Not Washington State, but I do landslide work so I made a stop at this landslide I spotted when driving Highway 6 in Utah. I see landslides all the time and can't help stopping. This slide was for a time famous in that it once posed significant threat to the City of Provo.
Thistle Canyon Landslide viewed fro Highway 6
The Thistle Canyon landslide (https://en.wikipedia.org/wiki/Thistle,_Utah) is an example that should be remembered about mountainous terrain. Steep mountain slopes can and do occasionally fall down. This mountain canyon slide caused a classic hazard. The slide itself of course was damaging, but the slide also blocked the Spanish Fork River forming a lake.
"Thistlelandslideusgs" by R.L. Schuster, U.S. Geological Survey - http://landslides.usgs.gov/learning/imagepreviews.php. Licensed under Public Domain via Commons