Date of Award
Master of Science (MS) in Biology
The Elwha River is the site of the largest dam removal project in the world to date and serves as a case study for the ecological effects of large dam removal. The 2012-2014 removal of two dams on the Elwha River exposed a cumulative 2.76 km2 of previously inundated surfaces. Environmental conditions including sediment texture and nutrients, slope-aspect, dispersal distance from the river and mature forest, disturbance, and elevation vary widely across the reservoir surfaces, causing significant variation in the plant community. The first objective of my study is to continue a long-term study of the passively restored vegetation community to see how vegetation has changed since dam removal, and to determine which environmental factors were associated with the recovery of a self-sustaining native plant community. I hypothesized that surfaces with higher proportions of fine sediments and organic matter would have the most successful native plant community. My second objective is to integrate this long-term study with one managed by the National Park Service. The National Park Service study includes actively restored areas in the reservoirs, and data collection in both studies was concurrent, but used different methods. Here, I hypothesized that actively restored areas would have the greatest native species richness. The vegetation communities colonizing the reservoirs varied significantly among the two reservoirs and among landforms within the reservoirs in 2012-2013 surveys, a trend that continued for the most part in 2016. The proportion of fine particles (silt and clay) in the soil and the amount of organic matter present in the soil significantly affected the vegetation community. Vegetation cover has increased on valley wall and terrace landforms, even as overall species richness has decreased in those areas. Active restoration that was implemented in parts of the reservoirs by the National Park Service took the form of planting or seeding native propagules. While these actions had no significant effect on total vegetation cover or native species richness, the actively restored areas did have a different overall species composition than passively restored areas. Vegetation in the Elwha River reservoirs appears to be successfully colonizing many newly exposed surfaces without anthropogenic assistance, but it is dependent on many environmental variables and warrants continued monitoring.
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Morgan, Olivia A., "Vegetation community development after dam removal on the Elwha River" (2018). EWU Masters Thesis Collection. 523.