The I-90 Snoqualmie Pass Project in Washington State involves reconstruction and lane widening of about 25 km of the existing highway and includes the construction of multiple wildlife crossing structures. The majority of these are culvert replacements with new bridges over stream crossings. Stream restoration work has involved culvert removals/replacements and construction of new stream channels that flow into a lake. Study objectives were to monitor the colonization of the new stream channels by fish and macroinvertebrates as well as to assess fish passage following the restoration work.
Native westslope cutthroat trout (Oncorhynchus clarki lewisi) and cedar sculpin (Cottus schitsuumsh) occur in most of the streams that cross the highway in the project area, but little is known about their populations or movements within and between streams. Fish in the study streams were sampled by electrofishing in newly constructed channels and at upstream unaffected areas to allow for a BACI study design. Fish densities and size structure were monitored in each stream and Passive Integrated Transponder (PIT) tags were implanted in cutthroat trout to monitor growth and movements during recapture events over several years (2010-2018). Benthic macroinvertebrates (BMI) were sampled with a surber sampler to determine abundance and community diversity.
Colonization by fish was documented at five sites where new stream channels were built following culvert removal and bridge construction. A single tagged cutthroat trout was documented to have migrated from a new channel in one stream into a new channel of another stream and several individuals in one stream had moved upstream above a previously impassable barrier following its removal and channel reconstruction. Indices of biotic integrity from the BMI samples indicate rapid colonization rates, but community diversity was very different between stream channels that were inundated with fluctuating lake levels as compared to those that remained free-flowing.
Results thus far indicate that the restored stream channels have been successfully colonized by fish and aquatic macroinvertebrates. Monitoring surveys will continue throughout the duration of the highway construction project to evaluate the efficacy of the restored stream channels in improving aquatic connectivity within the watershed.