Instream Flowstudy Guidelines: Technical and Habitat Suitability Issues including fish preference curves


Published: April 5, 2004

Revised: February 2008

Pages: 65


The Washington Department of Ecology (Ecology) is charged both with administering state water rights laws and the federal Clean Water Act. Chapters 90.54 and 90.22 RCW require Ecology to maintain instream flows sufficient to protect and preserve fish and wildlife habitat, scenic and aesthetic values, navigation and other environmental values. Water right permits will be conditioned with instream flow requirements to protect these values.

For projects requiring a federal license or permit involving a discharge into navigable waters, a section 401 Water Quality Certification will be required. Ecology is required to condition certifications to ensure compliance with state water quality standards and to prevent the degradation of existing uses such as fish and wildlife habitat, recreation and aesthetics. On hydroelectric projects Ecology has authority under the Clean Water Act to mandate minimum instream flows.

The Washington Department of Fish and Wildlife (WDFW) recommends instream flows to be conditions of water rights or Clean Water Act Section 401 certification (issued by Ecology) and hydroelectric power project licenses or exemptions (issued by the Federal Energy Regulatory Commission - FERC). When a major water project is planned, WDFW and Ecology request that the project proponent conduct an instream flow study in consultation with the agencies to provide adequate information on which to base an instream flow recommendation or requirement. WDFW defines a major water project is a project that:

  1. diverts at least 1.0 cubic feet per second (cfs), and
  2. changes flow by at least 10% of the monthly 90% exceedence flow at any point along the stream channel.

The purposes of WDFW's instream flow recommendation are:

  1. to avoid reduction of habitat for fish and wildlife;
  2. to ensure fish passage upstream and downstream; and
  3. to maintain macrohabitat features of the stream channel.

To address fish habitat, WDFW and Ecology request use of an instream flow method which estimates the amount of habitat available at different flows which might occur with and without the proposed project. In most cases, this request is met by using the Physical Habitat Simulation (PHABSIM), part of the Instream Flow Incremental Methodology (IFIM), following quality control and model limitations consistent with the Instream Flow Study Guidelines.

A consultation documentation form is provided on the following page. Consultation with appropriate WDFW and Ecology personnel and adherence to the attached IFIM study guidelines during all phases of the instream study is crucial to completion of the study. We request documentation of each step of consultation by signature of a WDFW Habitat Management Program employee on the form on the following page.