Sediment Retention Structure at Mount St. Helens

The Sediment Retention Structure at Mount St. Helens, shown here in 2025, is a massive earthen dam built on the North Fork Toutle River by the U.S. Army Corps of Engineers. Constructed in 1989 and located roughly 22 miles upriver from Castle Rock, Washington, it catches millions of cubic yards of loose volcanic ash and debris that still moves through the system after the 1980 eruption of Mount St. Helens. The SRS protects downstream communities from flooding and keep river channels clear.