LONG-TERM GEOMORPHIC EFFECTS OF THE GLINES CANYON DAM REMOVAL ON THE ELWHA RIVER, WASHINGTON, USA

The Elwha River once provided vital habitat for a variety of salmonid species, but after two dams were emplaced on the river in the early 1900s, habitat diminished, and salmon populations declined. From 2011-2014, the dams were finally removed to restore the Elwha ecosystem. To understand the long-term geomorphic impacts of the Glines Canyon Dam removal on the Elwha River, I quantified changes in four parameters: in-channel large wood, main channel sinuosity, channel braiding, and sedimentation. High-resolution imagery from 2012-2020 was used to map large wood and digitize main and secondary river channels, and field surveys were completed at study sites to assess sediment-size distribution six years after the completion of the dam removal. Analysis of large wood revealed that the number of individual logs peaked during the dam removal but decreased after the removal and remained low. Logjam area increased steadily throughout the eight-year study period while the number of logjams stayed constant, suggesting that individual logs were recruited into existing logjams over time. Main channel sinuosity increased during and after the removal. After peaking in 2017, sinuosity decreased but has yet to return to conditions present before the dam removal. Channel braiding peaked during the dam removal process, dropped, and remained relatively consistent for the remainder of the study period, reaching an equilibrium state that is more braided than before the dam removal. Six years after the completion of the dam removal, sediment bars contain a mixture of grain sizes, in contrast to the armored, coarse sediment when the dam was in place or the blanket of fine sediment released during the dam removal. The results demonstrate the complexity and interconnectedness of various geomorphic and ecological parameters and suggest that while some geomorphic parameters may establish a new equilibrium in the years following a dam removal, others will continue to evolve over longer timescales.