Monitoring Effects of Highway Expansion on Stream Habitat and Fish Passage: The Interstate 90 Snoqualmie Pass East Project in Washington State

A major highway project to expand Interstate 90 over Snoqualmie Pass, WA was recently initiated by the Washington State Department of Transportation. The project will include the construction of wildlife crossing structures and will replace several existing stream-crossing structures. A pre-construction study was conducted in 2008 and 2009 as well as in 2010 during the construction phase to assess stream habitat conditions and to monitor fish movement through existing culverts and bridges. We created treatment and control study reaches (~ 200 m in length) in each of nine fish-bearing streams within the project area. Treatment reaches included existing highway stream crossing structures, and control reaches were areas upstream and outside of the influence of existing stream crossing structures. Backpack electrofishing and underwater snorkel surveys were conducted to quantify relative fish abundance in each study reach. Additionally, we implanted PIT tags in individuals in four streams to monitor movements across potential barriers. Aquatic macroinvertebrate samples were taken upstream and downstream of culverts to characterize the invertebrate community on either side of the highway.  Stream habitat was quantified using several metrics including substrate composition, large woody debris counts, and stream channel morphometry. Nine species of fish were identified with native cutthroat trout being the most common species. Adfluvial bull trout inhabited one of the streams, and bridge construction over the stream did not appear to affect their spawning migrations. Bridge construction also did not appear to affect kokanee salmon as they were observed spawning in the stream under the construction site. Juvenile Chinook salmon were found upstream of a culvert in one of the study streams indicating fish passage had occurred during spring flows in each of the study years. Fish movements and stream habitat characteristics will continue to be monitored during and after completion of the highway expansion project to assess the efficacy of the new stream crossing structures.