Washington State has steep mountains that have very steep valleys. Could large mountain landslides happen here? That answer is easy. We have had several of various scales. The most recent was the Naches/Nile River slide in 2009. This slide destroyed several homes, buried a state highway and dammed a river.
In 1964 the Hope Slide blocked Highway 3 in southern British Columbia. Somewhere under the piles of rocks are buried cars where people who were at the wrong place at the wrong time were driving.
The Frank Slide in southwest Alberta killed 70 people, blocked a railroad and buried the entrance of a coal mine with people inside. The miners were able to dig their way out.
Several large landslides have been identified in the steep valleys of northwest Washington. The Church Mountain landslide buried the North Fork of the Nooksack River. The town of Glacier and other home subdivisions are built on top of the landslide debris form that slide. David Tucker provides a field trip guide to the Church Mountain slide at Here as well as a trip to a lake deposit up the Skagit Valley from a slide that dammed the Skagit River Here.
LIDAR images of the mountains in Whatcom County identify numerous mountain size failures. As I posted before Here numerous slides are located on the slope of Sumas Mountain including the active Swift Creek slide on the west side of the mountain described in part in this post Here.
Perhaps the most impressive and most dangerous is the Bonneville slide complex on the Columbia River. The Bridge of the Gods slide that dammed the Columbia River in the Columbia River gorge. This slide was described by native people living in the gorge and it was obvious to Lewis and Clarke that the slide was a recent feature. This slide altered politics on the Columbia River as it blocked passage on the river first as a dam and then as a steep rapids. There is evidence that a massive flood 80 feet above current flood levels took place downstream of the dam when the landslide blockage was breached.
Lewis' map of the slide area.
As can be seen in this Washington State Department of Natural Resources map multiple slides have taken place at the Bonneville site. The current lake is the result of Bonneville Dam, built on the slide deposits. Major power lines, pipelines and highways use this natural corridor. Another slide at this site will have profound impacts on Washington State and Oregon. The fact that the slide is not ancient history causes one pause to think how it impacted people living in the area a few hundred years ago. It had a profound impact then and it will when the slope fails next time.