Water depth, velocity, and the range of conditions at which a route remains usable can determine whether fish can find and traverse it. NOAA’s 2022 West Coast fish-passage manual focuses on anadromous salmonids; it is regional guidance, not a universal design standard. USFWS Region 1 guidance considers a broader set of Pacific Northwest fish issues. Project teams should identify which standards and permitting requirements apply to their site.
Understand what the existing structure must do
Document the barrier’s purpose and operating pattern: irrigation diversions and delivery requirements, flood levels and gate rules, navigation water levels, other water users, and any lawful abstractions. Also establish where fish currently encounter the structure and what happens at different flows. A solution that improves passage but compromises a required delivery or flood function may not be workable; conversely, preserving every existing operating practice without examining its effect on fish can foreclose better options.
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Set outcomes that can be checked
Translate broad goals into criteria that can guide design and later monitoring. These may include the species and life stages expected to pass, the range of flows and seasons when passage should be available, delivery or flood-control performance, and habitat or river-process objectives. The project should also define how those outcomes will be assessed and what operational or physical changes will follow if results fall short.
Compare the main approaches
Options range from restoring the river by removing a barrier to retaining a structure and adding or changing facilities. Their suitability depends on site conditions; the categories below are not interchangeable prescriptions.
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| Approach | What it can address | Key planning question |
|---|---|---|
| Barrier removal and river-process restoration | Removes the obstacle and can restore fluvial processes as well as a route for fish. | Can the structure be removed while meeting water-user, flood-risk, land, and other site requirements? |
| Fishway or rock ramp | Provides a route where an impounding structure must remain. | Can the design serve the local species and flow conditions without undermining the structure’s purpose? |
| Bypass channel | Routes fish around a barrier; it can be paired with a replacement water-control structure. | Is there enough suitable land and water, and how will the channel operate and be maintained? |
| Screened diversion with a return route | Helps prevent fish from entering an intake, canal, pipe, or penstock and directs them back to the river. | Does the system protect fish at the intake while maintaining safe return and passage through the main waterway? |
| Gate-operation changes or fish-friendly gates | Can improve the time and conditions during which fish can move past flood or tide gates. | Can operations preserve flood protection while keeping a usable route open during movement periods? |
For every viable option, compare passage for the target species, sizes, life stages, and movement directions; performance across current and projected future flows; effects on irrigation deliveries, other users, and navigation levels; flood performance; habitat and sediment processes; land and construction constraints; resilience; operating and maintenance demands; and the monitoring and intervention plan. Capital cost alone is not enough to distinguish alternatives if their operating responsibilities and long-term performance differ.
When removal is possible, consider the river process—not just the route
Barrier removal may restore more than a path around an obstacle. USFWS policy emphasizes barrier removal and restoration of fluvial processes where possible, and says full process restoration is expected to have a higher likelihood of long-term success than structures that require continuing operation and maintenance. That is a policy-level rationale, not a guarantee that removal is feasible or appropriate at every site.
Where a barrier has to remain, a fishway, rock ramp, or bypass may be more appropriate. New Zealand’s Department of Conservation (DOC) suggests a full-river-width natural rock-ramp fishway where feasible. If that is impractical, its guidance identifies a V-shaped broad-crested weir with a baffled surface or a bypass channel as alternatives. Those are options to assess against local fish, flows, space, and infrastructure needs—not designs to copy without site-specific work.
If a structure remains, agree in advance who is responsible for inspection and maintenance, which adaptive actions are available, and what conditions trigger intervention. USFWS policy calls for such a management plan when removal is not possible. This makes ongoing responsibilities part of the project decision rather than an afterthought.
Make irrigation screening part of a complete passage system
A screen prevents entrainment; a bypass returns fish
Fish screens are intended to keep fish from being drawn into diversion structures, canals, pipes, or penstocks. A screen by itself does not restore movement past a barrier, and a poor intake location or approach flow can leave fish exposed to risk even when the screen is present. Design also has to consider velocity at and past the screen, where fish go after encountering it, and whether the main waterway remains passable upstream.
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USFWS Region 1 notes that screened diversions commonly use a bypass channel to direct juvenile fish back to the river. The return route should allow fish to reach the main channel without injury. The screen and bypass should be assessed as one system, including how they perform at the flows when fish are present.
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Use numerical criteria only in their proper context
New Zealand DOC guidance gives illustrative intake criteria: through-screen water velocity of no more than 0.1 m/s; sweeping velocity past the screen greater than 0.5 m/s and at least equal to approach velocity; screen openings no larger than 2–3 mm; and a bypass that returns fish undamaged to the main stem. These are criteria from that guidance, not universal standards. Applicable limits can vary with jurisdiction, species, and facility, so the project team should confirm the governing requirements rather than treating these figures as a blanket specification.
Plan for shutdowns and changing bypass flow
Operations can undo the benefit of a carefully designed screen and return route. USFWS Region 1 warns that rapid reductions in bypass flow can strand fish in pools or shallow, disconnected habitats. Fish left behind may face poor water quality, predation, or drying. The passage and habitat plan should therefore address diversion shutdowns and flow-ramping schedules, including how quickly bypass flow may be reduced and what actions protect fish as operations change.
