Elwha River Dam Removal — Reconnecting 70+ Miles of Salmon Habitat and Restoring Sediment Flow (USA)

The Elwha and Glines Canyon dams, built in the early 1900s on Washington's Olympic Peninsula, blocked anadromous fish from over 90% of the watershed for about a century. Both dams were removed simultaneously between 2011 and 2014, the largest dam removal in US history at the time, reopening more than 70 miles of habitat (NOAA Fisheries).
About 19 million cubic metres of trapped sediment were expected to be released, the largest controlled sediment release into a river and marine waters recorded (USGS). USGS and NPS monitoring reports that salmon have begun recolonising habitat above both former dam sites, and that sand and gravel are again reaching the coast, helping reverse erosion.
Caution: recovery is gradual and depends on continued monitoring and adaptive management; early sediment pulses affected the lower river, and the sources cited here do not quantify total cost or current population sizes.

Read the full analysis: https://www.nps.gov/olym/learn/nature/elwha-research-efforts.htm