Mt Sulzer Debris Flow

A large debris flow on the White River, Wrangell-St Elias National Park and Preserve.

NPS/Jeff Tropp

Over the last several years, at least three large earth movements have occurred on the north side of Mt. Sulzer, in an unnamed tributary drainage of the White River within Wrangell-St. Elias National Park and Preserve (Figure 1). The tributary goes by the local name Flat Creek, and is in a remote area known mostly by outfitters, pilots, and hunters who spend time in the vicinity.

With assistance from geophysicists Chris Larsen, University of Alaska Fairbanks and Kate Allstadt, U.S. Geological Survey, as well as observations from ranger Luke Wassink and land specialist Josh Scott, both with the National Park Service; pilots Gary Green, McCarthy Air, and Paul Claus, Ultima Thule, geologist Michael Dokukin, Nalchik High-Mountain Geophysical Institution, outfitter Tom Vaden, Solo Creek Guide Service, LLC, and others I’ve been slowly piecing together what has happened out there. This briefing statement serves as an update on what I’ve learned, and as a primer for those who might be interested in helping to figure out why it’s happening.

Figure 1. General location of Flat Creek debris flows (red polygon) within Wrangell-St. Elias National Park and Preserve
(green boundary). Location of Glennallen, McCarthy, and Mt. Churchill are indicated.

Summary of Events

The history of earth movements (all appear to have been debris flows with a high water content) on Flat Creek was first determined roughly from interviews, satellite imagery, and historic photos, and now has been confirmed by seismic records from the area.

In order, the three known events were:

Event 1. August 21, 2013, 10:08 pm

Based on runout debris observed in satellite imagery, this was the first big debris flow at Flat Creek. Nobody witnessed it. The runout from this event overtopped a significant hill in the valley floor but did not reach the White River (see image at the top). From available images, it appears that the deposit was hummocky, unconsolidated, and has a lobate terminus that is several to tens of meters thick.

Interestingly, this event was preceded and followed by a cluster of small (<=4.1) earthquakes centered approximately 20 km southeast of Mt. Sulzer. They may or may not be related.

Event 2. July 30, 2015, 5:25 pm (with “aftershocks” at 5:34 and 5:57)

This was the largest debris flow at Flat Creek, and based on available imagery was the first sizable event to occur after the 2013 slide (Figure 2). The runout from this event climbed even higher on the hill overtopped by the previous event: over 67 meters (220 feet) above the valley floor. The runout reached the White River with a less lobate form than the previous event. The runout material was unconsolidated and included individual clasts up to nearly a meter in diameter. Details of the source area for the debris flow are uncertain, but if the event was sourced along the ridgeline, as appears likely, the along-path runout distance was approximately 11 km, and drop height was over 1500 m.

Event 3. August 10, 2016, 11:27 am

This flow was much smaller than the prior events, but is notable because it was witnessed by NPS rangers Peter Christian and Luke Wassink. Wassink shot video of the event, available online at https://www.youtube.com/watch?v=Xh23H3QApk8. Judging from the video and later photographs of the deposited material, this event was very ice-rich and apparently carried a lower concentration of sediment.

Figure 2. Oblique view of the runout of the July 2015 debris flow. The slide path is outlined in red, but note that details of the source zone for this event are poorly understood, and that the origin of the slide may be broader, or more localized, than what is shown here.

Image from Google Earth, September 7, 2015

Source of the Material

Because the first two, larger events were not witnessed, and because only the runout of the last event was seen, it is difficult to characterize the source of these debris flows. Some photos are available from aerial overflights of the source area in 2016, but there is no clear and evident scar to which the slides can be attributed. Sequential photos from before and shortly after the large 2015 event demonstrate clearly that this event left behind a large amount of ice on the steep, previously bare slopes at the head of the valley. That, along with extensive crevassing evident in the remaining glacier ice above those bare slopes, suggests that the event was associated with the release of substantial glacier ice from the slopes above that bare bedrock. It is unclear whether collapse of the glacier ice triggered the event, or conversely was caused by initial failure in the bedrock below the glacier.

In any case, by the time of an overflights in summer 2016, the barren slopes below the ice cliffs were again exposed. Images from two flights show exposed bedrock and sediment that appears weak, multi-colored, fine-grained, shot-through with dikes, and plastered in places with fragments of remnant ice. The rock in this area was mapped by Richter as Hasen Creek Formation (Early Permian)—fossil-rich marine argillite, shale, and sandstone; nonvolcanic origin, but to my knowledge nobody has been on the ground in Flat Creek to do any close analysis or sampling.

Perhaps importantly, the head of Flat Creek is very close (<1 km) to the mapped strike of the late Quaternary Totschunda Fault. Bedrock in the vicinity of this fault would be expected to be sheared, deformed, and weak (Figure 3). Also notable in the vicinity, though perhaps irrelevant, is the presence, 25 km to the south-southwest, of Mt. Churchill. Mt. Churchill is a volcano believed to have erupted twice in the last 2000 years, and is the source of a substantial ash deposit (“The White River Ash”) that overlies surficial sediments in much of the Flat Creek area.

Figure 3. Approximate outline of recent slide activity on Flat Creek (red polygon) in comparison with the strike of the Totschunda Fault (green line).

Base image is hillshade from IFSAR-based DEM acquired in 2014. Blue lines outline glacier boundaries from the Randolph Glacier Inventory v 5.0.

Related Glacier Surges?

Though perhaps unrelated, I’ll note here that glacier surges have been noted within the vicinity of Flat Creek very recently. A glacier in the valley immediately west of Flat Creek (~2 km away) has been surging for at least the last year, and there is some ambiguous evidence that tributary glaciers in the drainage basin immediately south (that flows west into the terminus of Russell Glacier, ~5 km away) have also been surging. Looped medial moraines on the lower Russell Glacier clearly attest to other, older surges occurring in the area.

Next Steps

We have taken some steps already to characterize this landslide. In addition to the interviews, photos, and satellite images already reviewed for preparation of this briefing, and the seismic data gathered to better characterize the timing of the events, NPS had the landslide flown with equipment necessary to create a digital elevation model and orthophoto using Structure from Motion (SfM). These data, acquired 1 June 2016, provide an extremely accurate and detailed overview of the slide path and most of the source area after the largest, 2015 debris flow. We have only completed very preliminary analysis of these data, however (e.g. calculation of runout height over the prominent hill), and more detailed characterization of the terrain is an important next step.