Does large dam removal restore downstream riparian vegetation diversity? Testing predictions on the Elwha River, Washington, USA

Abstract

Large dams and their removal can profoundly affect riparian ecosystems by altering flow and sediment regimes, hydrochory, and landform dynamics, yet few studies have documented these effects on downstream plant communities.Ecological theory and empirical results suggest that by altering disturbance regimes, reducing hydrochory, and shifting communities to later successional stages, dams reduce downstream plant diversity. Dam removal could reverse these processes, but the release of large volumes of sediment could have unexpected, transient effects. Two large dams were removed on the Elwha River in Washington State, USA, from 2011 to 2014, representing an unprecedented opportunity to study large dam removal effects on riparian plant communities.

Large wood supports Elwha revegetation by reducing ungulate browsing

“…Low abundances of ungulate-preferred species in Geyser Valley were unlikely due to effects of elevation, climate, or plant species pools, which are similar to those in the Mills valley (Table 3). Future work could evaluate more plausible factors of Geyser Valley successional history, recent flooding Brown et al, 2022), and heavy browse pressure during the century following wolf extirpation (Beschta and Ripple, 2008). Such work should consider that species preferred by ungulates were planted in Mills, but no seeding or planting occurred in Geyser Valley (Chenoweth et al, 2011;Chenoweth et al, 2021).…”

“…We evaluated these hypotheses in two valleys along the Elwha River: the recently exposed reservoir bed directly above the largest dam removal and a comparable valley upstream of both of the Elwha's former dams and reservoirs. The two valleys represent opposite Warrick et al, 2011;Brown et al, 2022). By working in the two valleys, we hoped to derive insights regarding browse impacts and LWD mediation from early years to late stages in riparian restoration.…”

Why does geography matter in big dam removal projects? Lessons from a comparison between the Sélune and Elwha River cases

“…The Elwha case study is based on an extensive review of scientific publications and grey literature (e.g. Duda et al, 2008;Pess et al, 2008;Crane, 2011;Sadin et al, 2011;Brenkman et al, 2012;Johnson, 2013;East et al, 2015;East et al, 2015;Warrick et al, 2015;Foley et al, 2017a;East et al, 2018;Brewitt, 2019;Morley et al, 2020;Hess et al, 2021;Quinn et al, 2021;Brown et al, 2022;Chenoweth et al, 2022), supplemented by a field visit to the restored sites and lengthy specific interviews with the project's managers (NOAA, NPS) conducted in April 2022. We did not meet with Lower Elwha Klallam Tribe representatives who were central players in the removal; we instead used various available peer-reviewed papers (e.g.…”

Effect of river damming on nutrient transport and transformation and its countermeasures

“…Dam removal has two distinct effects. As depicted in Figure 7, removing dams offers the opportunity for the ecological restoration of the river (Doyle et al, 2003;Brown et al, 2022;Leisher et al, 2022). However, dam removal exposes and transports huge amounts of sedimentary pollutants accumulated in the reservoir to downstream areas because of the greater flow when the dam is dismantled.…”