Using Phabsim to Investigate Effects of Altered Streamflow on Fish in the Chehalis River, Washington

Due to major flooding in the Chehalis River in recent years, flood reduction strategies are being considered that include construction of a dam the upper watershed. The dam would inhibit high flows in winter and increase base flows during summer. We examined potential impacts to aquatic species from altered streamflow specific to the Chehalis River. We used Physical Habitat Simulation (PHABSIM) to model available aquatic habitat in terms of Weighted Usable Area (WUA) for Pacific lamprey, largemouth and smallmouth bass, speckled dace, largescale sucker, western toad, and mountain whitefish, to identify species for which flow is limiting and estimate habitat changes caused by dam-altered streamflow. For comparison, we also examined WUA for Chinook, coho, and chum salmon, and steelhead which have previously established habitat suitability information. We found some species life stages are limited by flow (spawning Pacific lamprey), so increased summer base flows would increase WUA. Conversely, flow increases would diminish WUA for other life stages (rearing Pacific lamprey). Furthermore, WUA changes were not only dependent on species and life stage, but also river location and time of year. These results will inform future dam-construction decisions, playing a part in cost-benefit analyses and informing impacts to non-salmonid aquatic species.