Friday, April 18, 2014

Church Mountain and Sanford Pasture Landslides

LiDAR imagery does not extend all the way up to Glacier, Washington. As noted in my last post, the Church Mountain Landslide fills the valley floor of the North Fork Nooksack River near Glacier.
DEM (digital elevation model) of Church Mountain Landslide
Summit of Church Mountain is on the upper left
Lumpy area in lower center filling the valley is the thicker part of the slide deposit
The slide deposit extends nearly all the way to the left edge of the image
Note the change in image sharpness with LiDAR DEM on the left half

The Church Mountain slide is absolutely huge. Not to take anything away from the Church Mountain landslide, but the petty side of me has to point out that the Church Mountain landslide is not the largest landslide in the United States or even Washington State. The Sanford Pasture Landslide west of Naches, Washington is substantially larger. A very small secondary portion of this landslide collapsed a few years ago and blocked the Naches River. 
DEM of Sanford Pasture Landslide west of Naches

USGS topo map of Sanford Landslide

Aerial of Landslide

A final note: Sanford is not the Washington State champion; there is an even a bigger landslide south of Wenatchee.


Doug McKeever said...

Dan: Very interesting! Are there estimates of volumes of either the Sanford Pasture landslide, or for the even larger one you mention south of Wenatchee? Not wanting to minimize either of those,but talking in superlatives, how about the volume of the Mt. St. Helens debris avalanche of May 18, 1980, recognized as the largest historic landslide on Earth with a volume of 0.67 cubic miles! Then there is Heart Mountain in Wyoming with an original volume estimated at an unimaginably large 3250 cubic miles. If this is truly a slide block, and not a thrust, it compares with the largest identified submarine landslides such as the Storegga submarine slide off Norway (840 cubic miles)or the Agulhas submarineslide off South Africa
(4800 cubic miles, the largest volume mass movement event known on Earth). References are given from sources on Wikipedia articles.

Dan McShane said...

I have not been able to find a good volume estimate but I came up with approximately 0.5 cubic miles for Sanford Pasture. I assumed 5 miles long, .2 miles deep (1,000 feet) and 0.5 miles wide. I have to do some research on the reported #1 landslide near Rock Reach on the Columbia - it might be bigger than St Helens.