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Paper No. 53-7
Presentation Time: 3:20 PM
A BACI STUDY OF RIVER RESTORATION WITH LARGE-WOOD ADDITIONS USING PIT-TAGGED TRACER PARTICLES TO ACCESS BED-SEDIMENT MOBILITY ON THE NARRAGUAGUS RIVER, MAINE
Critical spawning habitat for Atlantic salmon (Salmo salar) along the Narraguagus River in Maine was impacted by historic log drives with resultant over widening and immobile, embedded sediments. A Before-After-Control-Impact (BACI) design was used to evaluate the ability of large wood (LW) additions to enhance sediment mobilization, decrease embeddedness and promote greater variation in depth to improve aquatic habitat. In 2016, ten cross-sections, spaced 5-m apart, were established and surveyed with a laser total station in each of two different study reaches. A grid of approximately 200 glass spheres embedded with passive integrated transponder (PIT) tags, with twenty alternating 25-mm and 40-mm size particles equally spaced along each of the ten transects, were placed to serve as point sensors to detect sediment mobilization within each reach. In 2017 and 2018 before the LW additions, the site was surveyed to document baseline tracer particle mobilization rates. In 2017, 94% to 99% of particles were rediscovered. Only 14% to 28% of particles showed evidence of movement more than 1.0 m following an 88.9 m3/s, <2-year magnitude flood event. The next winter, a 10-year magnitude flow occurred prior to the 2018 resurvey, which resulted in an 87% to 95% recovery rate with a corresponding entrainment rate of 41% to 69% at the two sites. Between 10% and 16% of the 25 -mm particles showed movement of at least 1 m both years, while 0% to 13% of the 40-mm particles were entrained each season. Average annual distance moved was 7.3 m to 8.7 m for the 25-mm tracers and 2.9 m to 6.4 m for the 40-mm tracers. A 97.8 m3/s, <2-year magnitude flood event occurred in spring of 2019 after post-assisted log structures (PALS) additions to one of the study sites. Tracer-particle recovery efforts in the summer of 2019 are used to evaluate the effectiveness of the PALS in enhancing sediment mobilization.