Fish passage is one of the first questions asked about in-water work, and for most bank protection the honest answer is that it is not the relevant concern.
What actually blocks passage
Three barrier types:
A drop. A vertical or near-vertical step a fish cannot ascend. Created by weirs, grade control structures, perched culverts, and any structure with a hydraulic fall.
A velocity barrier. A reach where flow exceeds what a species can swim against for the distance required. Created by constrictions, smooth high-gradient chutes, and culverts sized for hydraulic capacity rather than passage.
A depth barrier. Water too shallow to swim through, often at the same low flows when passage matters most.
Each is a property of the channel geometry and hydraulics, not of what the bank is faced with.
Why bank armor is usually not the issue
A revetment sits on the bank. It does not span the channel, it does not create a drop, and unless it substantially narrows the section it does not create a velocity barrier.
What it can do, if designed carelessly:
Narrow the channel. Material projecting into the flow raises velocity through the section. On a marginal reach that could matter for passage, and it is also a floodplain issue. See floodplain permitting and no-rise.
Simplify the margin. Natural banks have undercuts, roots, and pockets that provide refuge and low-velocity resting areas. A smooth armored face has fewer. A rough, permeable, unit-placed surface retains more of that complexity than a flat wall, which is one of the arguments in hard armor vs a wall, environmentally — but it is still a change.
Remove overhanging vegetation during construction, which is cover.
Those are real effects and they are about habitat quality rather than passage.
Where passage genuinely comes into it
Grade control structures and low-head weirs. These create a drop by design, and that drop is a passage question. The answer bank covers how units are transitioned around a grade-control structure or low-head weir; the passage design of the structure itself belongs to the engineer of record and usually to a fisheries specialist.
Culvert repairs. A perched outlet — where the culvert invert sits above the downstream bed, often because a plunge pool has scoured it out — is a classic barrier. Fixing outfall scour can raise the downstream bed and improve passage, which is a genuine co-benefit worth stating in a submittal. See culvert outfall scour.
Anything that changes bed elevation over a length, since it changes the longitudinal profile.
The construction-phase question
Frequently the bigger concern, and it is a timing question rather than a design one.
In-water work windows exist largely for this: to avoid spawning, incubation, migration, and rearing periods. Working inside the window is the primary control. See in-water work windows.
Dewatering requires fish salvage in most jurisdictions — capture and relocation before the area is drained, which is a specialist operation with its own authorisation. A method that avoids dewatering avoids that entirely, and it is worth stating as a minimisation argument. See what dewatering really adds.
Turbidity affects fish directly, which is why the limits exist. See construction turbidity control.
Noise, where pile driving is involved. Not applicable to unit placement.
Where consultation takes over
Where a listed species is present, this stops being a general question and becomes a Section 7 consultation, with effect determinations, an action area, and possibly a biological opinion. See Endangered Species Act consultation.
Getting to a “not likely to adversely affect” conclusion is easier for work that stays in the window, avoids dewatering, minimises footprint, and keeps a permeable rather than sealed margin. Those are the same measures that make sense anyway.
What we do not have
No data on fish use of installations, no passage studies, and no quantified habitat outcomes. What the site says about colonisation is qualitative for that reason. See what colonises an installation and when not to use rock bags.
If your project needs a passage assessment, that is a fisheries engineer’s work.