Tuesday, November 20, 2012

Glacier Peak and the Sauk - Stillaguamsih Divide

In the early fall I had a nice pass by of Glacier Peak. Glacier Peak is one of Washington's strato volcanoes. I have referred to it as the sneakiest as it does a fair job of hiding amongst the high peaks of the North Cascades Range. It is the 4th highest peak in the state at over 10,000 feet, but it has enough high peaks around it and it is far enough back into the main range and deep into wilderness that it gets overlooked. Unless you know what to look for it blends in with the other snow capped peaks when viewed from Seattle or Everett. Very different than the other big stratos in the state that are all much better known simply because they stand out well above everything else on the horizon.
 
Glacier Peak on October 9, 2012
 
In one regard the peak is well named as it is covered with glaciers. The peak always fascinated me ever since I saw Ira Spring's photographs of the peak in hiking guides.
 
I have never been up the mountain itself. I have done geology work of various sorts on the ridges and in the valleys all around the mountain. On each of the adventures I encountered evidence of huge and not very old (post last glacial period) eruptive events: piles of pumice fragments miles from the mountain itself. One of the first geology hazard projects I did at Stratum Group was assessing a steep terrace slope above the Sauk River flood plain a bit north of Darrington. Prior to arriving at the site I assumed the terrace would be underlain by glacial and alluvial sediments. Instead, I found the entire slope consisted of pumice fragments; the entire valley had been filled with pumice material and the river had incised down through the volcanic fill. This was an in your face evidence of a huge post ice age eruption.  
 
Glacier Peak's potential to cause problems in northwest Washington State is well illustrated in the image below and is summarized here:http://pubs.usgs.gov/fs/2000/fs058-00/
 
 
 
All that expensive flood management work on the Skagit River will be sorely tested in the event of a big mud flow from Glacier Peak. The pumice covered terrace I observed above the Sauk River near Darrington was associated with an eruptive event that erupted an estimated 5 times the amount of tephra Mount Saint Helens erupted in 1980.

One of the big post ice age eruptions from Glacier peak altered the route of the Sauk River. The Sauk formerly flowed west from Darrington down what is now the North Fork Stillaguamish River. The divide between the two rivers northwest of Darrington is a broad flat plain approximately one mile across.
 
LiDAR of the divide between the Sauk on the east and North Fork Stilliguamish on the upper northwest.

 It is interesting to contemplate what would happen if another large eruption would fill in the Sauk Valley and send the Sauk back down the Stilliguamish. The Sauk is a rather wild river subject to huge channel migrations during flood events along much of its lower course.

2 comments:

BAP said...

Dan,

Great post about Glacier Peak and the Sauk. I own a rafting company, www.adventurecascades.com, that runs trips on the Sauk and Skagit so we get to see the evidence of past eruptions from Glacier peak all the time.

Is there somewhere I can get a better version of the Lidar image on the Darrington area?

Thanks,

Brian

Dan McShane said...

Brian:

Snohomish County has an online map program that includes a LiDAR layer: http://gis.snoco.org/maps/snoscape/index.htm