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Why dams disrupt fish migration
Many migratory fish move between different parts of a river during their life cycle to reach spawning, feeding or rearing habitat. A dam can cut off those routes. NOAA Fisheries says more than 2 million dams and other barriers in the United States block upstream fish migration (NOAA Fisheries).
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Fish do not all migrate in the same way or at the same life stage. Passage plans therefore need to account for the target species and whether adults, juveniles or other life stages must get past the barrier.
How fish ladders let fish pass upstream
A fish ladder, or fishway, provides a route around or over an obstruction. NOAA Ocean Service defines it as “a structure that allows migrating fish passage over or around an obstacle on a river” (NOAA Ocean Service).
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A common ladder uses a series of pools with water flowing through them. Fish locate the current near the entrance, then swim or leap from pool to pool until they reach water above the dam. The arrangement and water conditions are designed around the barrier, river flow and fish expected to use it (NOAA Ocean Service).
What happens when a ladder is not suitable?
Other fishways and bypass channels
Some sites use nature-like channels or other engineered passageways. Federal guidance discusses nature-like fishways as an option where complete barrier removal is not possible (U.S. Fish and Wildlife Service). The design still has to match the species and site rather than assume one structure will serve every fish.
Trap and haul
Where a ladder is impractical, fish can be attracted into a capture facility, then transported in tanks or specialized vehicles and released upstream. NOAA describes this as a way to move fish around barriers when a direct route is not feasible (NOAA Fisheries).
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Dam removal
If removal is feasible, taking out the barrier can reopen the river without asking fish to find and use a passage structure. NOAA reports that its fish-migration projects have opened nearly 9,000 miles of rivers and streams through hundreds of projects; that is a cumulative tally of river access, not a measurement of fish population recovery (NOAA Fisheries).
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A fishway’s presence does not guarantee that every fish will find it or pass through successfully. In a 2012 review indexed by the U.S. Geological Survey, C. M. Bunt, T. Castro-Santos and A. Haro examined 19 monitoring studies covering 26 anadromous and potamodromous species in six countries, compiling 101 records. The reported mean passage efficiencies differed by fishway type:
| Fishway type | Mean passage efficiency in the review | Median passage efficiency |
|---|---|---|
| Nature-like | 70% | 86% |
| Denil | 51% | 38% |
| Vertical-slot | 45% | 43% |
| Pool-and-weir | 40% | 34% |
These are pooled historical results from the reviewed studies, not predictions for a particular dam. Reported ranges were broad, in some cases from 0% to 100%. Species, attraction to the entrance, slope, elevation change, local conditions and study methods all affect the result. The review authors also noted that available data often could not support firm design recommendations (U.S. Geological Survey).
Some fish need especially different designs. A USGS-indexed synthesis found that passage facilities designed for other fish have rarely worked well for sturgeon. Successful upstream approaches reviewed for sturgeon included large entrances, full-depth guidance, large lifts or wide passageways without tight turns (U.S. Geological Survey).
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Fish must also get back downstream
Upstream passage alone does not complete a migratory life cycle. Juveniles or returning fish may need to travel downstream through spillways, juvenile collectors or screened water passages with bypass systems. NOAA calls downstream passage a major challenge in reintroduction programs (NOAA Fisheries).
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One example: fish passage at Mirabel Dam
At Mirabel Dam on California’s Russian River, a renovation combined three features: a fish screen to keep fish out of overflow ponds, a reconstructed ladder for adult upstream passage, and a viewing chamber for counting fish and public observation. NOAA reported a project cost of approximately $12 million in December 2016; that historical figure is not a current cost estimate. Bob Coey, then NOAA Fisheries West Coast Region Branch Chief in Santa Rosa, described the three components as improving the screen, rebuilding the ladder and adding the viewing chamber (NOAA Fisheries).
What determines the right solution?
Choosing a passage approach involves more than selecting a ladder style. Relevant factors include:
- Which species and life stages need passage, and when they migrate.
- Whether fish can locate and safely use the entrance and route.
- River flows, seasonal timing, slope and the dam’s elevation change.
- Available space and the dam’s physical configuration.
- How juveniles and other fish will pass downstream.
- What monitoring is needed to determine whether the system works.
- Whether removing the barrier is feasible instead of building or operating passage infrastructure.
These considerations explain why fish passage is designed for particular rivers and barriers, rather than provided by a single universal solution (U.S. Fish and Wildlife Service; NOAA Fisheries).
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