Riparian restoration following dam removal on the Elwha River, Washington
Riparian vegetation is crucial for ecosystem functioning, and is significantly impacted by dams. Dams can reduce vegetation diversity by flooding impounded reaches and altering downstream flow, geomorphology, and hydrochory. With >50% of large U.S. dams beyond their life expectancy, dam removal is increasingly being considered to eliminate aging infrastructure and restore ecosystems. However, few studies have considered riparian ecosystems post-dam removal. Large dam removal can rapidly release large volumes of sediment, affecting downstream communities. Consequently, it is unknown whether dam removal will result in restoration or permanent change.
The removal of two dams on the Elwha River, Washington represents the largest dam removal project to date. Our objectives were to determine: 1) whether dam removal increases downstream vegetation and seedbank diversity, and 2) which factors drive vegetation recolonization in drained reservoirs. We sampled vegetation and environmental variables in 100-m2 plots along 15 transects above, between, and below the two dams pre- (2005-2010) and post-dam removal (2013-2014), and along 10 transects within the drained reservoirs (2013-2014). Seedbanks in bar landforms were assessed in 2014. Variation in species richness among reaches, years, and landforms was analyzed using mixed linear models. Structural equation models were used to relate variables to reservoir vegetation recolonization.
Prior to dam removal, we found that native plant species richness was ~45% lower downstream from dams compared to upstream, while exotic species richness was not different. However, downstream species richness did not increase two years following dam removal. Seedbank species richness in bars did not differ between reaches following dam removal, despite the presence of large volumes of new sediment in downstream reaches. Native, exotic, and seedbank species richness were higher in the downstream reservoir, which also had finer sediment with more organic matter than the upstream reservoir. The most important correlates to native species richness and cover in the two reservoirs were soil composition (particularly organic matter), followed by hydrology (particularly coarse sediment grain size). In contrast, exotic species richness was also strongly related to soil composition (particularly organic matter), while exotic cover was more strongly related to hydrology (particularly coarse sediment grain size). Overall, our results suggest that species diversity downstream from dams does not increase immediately following dam removal, perhaps due to sediment burial. Diversity could potentially require a longer time period to reestablish. Also, recolonization of reservoirs by native and exotic species is likely to vary substantially with soil conditions and hydrology.