Oso Landslide

I'll preface this blog post by acknowledging that I am not a landslide expert.  It's safe to say that I have an environmental science and geology background by education and profession.  With that, I can examine and research some of the factors that resulted in this catastrophic earth movement.  These factors are:

• Rainfall
• Geology
• Gravity

Rainfall has been of nearly record levels during March in our western Washington area.  Almost five inches of rainfall has been recorded during March in the Everett area and probably more than that fell in the landslide area.  Water is heavy and some of it soaks into the ground.  A wet sponge is heavier than a dry sponge.  Gravity then exerts more force on the heavier mass.  Combine the added mass with a steep slope and you have the potential for a landslide.

Here's a basic diagram of a typical slump block landslide, looking very much like the type of landslide that occurred here.

From the United States Geological Survey (USGS)

When I first looked at the area on Google Earth it became obvious that landslides had occurred in the past along both sides of the North Fork Stillaguamish River valley.  I could see some relict scarps, some fresher looking scarps, and hummocky surfaces where landslides had rolled out and came to rest.  Here's a Google Earth Image of the landslide area.  You can see the whitish area that could be a relict scarp from a previous landslide.  The actual landslide broke a little farther back from the assumed relict scarp marked by the whitish area and then flowed across the river and Highway 30 taking out everything in its path, including the houses seen in the foreground.

Pre-landslide view.  Note the whitish area at center which marks the area of the recent landslide.  (Google Earth Image used for a noncommercial purpose)

I was curious about the geology and so looked at some geologic maps available online.  On these maps, old landslides and landslide deposits are clearly mapped up and down the river valley on each side of the valley.  The stratigraphy of the valley reflects the recent glacial history and shows that the underlying geology was a significant contributing factor.

Let's look at the recent geologic history of the Puget Sound region.  During the last Ice Age, perhaps 16,000 years ago, the Puget Lobe of the great continental ice sheet moved into our area from the north, part of the vast ice sheet that covered much of North America.  The Puget Lobe essentially dammed up the river valleys along the front of the Cascade Range and even moved up these valleys.  This is somewhat counter-intuitive because one pictures glaciers moving down a valley, not up.  As these river valleys were dammed by the ice sheet, glacial ice margin lakes formed and in those lakes were deposited primarily clays and silts (Qglv - Advance glaciolacustrine deposits).  Then the ice sheet moved up and overrode these lacustrine deposits forming another layer of deposits called till (Qgtv), basically a mish-mash of silt, clay, sand, and gravel that is ground and packed under a glacier as it moves.  Both the till and advance glaciolacustrine deposits were densely packed under the hundreds of feet of glacial ice.

As the ice sheet receded some 13,000 years ago, it again left ice margin lakes with similar clay and silt deposits to the advance lacustribe deposits (Qgle - Recessional glaciolacustrine deposits).  The final top layer of the glacial sediment cake were the recessional outwash deposits (Qgoe) composed mainly of sand and gravel disgorged from the melting and receding glacier.  The recessional lacustrine and outwash deposits were never overridden and packed under glacial ice and are comparatively loose and unconsolidated.

As a refresher, the glacial deposits along the side of the river valley are from top to bottom (newest to oldest):

• Recessional outwash (sand and gravel; loose)
• Recessional glaciolacustrine deposit (silt and clay)
• Till (silt, sand, and gravel; dense)
• Advance glaciolacustrine deposits (silt and clay, dense)

The following map is a geologic map that shows the glacially-derived sediments discussed above and landslide deposits with arrows showing direction of historic landslide movement.

2003. Washington Division of Geology and Earth Resources. Geologic map of the Mount Higgins 7.5- minute quadrangle, Skagit and Snohomish Counties, Washington. Open File Report 2003-12. Dragovich, J.D., Stanton, B.W., Lingley, W.S., Jr., Griesel, G.A., and Polenz, Michael

The following photo shows what I interpret as the upper stratigraphic units discussed above.  At the top in light gray is the sand and gravel of the recessional outwash.  This is a relatively porous deposit that could hold a lot of water.  The darker gray band below the outwash is likely the recessional glaciolacustrine deposit composed of silt and clay.  With the clay and silt, the lacustrine deposit would be comparatively less permeable and would act as a perching layer holding up the rain-soaked recessional outwash and preventing water from infiltrating deeper.  The added mass of the rain-soaked recessional outwash at the top of the slope combined with the force of gravity have caused the slope to fail resulting in the massive landslide.  This is only an interpretation on my part.

Landslide main scarp. Note that the trees are probably 50 feet, if not 100 feet tall.  (Snohomish County)

As it stands now, the landslide has dammed the river and water is building up behind the dam.  The river will eventually overtop or work its way around the dam, either in a catastrophic flash flood event or perhaps a more gentle cutting of a new course.  Three people are confirmed dead and 18 are missing.  My hopes and prayers to all affected.