Selected Examples of Mass Wasting
on Mount Shasta

A Term Paper
by Bob Musgrove
Geography 581 Geomorphology
Professor: Dr. Mairs
Southern Oregon University 
 Introduction | Snow Avalanches | Jokulhlaups | A Unique Debris Flow | Conclusion | Sources

A UNIQUE DEBRIS FLOW


One Friday in August 1999 I was working as caretaker at Shasta Alpine Lodge, the climber's hut at 8,000 feet in lower Avalanche Gulch on Mount Shasta. The early evening brought an intense rain storm which tested the abilities of my tent, and the following afternoon four climbers reported a mass wasting event to me. The climbers told me that a slow-moving slide consisting of small boulders, debris, and water had run past them as they were descending the mountain. The slide made a loud, rushing noise which first attracted their attention, and the party actually had to dash out of the slide's path as it overtook them. Subsequent interviews of U.S. Forest Service rangers informed me that the flow continued, off and on, for "a couple of weeks" and that water had been observed in the slide path (Towner 1999). My own observations of the mountain yielded two more new slide scars, both on Shastina, that had originated at similar high altitudes (above 12,000 feet). I decided to first climb up to and photograph the Avalanche Gulch slide, and to later interview Forest Service soil scientist Peter Van Susteren.


Figure 9. This USFS aerial photograph shows the debris flow in red. The green line is the path of the January 1997 avalanche.

     When I hiked up to the debris flow I found that it originated in the Trinity Chutes on upper Casaval Ridge, flowed down to an area below the Heart, and then followed the path of least resistance to an area just above Lake Helen. Most of the debris flow track consisted of a trench about 1.5 to 3 meters wide and about 1.5 meters deep with well-defined, miniature levies. Apparently the debris flow did not have a great deal of momentum because once the slope angle leveled off above Lake Helen, the track spread out and deposited material in an a pattern that resembled several overlapping alluvial fans. The debris flow started at an altitude of approximately 12,800 feet and slid down to about 10,800 feet over a distance of 1.4 kilometers.


Figure 10.
Photo taken from about 11,000 feet. The 1.5 meter ski poles are in the trough carved by the debris flow. The large rocks were left by viscous material dropped the rocks as it flowed down the trough.

Figure 11. The point of origin for the debris flow. This part of the flow is show in red in Figure 10.

Causes of the August 1999 Debris Flow on Mount Shasta


The rainstorm the night before the slide, the properties of the volcanic materials previously deposited at high altitude, and the slope angle may have all contributed to the event. As stated earlier in this paper, shear strength holds material in place on slopes, and shear failure can occur if the stress is strong enough to overwhelm the forces of cohesion and resistance in the layers of material (Easterbrook 1999). In this case, the addition of rain water to the volcanic debris which was supporting itself may have contributed to shear failure by lessening the cohesion of the debris, causing the material to reach its plastic limit (Van Susteren).


Figure 12. A scanning electron microscope image of the finer material from the debris flow. The blue line scale in 0.5 mm in length. Fine particles such as these transported the large rocks in Figure 10.


Figure 13.
This energy dispersive x-ray analysis for the material imaged in Figure 12 was produced with the SEM. The spike shows that silicon is the most common element in the sample.

 

 

 Introduction | Snow Avalanches | Jokulhlaups | A Unique Debris Flow | Conclusion | Sources

web page authored by Bob Musgrove