PIT-TAGGED PARTICLE STUDY OF BED MOBILITY ASSOCIATED WITH LARGE-WOOD ADDITIONS ON THE NARRAGUAGUS RIVER, MAINE

Sediment continuity, a key tenet in dynamic equilibrium, depends on upstream sediment delivery to balance localized erosion. The Narraguagus River in Maine, a key Atlantic salmon (Salmo salar) river, is still responding to quaternary-age features that include mid-stream glacial kettle lakes, which create a series of sediment traps, or discontinuities in sediment transport. Habitat along the river was impacted by historic log drives with resultant over widening and immobile, embedded sediments. A challenge for restoration efforts stems from limited ability sediment deposition along banks and bars because of these sediment discontinuities. Large wood (LW) additions were performed to enhance sediment mobilization, decrease embeddedness and promote more channel change. In 2016, a grid of approximately 200 glass spheres with passive integrated transponder (PIT) tags, equally spaced along each of the ten transects, spaced 5-m apart, in each of three different study reaches with alternating 28-mm and 40-mm size particles served as point sensors to detect sediment movement. In 2017 and 2018, reaches were surveyed before LW additions in the lower reach to document baseline tracer particle mobilization rates. Surveys were repeated in 2019 and 2022 in two reaches after LW additions. PIT-tag tracer recovery rates varied from 79.5% to 99.0%. From 2016 to 2018, entrainment rates in the reaches decreased in the downstream direction from 70% to 29% for 28-mm diameter particles, and 62% to 33% for 40-mm diameter particles. Overall, 46.0% of all particles moved from 2016 to 2022 in the treatment reach. Pre-treatment entrainment rates were 29.0% and 33.0% for 28-mm and 40-mm diameter particles respectively. After treatment, entrainment rates were 29.2% and 28.6% for 28-mm and 40-mm diameter particles respectively. The overall low rates of movement, and higher entrainment rates and step-lengths for larger particles highlight the presence of a mostly immobile armor layer, or channel pavement. With limited sediment supply because of sediment discontinuities, and effective sheltering of subpavement sediments below the surface pavement, lack of sediment supply will be a major impediment to the rivers ability to rebuild channel banks and mid-channel islands disrupted by logging activities.