Publication Summary

The Washington State Department of Ecology (Ecology) conducted an instream flow study on the East Fork Lewis River using the Instream Flow Incremental Methodology (IFIM). In addition, we collected Toe-Width information on 13 streams in WRIA 27. These studies provide information about the relationship between streamflows and fish habitat which can be used in developing minimum instream flow requirements for fish in the East Fork Lewis River and the 13 chosen streams in WRIA 27.

The Washington State Department of Ecology (Ecology) conducted an instream flow study on the East Fork Lewis River using the Instream Flow Incremental Methodology (IFIM). In addition, we collected Toe-Width information on 13 streams in WRIA 27. These studies provide information about the relationship between streamflows and fish habitat which can be used in developing minimum instream flow requirements for fish in the East Fork Lewis River and the 13 chosen streams in WRIA 27. For the IFIM study on the E.F. Lewis River one site, composed of eight transects, was chosen. The site was located at approximate River Mile 10.8 at Daybreak County Park. Streamflow measurements and substrate information were recorded at high, medium and low flows. This information was entered into the IFG4 hydraulic model to simulate the distribution of water depths and velocities with respect to substrate and cover under a variety of flows. Using the HABTAT model, the simulated information was then used to generate an index of change in available habitat relative to changes in flow; this index is referred to as "weighted usable area" (WUA). Determination of a minimum instream flows for WRIA 27 will require setting priorities for river reaches, fish species and life stages. Different fish species and life stages exist simultaneously in the river and each has a different flow requirement. There is no single flow that will simultaneously provide optimum habitat for all fish species and life stages.

In addition, minimum instream flows must include flows necessary for incubation of fish eggs, smolt out-migration, fish passage to spawning grounds, and prevention of stranding fry and juveniles. Other variables to be considered include water temperature, water quality, and sediment load. These variables were not addressed in this study.

No instream flow recommendations were made in this report. Those recommendations would be the next step after this study. Instream flow determinations would require an evaluation of the environmental variables listed above combined with the long-range fishery management objectives of the state and federal natural resource agencies and affected Tribes.