Cooperative Fish and Wildlife Research Units Program: Oregon
Education, Research and Technical Assistance for Managing Our Natural Resources

The Willamette River, Oregon, is home to two salmonid species listed as threatened under the Endangered Species Act (ESA), Chinook salmon and steelhead trout. Streamflow along the Willamette River is largely controlled by upstream dams, 13 of which are operated by the U.S. Army Corps of Engineers (USACE) as part of the Willamette Valley Project. In 2008, these dams were found to have a deleterious effect on ESA-listed salmonids (NOAA Fisheries, 2008), resulting in USACE taking mitigatory actions. Included in these actions was setting seasonal streamflow targets at various locations along the river to improve survival and migration of juvenile salmonids. Though these targets were established with the best available information at the time, recent data and models have advanced understanding of river dynamics and allow a more robust analysis of the affect of streamflow on downstream habitat. This study leverages these recent datasets to build high-resolution models of useable habitat for juvenile Chinook Salmon and steelhead trout to assess how this habitat varies with streamflow. Habitat models rely on hydraulic models developed in White and Wallick, 2021, and temperature models developed in Stratton Garvin and others, 2021b, both paired with literature supplied data on habitat preferences. Findings a non-uniform response to streamflow, where habitat in some reaches of the Willamette consistently increases with additional streamflow, while other reaches see habitat decrease when streamflows increase from low to moderate flows. These differences are explained by local geomorphology in each reach, particularly the transition of the river from a relatively dynamic, multi-threaded river to a simpler and more confined river downstream. The upstream reaches generally have more habitat available per unit stream distance than their counterparts, but all reaches display greatest amounts of habitat at the highest streamflows. Finally, results show that water temperature in summer greatly reduces the amount of habitat available to the focal species, particularly downstream of Corvallis. Together, these findings serve to inform flow management by providing a quantitatve assessment of the affects of streamflow on habitat.