Protecting stream flows

Instream flows are a specific flow level measured at a specific location in a given stream. Seasonal changes cause natural stream flows to vary throughout the year, so instream flows usually vary from month to month rather than one flow rate year-round.

Once instream flows are set, when actual streamflows drop below the adopted instream flow levels, junior water rights (those established after the instream flow) can be temporarily interrupted to keep the water in the stream. There are ways for new uses to prevent interruption:

• Hooking up to an existing water system that has “senior” water rights (that predate the rule).
• Transferring a senior water right to the new use.
• Mitigating to prevent impacts to streams.

The natural environment and communities rely on healthy streamflows:

• To support sustainable groundwater levels and surface water levels in wetlands, lakes, and ponds.
• To provide habitat for fish, support wildlife, and help maintain healthy ecosystems.
• To support recreation, stock watering, navigation, and scenic and aesthetic values of natural settings.
• To maintain water quality, including temperature, dissolved oxygen, and the concentration of impurities.
• To preserve existing farming, industry, and residential water supplies.

Many consider Washington a water-rich state, but water shortages often happen when and where it is needed the most. Many streams around the state often fall below critical levels, especially in late summer. In recent drought years, a large portion of the state’s streams dropped to record-breaking lows. Population growth and effects of climate change continue to increase the stress on this crucial and finite resource.
 

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

See more information:

• An introduction to: Instream flows and instream flow rules
• Instream flow study methods used in Washington State
• Instream flow study guidelines: Technical and habitat suitability issues

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.
 

An instream flow is not the lowest amount of water that has occurred in the stream. Protecting only the lowest flows would over time result in fish always living in drought conditions. Fish populations would drop as streamflow and available habitat dropped. Eventually, the fish population would collapse.

State law is clear that instream flows are to protect and preserve fish and other instream resources over the long-term.
 

If the instream flow number is high relative to the average summer streamflow, it means that the stream will provide more fish habitat in wet years than in dry ones.

Protecting the occasional “good water year” is needed to preserve a healthy population of fish. If we want to protect the habitat available in those good wet years, then the instream flow needs to be set at that higher flow level.
 

Washington water law is based on the prior appropriation doctrine, or “first in time, first in right.” Each water right has a priority date, which determines its ranking in relation to all other rights from the same (or connected) source. In times of water shortage, older (senior) water right holders have the right to the full measure of their water right before later (junior) water rights are served.

Instream flows have a priority date like all other water rights. Therefore, adopting instream flows into rule has no effect on existing rights that predate the rule. This includes water rights established through water right permitting and existing uses of a “permit-exempt” well. (See RCW 90.44.050.)

As most cities and towns hold enough water rights to serve many years of future growth, further development in urban and suburban areas is generally unaffected by instream flows. Adopting new instream flows is most likely to affect those wishing to build homes in rural areas that would rely on a permit-exempt well.

Although we have attempted to provide provisions in each instream flow rule to allow some level of future rural development, recent court decisions have restricted many of these provisions. We continue to work with the Legislature to find workable solutions to resolve this issue. Meanwhile, it is increasingly difficult to find adequate water supplies for new uses in many areas of the state — with and without instream flows.
 

Most groundwater in Washington is hydraulically connected to nearby lakes and streams. In fact, during much of the year, groundwater discharge may make up most of the water in a stream.

When groundwater is pumped and water is put to use, this often leads to a decrease in streamflow. Many factors influence where and how much a groundwater use may affect a stream, such as the nature of the aquifer, the distance between the well and the stream, the well depth, and the type of water use (for example, lawn watering consumes much more water than in-house use that discharges to a septic system). However, at least some portion of the well water used in a home typically has a negative impact on nearby streams.