The Science Behind Instream Flows

Resources and Studies   This webpage provides basic information about the scientific methods used in Washington to determine instream flow levels.

You can also refer to Ecology publication titled Focus on Instream Flow Study Methods Used in Washington State.

Terms in green are linked to a glossary.

Instream Flow Study Methods

Photo of Fish Spawning

One of the most important water management tools we have to protect stream flows is to set stream flow levels by regulation, called “instream flows.”

How are instream flow levels determined?  You will find that a lot of discussion centers on fish needs.  This is because fish are considered an “indicator species” – if the fish are doing well, then generally other instream resources are too.  And fish needs can be more easily quantified by existing methods than other instream values.  This is why fish studies are often the basis for determining instream flow numbers.

The Science Behind Instream Flows  

In Washington State, the four most commonly used stream flow study methods are:

See a brief side-by-side summary and comparison of these four methods.

Instream Flow Incremental Methodology (IFIM) / RHABSIM

Photo of Biologist Measuring Streamflow

IFIM is used nationwide and is accepted by most water resource managers as the best available tool for determining the relationship between stream flows and fish habitat (living space).  The method is relatively time-consuming and expensive, and is therefore best used for rivers where there is a lot of water and therefore a lot of controversy.

IFIM is a series of computer-based models which calculate how much fish habitat you gain or lose as you increase or decrease stream flow.  It is based on the understanding that fish prefer water with a certain depth and velocity (how fast the water is flowing).  These preferences vary for different species of fish, and for each of their life stages.

Experts collect data on the river at specific representative sites and this in turn is put into a group of models called RHABSIM (Riverine HABitat SIMulation), a part of IFIM.  Data on fish habitat preferences are then entered, and the final calculation results in “Weighted Usable Area” (WUA), the stream flows specific fish species at specific life stages prefer, based on the depths and velocities they prefer.

For a more detailed explanation of IFIM, refer to Ecology publication Q-WR-95-104.


Photo of Biologist Taking Toe-width Measurements

Most of the over 250 instream flows set by rule in Washington State were done with Toe-width.  Quick Toe-width estimates may be adequate for management purposes and results compare favorably with those from IFIM.

Toe-width is a simple measurement and calculation.  Based on the width of the stream, it provides a stream flow number useful for protecting salmon and steelhead spawning and rearing. The “toe” of the stream refers to the point in a stream where the side of the stream meets the bottom. The toe-width measurement is the distance from the toe of one streambank to the toe of the bank across the stream channel.

In this approach, the toe-width of a stream is measured and put into an equation to get a reasonable stream flow number for protecting fish.

Compilations of preferred stream flow levels for spawning and rearing for salmon and steelhead provide a basis for Toe-width studies.

For more information, refer to these two scientific references on how Toe-width measurements were created:

Wetted Width (Wetted Perimeter)

Wetted Width example graph

This method is used to help determine an instream flow recommendation for low flow periods, based on adequate fish rearing and migration flows.  The wetted perimeter is the portion of the channel that is “wet.”  The method assumes a connection between reducing wetted width and the loss of fish habitat.  It involves measuring from water’s edge to water’s edge over a number of site visits.

Graphs are developed to show the relationship between actual stream flow and wetted width.  There can be a clear point where the graph levels out – when you’re no longer rapidly gaining width with increases in flow.  This point is referred to as the breakpoint or “inflection point.”  It indicates the stream flow level at which habitat is stable, and therefore where sufficient food is protected to maintain the existing fish population at an acceptable level.  This point becomes the stream flow recommendation.

The technique was modified by Dr. Hal Beecher (Washington Department of Fish & Wildlife) who added a measurement of width where depth is at least 6 inches.  This indicates where the width is adequate for fish passage and rearing habitat.  Young salmon are seldom found in water shallower than 6 inches.

Hatfield and Bruce

Hatfield and Bruce developed a series of equations to estimate the stream flow that maximizes the weighted usable area (WUA) based on previous IFIM studies, for up to four life stages of certain salmonids (such as salmon and trout).  This method can be done entirely from the office; no field work is required.  The user looks up the yearly average stream flow, longitude and latitude for the river, and enters these into equations on a calculator or computer.

This method is a quick, high-level technique using IFIM study results.  It does not replace a detailed analysis of a watershed and river.

Instream Flow Study Guidelines

This technical report provides guidance on designing and conducting an instream flow study to estimate the amount of fish habitat available at different flow levels.  This document will help consultants and resource managers evaluate the impacts of altered flow on instream resources, including fish habitat and production, for the purpose of making decisions or recommendations on water use.

Completed Studies

River and Flow Monitoring

Instantaneous stream flow information is available through the Stream Hydrology Technical Coordination Team of Ecology’s Environmental Monitoring and Trends section.  You can also refer to the USGS site for flow data.

These stream flow data are an important part in determining how much water is needed for instream resources, and how much water is available for out-of-stream uses.