Landslide!

On Saturday, January 25, 2014, a group I was touring with along the west shoreline of the White Salmon River noted that a recent landslide had occurred on the vertical canyon wall approximately ¼ mile above the Mill Creek confluence.  The site was visited again today to get photos and the estimated size of the slope failure.

This photo, taken from an upstream vantage, shows the rust-stained surface of the newly exposed basalt, and tell-tale absence of the horizontal lines that betray the old Northwestern Lake water level (click on image to enlarge).  In the river is the conical pile of rock rubble that now slightly restricts river flow.  The estimated height of the pile is 15+ feet, and the diameter is 50 feet.  Assuming these dimensions are fairly close, the size of the landslide was about 300 tons.

Such an event obviously leads to speculations of cause.  Three ideas all relate to the current absence of Northwestern Lake.  First, the cliff rock was buoyed by reservoir water prior to 2010.  Since the weight of rock is much less underwater, the downward and outward forces that can lead to slope failures are much higher now that the reservoir is drained.  Second, it’s possible that water in the hillside is still adjusting to the reservoir’s absence, and hydro-static forces once balanced by the pooled water may still be pushing the rock outward.  Third, the rock face is now subject to freezing and thawing that can pry and destabilize the rock (see Dan McShane’s comment on this post).

Regardless of cause, landslides and other types of earth movement are very common geologic events, which can be harmful if people and structures are proximate.  Increasingly though, watershed and fisheries studies have shown that landslides are important sources of mineral and organic matter entering rivers and streams.  Input of these materials is important to providing adequately sized spawning gravel for fish, and nutrients for aquatic plants and animals.  The addition of these materials through landslides and other mechanisms is also critical to the proper functioning of floodplains, which are vital to regulating peak flows during floods.  In this way, landslides and other types of geologic mass movements may protect human lives and property.

Pre-Dam Removal Aerial Photo Showing January 2014 Landslide Location

Great “Pre-Slide” Oct13 Photo of Rock Wall by Darryl Lloyd — Thanks Darryl