File photo of a landslide.

File photo of a landslide.

Landslide risk in the Snoqualmie Valley

Rains have pelted King County in recent weeks, saturating the soil and increasing the risk of landslides in the Snoqualmie Valley.

Landslides are a risk throughout the valley, especially along the Tolt River, where both sides of the river valley have experienced historic landslides. The west face of Mt. Si, as well as areas north of Snoqualmie Ridge and Fall City, are also prone to significant landslides.

The risk increases for landslides as soil becomes saturated with water, especially during heavy winter rains. As of Jan. 13, the risk of landslide in the Seattle area was high, according to the U.S. Geological Service gauge system.

But not all landslides are created equal in terms of magnitude. Any time soil or rocks slide down slopes, it’s considered a landslide, said King County Water and Land Resources Division Geologist John Bethel. The most common type of landslides in the Pacific Northwest are shallow landslides, where a few feet of soil becomes saturated with water, then slides.

These are generally small, and are commonly seen blocking roads around King County, including the valley. Similar to those seen across the entire region, these shallow landslides are also the most common in the Snoqualmie Valley, Bethel said.

But another common form of landslides locally are debris flows. These typically occur in drainage systems. Blockages like beaver dams or log jams can retain water, and when they fail, a pulse of water moves down drainage ditches or pipes. As it goes, it accumulates more water and sediment.

Rock fall is another type of landslide. While there hasn’t been a recent history of rock fall, Bethel said there’s historical evidence of it happening. Moon Valley Road near North Fork is one such area. It has visible angular broken rocks, indicating they fell off the mountainside.

A screenshot from King County iMaps showing landslide risk in and around the Snoqualmie Valley.

A screenshot from King County iMaps showing landslide risk in and around the Snoqualmie Valley.

A fourth kind of landslide that is also common throughout King County is known as a deep-seated slide. This is where chunks of land that may be 100 feet thick or more begin sliding down slopes. These tend to move slowly.

The Tolt River valley has evidence of this type of landslide on either side of the river valley. Most of the time, these landslides aren’t active, but could reactivate.

“In some places in the county, there are big subdivisions or houses built on the big, old landslides,” Bethel said.

This includes areas of Renton and Issaquah.

“One of the spooky things about these big landslides is there’s a lot of them. Many of them are heavily developed,” Bethel said.

In the valley, an example of a deep-seated landslide can be seen across from the observation area at Snoqualmie Falls where there used to be a railroad track.

The Oso landslide was an example of a deep-seated landslide, but it was also unique, Bethel said. It started as a slow-moving deep landslide, but because of soil saturation, it collapsed and turned into a debris flow.

In 2016, King County began looking for similar risks in the county and didn’t find any.

“We did not find evidence of that kind of failure in the areas that we looked at,” Bethel said. “That’s a somewhat unusual manifestation.”

The best way to deal with landslides is to avoid them. For property owners, this means doing research before buying or building on property. Trees and vegetation on slopes can also help keep soils in place.

Homeowners can keep an eye out for deep-seated landslides by looking inside their existing homes. Houses built on areas that are experiencing this kind of landslide often have doors and windows that begin sticking, cracks in drywall or houses that separate from their foundation.

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A landslide in December 2019 created a crack in this Fall City road, allowing for a one lane entry and exit. Courtesy of King County Road Services

A landslide in December 2019 created a crack in this Fall City road, allowing for a one lane entry and exit. Courtesy of King County Road Services

File photo shows a 2017 landslide at Canyon Springs

File photo shows a 2017 landslide at Canyon Springs

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