From sediment pulse to ecological recovery: A nine-year study of benthic macroinvertebrate response to Elwha River dam removal

Sarah A. Morley; Jeffrey J. Duda; Holly J. Coe; Roger J. Peters; Martin C. Liermann; George R. Pess; Michael L. McHenry; Allyce R. Miller

doi:10.3389/fevo.2026.1914975

USGS · 70283444

From sediment pulse to ecological recovery: A nine-year study of benthic macroinvertebrate response to Elwha River dam removal

Sarah A. Morley·Jeffrey J. Duda·Holly J. Coe·Roger J. Peters·Martin C. Liermann·George R. Pess·Michael L. McHenry·Allyce R. Miller

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Abstract

Large dam removals are increasingly important river restoration actions, but long-term ecological responses remain poorly understood, especially for removals involving large sediment releases. We evaluated benthic macroinvertebrate (BMI) responses following removal of the Elwha and Glines Canyon dams on the Elwha River, Washington, USA. At the time of removal (2011–2014), these were the largest dams to be dismantled worldwide. Dam removal reconnected over 90% of the watershed to anadromous fish, released more than 20 megatonnes of reservoir sediment downstream, and restored natural sediment and wood transport processes. Using nine years of monitoring data spanning periods before, during, and after dam removal, we quantified changes in BMI density, taxa richness, and community composition across five river sections exposed to different combinations of sediment disturbance, reservoir succession, and fish reintroduction. Sediment release substantially altered downstream physical habitat and was accompanied by pronounced declines in BMI density and richness, particularly downstream of both dams. Density declined by up to 95% during active dam removal but rebounded to near baseline levels within three years after peak sediment transport subsided. In contrast, taxa richness and community composition remained significantly altered three years after dam removal, with persistent reductions particularly evident among Trichoptera and long-lived taxa. Fine sediment was the environmental variable most strongly associated with changes in BMI community structure. Our results demonstrate that large dam removal can cause severe but largely transient reductions in BMI abundance while producing longer-lasting changes in taxonomic diversity and community composition. These findings highlight the importance of distinguishing between numerical and ecological recovery and suggest that recovery trajectories following large dam removal extend well beyond the period of active sediment release.

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Frontiers in Ecology and Evolution Article

Source-reported bounding extent: 47.72932147001819° to 48.15066713800513° latitude; -123.62981757237395° to -123.38354341008477° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Sarah A. Morley, Jeffrey J. Duda, Holly J. Coe, Roger J. Peters, Martin C. Liermann, George R. Pess, Michael L. McHenry, Allyce R. Miller. 2026-10-20. From sediment pulse to ecological recovery: A nine-year study of benthic macroinvertebrate response to Elwha River dam removal. https://doi.org/10.3389/fevo.2026.1914975

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