How Does a Salmon Get Past a Dam? A Staircase Made of Water
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A series of stepped pools lets migrating fish climb past a barrier by jumping or swimming from one to the next. They work considerably less well than the numbers of fish passing through them suggest.
What the structure does
A dam or weir presents a vertical drop that migrating fish cannot pass, which cuts them off from spawning grounds upstream and eliminates the population above it. A passage structure divides that drop into a series of small steps, each one a pool separated from the next by a low barrier with an opening, so a fish moves upstream by swimming or leaping from pool to pool, resting between efforts. The critical requirement is that the water velocity through each opening stays within what the species can swim against, and that the drop between pools stays within what it can ascend, both of which vary enormously between species and between adults and juveniles.
The main designs
Several arrangements exist and each suits different fish and different drops:
- •Pool and weir, the classic stepped pools with water spilling over each barrier
- •Vertical slot, with a full-depth gap letting fish pass at any depth they prefer
- •Denil, a sloping channel fitted with baffles that slow the water without pools
- •Rock ramp, a roughened slope imitating a natural rapid, which suits the widest range of species
- •Fish lifts, which raise fish mechanically where the drop is too great for anything else
- •Trap and truck, where fish are caught, loaded into tanks and driven upstream
Why they underperform
Counting the fish that pass through gives a flattering picture, and studies following individuals tell a different story. Many fish never find the entrance, since a structure delivering a modest flow alongside a dam discharging enormous volumes is hard to locate, and attraction flow is the single biggest design problem. Those that find it may take days to ascend, which delays spawning and exhausts energy reserves that the fish cannot replace, since several migrating species do not feed. Cumulative losses compound along a river with many barriers, so a passage rate of eighty per cent at each of eight dams delivers less than a fifth of the fish to the top. And most designs were developed for strong-swimming salmon and work poorly for eels, lampreys and weaker species.
The fish that cannot use them
Designs built around salmon suit a strong leaping fish and suit almost nothing else, which is why passage for other species has become a separate field. Eels ascend by climbing wet surfaces rather than swimming against flow, and they need brush or bristle-lined ramps that give them something to grip, which conventional structures do not provide. Lampreys attach with their mouths and rest, so they need resting places and rounded corners rather than sharp turns. Shad and sturgeon swim strongly and do not leap, and they refuse plunging flows that salmon accept. Many small river species are weak swimmers that simply cannot pass anything with a significant velocity. Modern practice therefore designs for the whole community present rather than for one commercially valuable species.
The problem going the other way
Downstream passage receives far less attention and kills more fish. Juveniles migrating to sea must pass the same barriers, and the obvious route is through the turbines, which kills a substantial proportion by physical strike and by rapid pressure change. Screens and bypass channels divert them where they are fitted, and fitting them to existing installations is expensive. Slack water in reservoirs above dams slows the journey and exposes young fish to predators concentrated there. The growing recognition that no passage arrangement restores a river fully has driven a shift towards removing obsolete barriers outright, with several thousand removed in the United States and Europe, and rivers recolonised by fish within a remarkably short time afterwards.
The takeaway
Stepped pools divide an impassable drop into steps a fish can ascend, with water velocity through each opening being the critical constraint. Counting fish that pass flatters the result, since many never find the entrance, ascent takes days that cost irreplaceable energy, and losses compound along rivers with many barriers. Downstream passage through turbines kills more, and removing obsolete barriers outright is now the preferred answer.