The May 1980 eruption of Mount St. Helens generated massive lahars, volcanic mudflows consisting of melted glaciers and snowpack that mix with mud and debris. A particularly large lahar raged down the Toutle River to the Cowlitz River and eventually into the Columbia River, destroying logging camps, seven bridges along the way, and directly causing millions of dollars in lost revenue to the Port of Portland. Following the eruption, the U.S. Army Corps Engineers build the 1,888-foot-wide and 184-foot-high sediment dam on the Toutle River to stem the flow of additional sediment from the deforested, erosion-prone blast zone.
The hike to the sediment dam starts from the parking lot on Stewart Dam Road, 31 miles west of Johnston Ridge Observatory along Spirit Lake Highway (WA-504). Just a few yards from the trailhead is a viewing platform. Another mile of hiking along a quiet forest path lined with sword ferns brings you to a the dam itself. You can continue along the dam until the gated maintenance entrance to the spillway. Looking upstream you have the chance to spot migratory birds in the sediment basin. Downstream is a meadow where lucky observers may spot larger fauna.
While the sediment dam might not make it onto most a shortlist of adventures around Mount St. Helens, anyone interested in truly understanding the geology and history of the 1980 eruption should not overlook this short hike.