The Elwha's insects returned in number before their river community recovered
WHAT THE EVIDENCE SAYS
Following removal of the Elwha and Glines Canyon dams, more than 20 million tonnes of reservoir sediment moved downstream as passage reopened to more than 90% of the watershed. Across five river reaches and nine years of observations, benthic invertebrate density fell as much as 95% during active removal, then returned near baseline within roughly three years of peak sediment transport. Taxa richness and community composition, notably some caddisflies and longer-lived groups, remained altered after that interval.
WHY IT MATTERS / OUR READ
The ecological signature of a restoration project can contain immediate disturbance, recovery in one metric and prolonged change in another. Restoration success criteria should distinguish connectivity, organism numbers, sensitive taxa and community composition; the years after construction are part of the intervention, not an optional follow-up.
CAVEATS / UNCERTAINTY
Observational before-during-after monitoring was conducted in one very large dam-removal system, with fish recolonization, sediment and habitat succession interacting. A numerical rebound does not prove all functions or populations are restored; these outcomes do not imply dams should remain.
SOURCE
Frontiers in Ecology and Evolution — Morley et al. (2026)
View original evidence ↗
Nine years of monitoring after two large dam removals show that the rebound in insect abundance and the recovery of community composition are not the same milestone.
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