Engineered logjams (ELJs) have become popular as an alternative to riprap for bank stabilization due to their perceived ecological benefits, which could potentially limit mitigation requirements for project proponents. This, along with the fact that many riprap bank stabilization projects have failed, led the Washington State Department of Transportation (WSDOT) to use ELJs to stabilize chronically eroding banks of the Hoh River near milepost 174.4 of Highway 101 (ELJ site). WSDOT also proposes to use ELJs to stabilize Hoh River banks near milepost 175.9 (RR site), where riprap has chronically failed. Although ELJs are expected to provide ecological benefits, they have not been thoroughly evaluated. The Hoh River sites offer an opportunity to use a before-after-control-impact design to evaluate the ecological benefits of ELJs.
The study objectives were to 1) collect baseline data that would allow future comparisons of habitat diversity, fish species diversity, fish abundance, growth, and survival in areas stabilized using riprap and ELJs, and 2) to evaluate fish habitat use and movement at the ELJ site. We collected pre-project data for fish habitat and fish abundance at both sites during the summer of 2009 and 2010, and fish abundance data during the winter of 2011.
There were more channels (i.e., main, braid, side) and primary (i.e., pools, glides) habitats at the RR site than the ELJ site; however, these differences were likely related to larger scale geomorphic factors than the bank stabilization. In contrast, there were more secondary habitats, which are smaller distinct units within primary habitat, at the ELJ site, which contained much more eddy habitat. Species diversity at the RR and ELJ site was variable. Chinook salmon, coho salmon, steelhead, mountain whitefish, and sculpin were the most common species or genus collected at both sites. Differences in fish abundance, size or growth at the two sites were quite variable. Apparent survival of PIT-tagged coho salmon (Oncorhynchus kisutch) was greater at the ELJ site than the RR site; however, the results could not be compared statistically since only one ELJ and RR site were sampled. Acoustic tracking data showed that steelhead parr (O. mykiss) and juvenile Chinook salmon (O. tshawytscha) used a large portion of the study area, often within a 24-hr period. The acoustic tracking system provided quality tracking data for just over 50 percent of the time the fish were expected to be present in the array. Steelhead parr selected primary pools and secondary eddy habitats.
They generally selected areas that were intermediate in depth (~0.6 to 3.5 m) and distances (~4-8 m) from the river bank. They generally did not use areas directly under the ELJs, which we hypothesize was related to the turbidity of the Hoh River. Turbidity likely provides cover thereby reducing the reliance on instream structures for protection from potential predators. The habitat and fish data collected will be useful for completing a before-after-control-impact assessment of the benefits of ELJs in the Hoh River. The movement data suggest that the reach scale may be the most appropriate spatial scale for monitoring ELJ projects, which is larger than the primary unit scale we used during this study. The habitat use results suggest that the ELJs will provide better habitat for juvenile steelhead, which preferred eddy habitats which were more abundant at the ELJ site than the RR site.