Coastal habitat and biological community response to dam removal on the Elwha River

Abstract

Habitat diversity and heterogeneity play a fundamental role in structuring ecological communities. Dam emplacement and removal can fundamentally alter habitat characteristics, which, in turn, can affect associated biological communities. Beginning in the early 1900s, the Elwha and Glines Canyon dams in Washington, USA, withheld an estimated 30 million Mg of sediment from river, coastal, and nearshore habitats. During the staged removal of these dams, the largest dam removal project in history, over 14 million Mg of sediment were released from the former reservoirs. Our interdisciplinary study in coastal habitats, the first of its kind, shows how the physical changes to the river delta and estuary habitats during dam removal were linked to responses in biological communities. Sediment released during dam removal resulted in over a meter of sedimentation in the estuary and over 400 m of expansion of the river mouth delta landform. These changes increased the amount of supratidal and intertidal habitat, but also reduced the influx of seawater into the pre‐removal estuary complex. The effects of these geomorphic and hydrologic changes cascaded to biological systems, reducing the abundance of macroinvertebrates and fish in the estuary and shifting community composition from brackish to freshwater‐dominated species. Vegetation did not significantly change on the delta, but pioneer vegetation increased during dam removal, coinciding with the addition of newly available habitat. Understanding how coastal habitats respond to large‐scale human stressors, and in some cases the removal of those stressors, is increasingly important as human uses and restoration activities increase in these habitats.

Dam removal: Listening in

“…; Bellmore et al . ; Foley et al ., ]. In turn, these holistic studies could inform the development of standardized metrics to be measured at all dam removal studies, increasing our ability to analyze responses across different removal types.…”

Rapid Recolonization and Life History Responses of Bull Trout Following Dam Removal in Washington's Elwha River

“…Although we cannot discern the exact mechanisms, Bull Trout as small as 150 mm were observed foraging in the estuary (Foley et al. ; Lincoln et al. ), telemetry data strongly suggested use of the estuary, and marine prey contributed to their trophic ecology (Quinn et al.…”

“…We analyzed stomach contents of Bull Trout that were captured during beach seining operations in the Elwha River estuary as reported by Foley et al. (). The beach seine (38 m long, 2 m deep) was deployed from bank to bank by using a small skiff and then was pulled to shore.…”

Using Structured Decision Making to Overcome Scale Mismatch Challenges in Barrier Removal for Watershed Restoration

“…For example, increased freshwater flows and sedimentation resulted in changes in invertebrate and fish community composition in coastal waters after two dams were removed on the Elwha River, Washington (Foley et al. 2017b). Additionally, removal of barriers in one watershed could lead to undesired changes in fish abundance across multiple watersheds, such as the increase of invasive Sea Lamprey in the Great Lakes (Jensen and Jones ) or the reduction of fish populations in other watersheds through connections among local populations (Lin and Robinson ).…”

“…River restoration is a dynamic process in which the ecosystem undergoes transitional stages before reaching a postremoval stable status (Foley et al. 2017a, 2017b; Figure ). Species abundance may decrease rapidly after barrier removal and then gradually recover to a pre‐dam or other long‐term stable level.…”