Sunday, March 30, 2014

Eastern Washington has Landslide Hazards Too

As follow up from yesterday; I would note that landslides are by no means restricted to western Washington. Late this last week I took a look at some landslide sites in Spokane County. Unconsolidated sediments in Spokane County undermined by Latah Creek. The bluff slopes above the creek valley provides fine views, but in places where the creek is hard up against the valley wall the bluffs are undercut and shallow landslides develop.

Lower Latah Creek in Spokane County

The side of the creek has been armored with rock and/or wood cribbing, but high flow events like one this winter still reach the toe of the slope. The whole scenario is very much like many shoreline bluffs around Puget Sound.

Some of the biggest recent landslides in Washington State have been in Franklin County. Proof you do not need high mountains, glacial sediments or high rainfall to have slope failures.
Headwall scarp of landslide in Franklin County near Mesa
View of slide run out
This slide permanently closed a county road and altered an irrigation water route
Franklin County identified a specific unit within the Ringgold Formation as a landslide hazard area. The specific soil properties of the unit are associated with numerous very large landslide locations in Franklin County ranging from very ancient to ongoing.
I had an early morning project at another eastern Washington site and then had some excellent light to see a very large landslide block on the east end of Umtanum Ridge near Vernita.

This slide is within the highly fractured basalt associated with tight folding and faulting. The slide block likely dates to the ice age floods that swept along the lower slopes of the ridge on multiple occasions. But I would note that the alluvial fan just to the right of the rotated block has been active post ice age floods and likely based on vegetative cover on the slopes above and on the fan itself is a landslide hazard site. 


susan said...

Central and eastern Washington have had, and still have, LOTS of landslides. I suppose the Nile Valley slide of 2009 is already history it was so long ago. And the classic 1961 Landslides Along the Columbia River Valley Northeastern Washington discusses more than 300 along the upper 200 miles of what was once the Columbia River. Oh, and when part of a mountain slid into and blocked the Columbia River in 1872. And the place called "Slide Ridge on the south side of Lake Chelan. Gobs more. Lookout Mtn landslide near Cle Elum. The Malaga Slide, downriver of Wenatchee. Ad infinitum...

Dan McShane said...

Thanks Susan. You provided excellent emphasis to my point and the links are great.