An ongoing interstate highway reconstruction project in Washington State (I-90 Snoqualmie Pass East) includes culvert removal and construction of new stream channels along a 25 km reach over the Cascade Mountains. Culvert removal and new stream channel construction were designed to improve fish passage and aquatic connectivity in an adjacent lake (Keechelus Lake) and river system (Yakima River). Pre- and post-construction monitoring was conducted to assess the quality of newly constructed stream channels using fish passage and macroinvertebrate colonization over several years following the restoration work as measures of successful connectivity.
Native Westslope Cutthroat Trout (Oncorhynchus clarki lewisi) populations occur in Keechelus Lake tributary streams that cross the highway in the project area and Bull Trout (Salvelinus confluentus) inhabit the lake as adults and use tributary streams as rearing areas. 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. Cutthroat Trout were tagged with internal PIT tags and external colored elastomer tags to monitor movements during recapture events over several years. Bull Trout were tagged with PIT tags as part of a collaborative study with US Fish and Wildlife Service and Yakama Nation. Benthic aquatic macroinvertebrates were sampled in the streams using a Surber sampler to determine abundance and community diversity.
Colonization by fish and macroinvertebrates occurred within one year at five sites where new stream channels were built following culvert removal and bridge construction. Tagged Cutthroat Trout and Bull Trout were found in restored channels with some individuals moving upstream above a previously impassable barrier following its removal. Indices of biotic integrity from the macroinvertebrate samples indicate rapid colonization rates, but community diversity varied between stream channels that were inundated with fluctuating lake levels as compared to those that remained free-flowing.
Results thus far indicate that aquatic connectivity within the watershed improved as evidenced by successful colonization by aquatic macroinvertebrates and fish movements within and between restored stream channels.