> Water Resources Home > Instream Flows In Washington > The Science Behind Instream Flows

The Science Behind Instream Flows

Flow Measurement Methods

IFIM/PHABSIM and Toe-width are the two most commonly used flow measurement methodologies used in Washington state.  For an overview of both, go to:

• Instream flow study methods used in Washington State (22kb pdf)

Instream Flow Incremental Methodology / Physical Habitat Simulation (IFIM/PHABSIM)  IFIM is the most commonly used method for determining how much water fish need. IFIM is a series of computer-based models that quantify the amount of fish habitat at different flow levels in a river or stream.  By matching those depths and velocities to fish preferences for those factors, the model will tell you how many square feet of habitat will be available at different flow levels.  Physical Habitat Simulation (PHABSIM) is the subset of IFIM used extensively in Washington state.

• Q & A: An Overview of the Instream Flow Incremental Methodology

Toe-width Methodology  Toe-width is a quick habitat assessment tool used primarily for small streams.  The measurement from the toe of one stream bank to the toe of the other is put into an equation and an estimation of flows needed for salmon and steelhead spawning and rearing is derived.

• Preferred stream discharges for Salmon Spawning and Rearing in Washington State
• Preferred stream discharges for Steelhead Spawning and Rearing in Washington State

Instream Flow Study Guidelines

Instream Flow Study Guidelines: Technical and Habitat Suitability Issues  This technical report gives guidance on designing and conducting an instream flow study which estimates the amount of habitat available at different flows.  This document will also 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 and Assessments

• Links to IFIM studies and watershed assessments

River and Stream Flow Monitoring

The Stream Hydrology Unit of Ecology's Environmental Monitoring and Trends Section provides timely and accurate instantaneous stream flow for various instream actions with River and Stream Flow Monitoring.  These instantaneous stream flows are an integral element in determining the available instream resources (for fish) and the available out-of-stream resources (for people). The driving forces behind the increasing need for accurate and timely stream flow data are the federal Endangered Species Act (ESA), local salmon recovery efforts, and an increased focus on water resource management in Washington state.

Ecology Home >  Water Resources Home > Instream Flows In Washington > The Science Behind Instream Flows