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Date of Award


Document Type

Thesis: EWU Only

Degree Name

Master of Science (MS) in Biology




"Riparian zones, the interfaces between terrestrial and aquatic ecosystems, provide numerous ecosystem functions such as wildlife habitat, erosion control, nutrient and energy storage, buffering of sediment and pollution, and maintenance of biodiversity. Worldwide, riparian ecosystems are threatened by a variety of human-induced disturbances including climate change, development, invasive species, and flow alteration by dams. Vegetation in riparian zones can be altered by the damming of rivers that disrupts natural fluvial processes that change the flow regime, trap and reduce sediment transport downstream, and limit the dispersal of seeds in the water column. On some rivers, dams reduce floristic diversity downstream and create fragmented communities. Dam removal is becoming more prevalent for economic, safety, and ecological reasons and is predicted to restore natural flow and sediment regimes, which could potentially reverse downstream impacts on riparian ecosystems. However, the removal of large dams (>10 m tall) may negatively affect downstream vegetation due to large amounts of mobilized sediment from the drained reservoirs. To date, few dams greater than 10 meters tall have been removed or studied. Consequently, the effects of large-scale dam removal on downstream riparian vegetation is not well understood. The largest dam removal project to date is the removal of the 64 m Glines Canyon and 32 m Elwha dams on the Elwha River in Washington State. Collectively, the two dams held upwards of 21 million m³ of sediment. Dam removal commenced in 2011 and was completed by the summer of 2014. Prior to dam removal, native plant species richness was approximately 45% lower and species composition was different downstream from the dams relative to the upstream reference reach. Also, the abundance and richness of hydrochorous seeds (dispersed by water) were much lower downstream from Glines Canyon Dam prior to removal. My research objective was to determine how hydrochory and downstream vegetation patterns changed in the first two years following dam removal. Specifically, I addressed the following questions: (1) Will hydrochory increase downstream of Glines Canyon Dam following removal? (2) How will dam removal influence plant species richness, particularly in areas with new sediment deposition from the drained reservoirs? (3) Will communities become more similar following dam removal or will they become more dissimilar due to reservoir sediment deposition? I predicted that hydrochory would be restored downstream of the Glines Canyon Dam and seed abundance and richness would become similar to the upstream reference reach. Second, I predicted that dam removal would increase native species richness downstream of the dams, and that plots with new sediment could show an increase in nonnative species. Finally, I predicted that community composition would become more similar to the upstream reach, and that newly deposited reservoir sediment would alter communities downstream of the removed dams. To determine how dam removal affected vegetation, I used a Before-After- Control-Impact Study design in which I compared vegetation sampled before (2005 and 2010) and after (2013 and 2014) dam removal in an upstream reference reach and in two impacted reaches located between and downstream from the dams. Within each reach, vegetation surveys were conducted in 100 m² plots located along five transects per reach. Within each transect, plots were located across bar, floodplain, and terrace landforms in a stratified random manner. Hydrochory was measured using drift nets in July and August of 2014 above and below Glines Canyon Dam and compared to pre-dam removal levels sampled in 2005 using general linear models. To assess the response of species richness and cover pre-and post-dam removal, I used mixed linear models. I used multivariate analyses (non-metric multidimensional scaling, permutational multivariate analysis of variance, and an indicator species analysis) to compare community composition across years (before and after removal), reaches, and landforms . My experimental results showed that hydrochory (species richness and abundance of seeds) was restored downstream of the Glines Canyon Dam to levels more similar to the upstream reach. I found that species richness (native and nonnative) did not initially increase downstream following dam removal nor was species richness influenced by the deposition of reservoir sediment on bar and floodplain landforms. Species composition also did not significantly change following dam removal. Although downstream riparian vegetation has not measurably changed in the first two years following dam removal, I expect that in the long-run, vegetation will respond slowly to ongoing sediment deposition and the restoration of fluvial processes. Reservoir sediment is predicted to continue eroding from the drained reservoirs and be deposited downstream. The amount of deposition in future years will be dependent on flood magnitudes and sediment stabilization in the upstream reservoirs. I predict that future riparian vegetation patterns will be influenced by the transport and deposition of reservoir sediment, flood events, and the restoration of hydrochory and the natural sediment regime"--Leaves iv-vi .

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