When deciding what flow levels to protect, a lot of the discussion centers on fish needs since there are state laws that specifically call for protecting fish. We have found that if the fish are doing well, then generally other instream resources are too. Fish needs are also easier to measure than some other instream values. This is why we usually start with instream flow or “fish-flow” studies to determine instream flows.
In Washington, the four most common instream flow study methods are:
- Instream flow incremental methodology (IFIM)
- Toe-width
- Wetted width (a.k.a. wetted perimeter)
- Hatfield and Bruce
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Data from streamflow studies are used together with the known streamflow needs of key fish species at varying stages of their lives (i.e., spawning, incubating, rearing, juveniles, and adults, as well as in-migration and out-migration for anadromous species).
Once fish biologists have determined which seasonal flow levels in a stream provide the best fish habitat, other considerations are brought into discussions with local agencies, tribes, stakeholder groups, and the public.