The I-90 corridor passes through a relatively narrow bottleneck of public land in Washington’s Cascade Mountains, joining extensive areas of habitat to the north and south that are protected within national forests, wilderness areas, and national parks. As such, it is a critical zone for Pacific Northwest wildlife populations. The I-90 Snoqualmie East Project (SPE), encompassing a 25-km stretch of I-90, aims to improve safety and ecological connectivity through creation of 27 wildlife crossing structures with associated habitat features, fencing, and revegetation. Washington Department of Transportation (WSDOT) began construction on the first structures in 2011 and plans to finish the last ones in 2029. As of fall 2022, 11 structures were finished, 11 were poised to begin construction, and work on the remaining five is planned to begin in 2026. The structures include two overpasses (one completed and one planned), as well as 25 underpasses, ranging from relatively small box culverts (minimum width 2m) to multi-span bridges (maximum width 283m). Use of wildlife crossing sites before, during, and after construction has been intensively monitored via wildlife cameras since 2008, yielding over 18,000 structure-days of pre-construction monitoring and over 18,000 structure-days of post-construction monitoring across all sites. Camera images examined to date have yielded over 35,000 detections of identifiable wildlife, including evidence of over 16,000 successful crossing events. Detections include at least 23 mammalian species, ranging in size from elk (Cervus canadensis) to weasels (Mustela spp.) and American pika (Ochotona princeps). Generalized linear mixed-models are being used to assess patterns in wildlife use of the structures, including associations with structure characteristics, habitat features, traffic, weather, interspecific interactions, time of day, season, and geographic location. Preliminary results show strong, species-specific differences in the use of various structure types, seasonal and daily patterns, and association with environmental factors such as traffic, snow depth, and recent presence of humans. Preliminary results also show a reduction in wildlife-vehicle collisions following construction of structures in the study area. Results are relevant to many SPE partners, including state agencies (WSDOT and Washington Department of Fish and Wildlife), federal agencies (US Forest Service and US Fish and Wildlife), and numerous non-governmental organizations. They will be used to guide adaptive management of the project area and adjustments to future construction plans.
wildlife crossing structure