Posted By Toutle Trekker

Where is the missing top of Mount St. Helens?  Many people think that the top of the volcano was blasted into the sky and became the cloud of ash that circled the world.  But that isn't true.  The ash that was erupted was "new" material from deep within the earth.  The "old" top of Mount St. Helens slid into the Toutle River Valley.  This landslide filled the valley up to 300 feet deep with chunks of the old summit.  The material consisted of loose layers of rock and ash along with pieces of glacier.  The largest mudflow resulted from the de-watering of this huge landslide. These "lahars" inundated all the low lying areas along the Toutle, filled the Cowlitz, and clogged the Columbia River shipping channel.  

In response, mass-dredging ensued. The Army Corps of Engineers also quickly built a sediment dam across the North Toutle valley in an attempt to hold the material in place.  This first dam (called N-1) was quickly overwhelmed.  Over the years, the Toutle River has continued to erode this material downstream, creating big problems for people along the river.  The sand along the Toutle and Cowlitz Rivers is old ash and rock from volcanic activity.

In 1990 the Corps came up with a solution.  They would build one huge sediment dam on the North Toutle and stop the erosion! The idea was simple; slow the river down with a shallow lake behind a big dam.  The sand drops out, and the water flows out over a spillway.  Now towns could be safe. Cowlitz County lifted the building moratorium on mudflow areas and housing developments popped up along our rivers, protected by the dam and higher levies. 

Anyone could see, however, that this "solution" didn't stabilize the river or get rid of the landslide material, it just held it in the upper valley a little longer.  And that is where we are today.  The dam is full, the river above the dam has become a shallow bay of mud, and the Cowlitz River is still clogged with sand.  The Toutle has no stable channel, and wanders over the sediment plain, now picking up material and moving it downstream.  The spillway on the dam has already been raised once to hold more material, with two more raises planned.  Oh, and the dam has no fish ladder.  Returning endangered salmon must be trucked around the mess.  Baby salmon (smolts) must navigate a web of shallow muddy channels downstream on their way to the ocean. Recently, agencies have tried a few creative ideas to hold sediment in place and to reduce the erosive action of the river.  You can see log dikes and piles along the Toutle River in places like Harry Gardner Park or the Mudflow Wildife Area.  These features are designed to improve fish habitat and hold the loose material in place long enough for vegetation to establish.

Learn More: https://www.nwp.usace.army.mil/mount-st-helens/overview/

See it:  Harry Gardner Park is the best place to see the erosion control and habitat enhancement structures.  Along Interstate 5, north of Castle Rock, you may notice large piles of dredge spoils along the Toutle and Cowlitz River.  Some of these sites are publicly owned.  In the upper Toutle Valley,  scan the sediment plain with binoculars for log structure and other erosion control measures. From the Hoffstadt Bridge area the remains of N-1 dam are visible in the valley.  

Trail across sediment dam
 

Hike it:  From Kid Valley travel east on 504 to Sediment Dam Road, which is actually the old Spirit Lake Highway.  Travel about 2 miles to the parking lot at the end.  The trail starts past the restroom, and leads to a dam viewpoint and continues to the dam itself.  It's a nice walk, half on dirt trail and half on old road.
The view from the dam shows the massive expanse of sediment held in place there.  Elk are also common, so be aware.  Sometimes herds of elk graze in the grassy field around the dam.  If you follow the signs and the old roads, the hike makes a nice 1.2 mile loop.  

 


 
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