Coordinate flood gates with fish movement
Flood and tide gates can block passage depending on how long they remain open, the size of the opening, water depth and velocity, and when they close. Operating rules are therefore part of passage design, not merely a matter for operators after construction.
New Zealand DOC recommends automated active gates that close only when water reaches a critical level where feasible. Where automation is not feasible, it recommends fish-friendly self-regulating gates kept open as long and as far as possible, particularly during periods of upstream fish movement. A site-specific plan must still account for flood-risk requirements and the conditions under which gates need to close.
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Reconnecting floodplain habitat can support habitat and natural river processes, but it interacts with flood-risk management, land, water allocation, and local permissions. In England, Environment Agency RPS 374 gives a specific regulatory example for certain nature-recovery abstractions: a qualifying bypass on a main river requires an applicable flood-risk activity permit before abstraction; the bypass must maintain fish-passage flow and return abstracted water to the same watercourse. Under the specified conditions, written agreements with landowners, riparian interests, other water users, and the owner of the impounding structure are also required. This is an England-specific example, not a rule for other jurisdictions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Design for future flows and long-term operation
Hydrology can change over a structure’s life. NOAA recommends considering projected future flows from the beginning of design, rather than treating them as a late-stage check. Teams should ask whether the passage route and water-control system will continue to function across the expected range of flows, and how changing conditions could affect delivery, flood operation, and fish access.
Monitoring should be tied to the project’s stated success criteria. The plan needs to say who collects and reviews information, who can authorize operational changes, and what action follows when passage or infrastructure objectives are not met. Adaptation might involve changing how a gate is operated or addressing a maintenance problem; the exact response depends on the project and its design. Without named responsibilities and intervention criteria, monitoring risks documenting a problem without creating a route to resolve it.
Planning should also account for the ongoing work required by a retained structure. Maintenance, adaptive actions, and decision authority need to be assigned before implementation. The alternative is not simply “build” versus “do not build”: it is a choice between solutions with different construction, operating, ecological, and long-term management obligations.
What two projects show—and what they do not
Lower Yellowstone: pair the fish route with water delivery
The U.S. Bureau of Reclamation and U.S. Army Corps of Engineers evaluated passage options at the Lower Yellowstone Intake Diversion Dam. A screened headworks structure entered operation during the 2012 irrigation season. After design, constructability, sustainability, and information about pallid sturgeon movement prompted reconsideration of the proposed rock ramp, the agencies selected a bypass-channel alternative in 2016.
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The selected package included a planned 11,150-foot bypass channel for pallid sturgeon, a replacement weir to maintain water deliveries to the Lower Yellowstone Project, and an adaptive management and monitoring plan. Reclamation describes itself as leading implementation of the plan in coordination with USACE and USFWS, and describes an eight-year plan. The project page also includes historical expected completion dates; those dates should not be read as confirmation of current completion or performance.
The practical lesson is that the fish route, the delivery structure, and the plan for monitoring and adjustment can be designed as a package. The project does not establish that a bypass channel—or an 11,150-foot channel—is the right choice at another site.
River Wey: fish access alongside navigation and flood objectives
The Environment Agency’s River Wey scheme describes refurbished historic weirs and new fish passes alongside continued navigation water levels and flood-risk objectives. The agency says the scheme aims to reduce flood risk for 388 residential and 236 non-residential properties, and that its first phase will open over 100 km of river to fish. These are reported objectives and outcomes for this named scheme, not typical rates for other projects.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsThe agency project page also gives a scheme cost estimate close to £13 million. It is a project-page estimate that may be dated, not a current general cost benchmark. The useful comparison is how the scheme combines functions; its stated figures should not be used to predict what another fish-passage project will cost or achieve.
A practical sequence for choosing and delivering a project
- Define the passage objective. Identify target species, body sizes and life stages, movement direction, and the seasons and flows when passage is needed.
- Map the water and infrastructure obligations. Record irrigation deliveries, other abstractions and users, flood-control rules, navigation requirements, land constraints, and the barrier’s current operating pattern.
- Establish ecological and hydraulic conditions. Evaluate the existing barrier and routes under relevant flows, including intake velocities, gate openings, bypass conditions, and habitat connections.
- Develop alternatives as whole systems. Consider removal, a fishway or rock ramp, a bypass with any needed replacement control structure, a screened intake and return route, gate changes, or combinations of these.
- Compare consequences and responsibilities. Assess passage and habitat objectives alongside delivery and flood performance, constructability, maintenance, resilience, monitoring, and who can make operational changes.
- Confirm permits and agreements. Apply the requirements of the project’s jurisdiction. Where relevant, secure permissions and agreements with landowners, water users, riparian interests, and structure owners before construction or abstraction.
- Set monitoring and intervention criteria. Decide how success will be judged, who will review results, and which operational or physical actions follow if objectives are not met.
This sequence does not select a design in advance. It ensures the project compares ecological performance and water-management duties on the same terms, and carries its operating and maintenance commitments into implementation.
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