Both approaches have been used to stabilize eroding banks, especially when infrastructure such as roads and houses is threatened. Several reports, however, suggest that the more commonly used method of stabilizing river banks, rock riprap, negatively impacts fish habitat complexity and aquatic communities including fish. Additionally, several reports suggest that wood is an important ecological component of aquatic systems, and fish and other aquatic life often occur in greater densities and diversity at locations associated with wood. As a result, large wood complexes, such as engineered logjams, are viewed as an alternative tool for bank stabilization and stream restoration. Therefore, bank stabilization with engineered logjams may provide the dual benefit of protecting infrastructure and conserving aquatic natural resources, such as fish populations, when compared to alternative stabilization techniques.
USFWS and WSDOT are conducting the Hoh River study to compare fish habitat diversity, fish density, growth, survival, movement, and species diversity in Hoh I and Hoh II. Fieldwork started in August 2009 with two study components. Fish survival in both Hoh I and Hoh II was estimated and compared between sites by capturing, tagging, and recapturing fish at a later date. A total of 495 fish were tagged with passive interrogated transponder (PIT) tags between the two sites. The second study component relied upon acoustic tagging technology to provide spatial information in order to identify how fish utilize the engineered logjams at the Hoh I site. In this second effort, USFWS technicians and biologists deployed a series of sound recorders called hydrophones on the river bottom. Twenty-nine fish were captured and surgically implanted with an acoustic tag. The hydrophones passively tracked the location and movement of the tagged fish over a couple of weeks. Preliminary results from Hoh River study indicate that engineered logjams are a valuable technique for bank stabilization projects and can provide favorable habitat for juvenile salmonids when compared to sites utilizing other bank stabilization methods.
In August 2010, we plan to PIT tag more fish at both sites. We are also preparing to collect habitat information in order to describe differences in the habitat quality and quantity from the perspective of fish in the study areas. Check back for postings of our upcoming fieldwork!