Spirit Lake Memorial Highway
Sediment Retention Structure, WA

Racing water pours through this structure as sediment builds up on the other side. After the eruption of Mount St. Helens, a 17-mile avalanche of about three billion cubic yards of volcanic rock, mud, and ice pounded into the North Fork Toutle River valley. To stop the debris from moving downstream and causing flooding and impeding navigation, a long-term resolution was needed. Visitors can view the 1,800-foot-long solution at the Sediment Retention Structure (SRS), part of an unprecedented solution for this problem.

The SRS does not store water as a conventional dam would. Instead, it slows the water sufficiently to allow the sediment to drop out. The sediment then builds up behind the structure in a single manageable deposit, while water and fish pass freely through. Over the 50-year life of the project, the lake is expected to fill with 258 million cubic yards of gravel, sand, and sediment.