Initial riparian vegetation response following dam removal on the Elwha River, Washington

Background/Question/Methods

Riparian zones, the interfaces between terrestrial and aquatic ecosystems provide numerous ecosystem functions such as wildlife habitat, erosion control, nutrient and energy stores, the buffering of sediment and runoff pollutants, 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. Dam removal is becoming more prevalent for economic, safety, and ecological reasons and is predicted to restore natural flow and sediment regimes to riparian ecosystems which could increase substrate heterogeneity and seed dispersal through water, hydrochory, increasing vegetation species richness downstream. The largest dam removal to date is on the Elwha River, Washington with the Elwha and Glines Canyon dams, 33 meters and 64 meters, respectively. Our objective is to determine vegetation and substrate responses in the riparian zone following dam removal. We sampled vegetation in plots above, between, and below the two dams pre (2010) and post (2013) dam removal to determine vegetation species richness. We hypothesized that species richness would increase between and below the two dams following dam removal and that the release of large quantities of sediment from the reservoirs would create new surfaces for vegetation establishment.

Results/Conclusions

Before dam removal, species richness was significantly lower in the reaches between and below the two dams. Contrary to our prediction that species richness would increase following dam removal, initial results show that species richness significantly decreased in the river reaches between and below the two dams from 2010 to 2013. Low diversity downstream could be influenced by the newly deposited sediment that originated in the reservoirs. We documented the creation of new substrates on low surfaces in the downstream reaches, and an increase in young geomorphic surfaces, open bars. Early assessments of reservoir soil found it to be low in nutrients, organic matter, and lacking microbial communities. It may take years or decades for species richness to respond to the restoration of the natural flow regime, seed dispersal through hydrochory, and soil development on new substrates. Furthermore, development of the downstream river reaches could be influenced by global climate change that is predicted to decrease vegetation diversity and increase coverage of non-natives in riparian ecosystems. Future monitoring will shed light on possible trajectories in riparian vegetation communities following large-scale dam removals.