Date of Award


Document Type


Degree Name

Master of Science (MS) in Biology




"Riparian ecosystems are important for ecological functioning of rivers, and are significantly impacted by dams. With over 50% of large dams in the U.S. beyond their life expectancy, dam removal is increasingly being considered to eliminate aging infrastructure and restore ecosystems. There have been few large dam removals to date, so studies assessing vegetation succession on exposed reservoir sediments are limited. My research aims to assess how environmental factors within exposed reservoirs affect vegetation succession following removal of two dams on the Elwha River, Washington. In addition, I compared patterns of vegetation among the two reservoirs and their landforms. To do this, I sampled 67 100m² plots in 2013 and 60 100m² plots in 2014 along 10 transects within Mills and Aldwell Reservoirs. In each plot , I recorded vascular plant species composition and woody species height. I collected and pooled 8 soil samples (20 cm) / plot to assess percent organic matter, nutrients, and percent sand, silt, clay, and conducted a Wolman Pebble Count. I used a structural equation models to show how environmental factors related to hydrology, soil nutrients, and dispersal distance affect species diversity and cover. I compared environmental factors and vegetation responses among the two reservoirs using general linear models. Structural equation models showed that soil nutrient levels, sediment texture, ground cover, and landform were the environmental factors most related to reservoir revegetation patterns. Native species richness and cover, and exotic species cover were highest on valley walls and were positively related to high percent organic matter and % silt, but negatively related to % litter, D50, Mg, and P. In contrast, exotic richness was highest on terrace and riparian landforms with low % litter, Mg, and P and high % organic matter that were furthest away from established forest communities. Sediment nutrient indicator variables organic matter, Mg, and P were co-correlated with other sediment variables and act only as a surrogate for those variables in these models. In total, 147 vascular plant species were sampled in the two reservoirs of which 47 (31%) were exotic. Aldwell reservoir contained higher native and exotic species richness, cover, and woody species growth, and had finer textured sediments, deeper sediment depth to refusal, and higher % litter ground cover than Mills reservoir in 2013, while Mills reservoir had higher % gravel ground cover. By 2014, the only significant difference between reservoirs was woody species height, which was higher in Aldwell reservoir. Native species richness and cover were higher than that of exotic species in both reservoirs; however, exotic species are increasing, particularly along riparian zones within both reservoirs and on the most fertile sites along Aldwell valley walls and terraces. The increase in exotic species occurred despite active management to control them, and should be a concern to Olympic National Park because the reservoirs could become a gateway of exotic species invasion into a relatively protected landscape. Over time, I expect multiple vegetation communities to form within each reservoir associated with landform. Valley walls will likely return to the composition and structure of surrounding upland forests, while riparian zones will likely come to resemble the upstream Elwha River reaches not affected by damming. Terraces, on the other hand, will likely form novel vegetation communities dependent on environmental factors that will differ between the two reservoirs. The results of my study highlight the effect of varying environmental conditions on vegetation recovery rates and can help inform the Elwha River restoration project as well as any future dam removal projects"--Leaves iv-v.

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