16 December 2020
Bute Inlet: a very long runout proglacial landslide in Canada
News has emerged over the last few days of a recent very large landslide close to Bute Inlet in British Columbia in Canada. This is a really big one – I think the runout is in the order of 13 km, based on a back of the envelope calculation from Google Earth. This landslide has been reported as a variety of phenomenon including a glacial lake outburst flood (GLOF) and a tsunami, and as loyal reader Hig points out below there is an element of these in the chain of events. The location is approximately 50.975, -124.609.
A good starting point is a set of videos posted to Facebook by 49 North Helicopters, who I think discovered the landslide. These show the track of the lower part of the landslide, which was confined within the valley below the glacier. You should be able to access them via the embedded tweets below:-
— Brent Ward (@GeoBrentatlarge) December 14, 2020
Brent Ward from the Department of Earth Sciences at Simon Fraser University has been tweeting extensively about this landslide, and he has also appeared on the TV News to discuss the slide. His interpretation of the events is I think spot on, and my description here is based on this.
The full extent of the landslide can be seen on the Google Earth image, obviously captured before the failure, below. I have annotated the location of the original landslide, the position of Elliot Lake, the track of the landslide and the location of the outwash deposits:-
Planet Labs has imagery from 2 December 2020 that captures the aftermath of the landslide – clearly the event occurred before this date. This is the source area of the landslide and the remains of Elliot Lake:-
Note the scale bar in the bottom right corner of the image – this is a very large failure. The interpretation is that a large failure occurred on the rock slope to the west of the front of the glacier. The very large mass entered the lake, driving a huge displacement wave and entraining both the water and lake sediment to form a catastrophic debris flow (or a hypersaturated flow?) down the valley. The track of the flow is captured very nicely in the 49 North Helicopters videos. This is a still from one of them:-
At the mouth of the valley there is extensive deposition of sediment:-
The flow then travelled westwards down the Southgate River into Bute Inlet, where a large amount of floating timber was observed.
This is a classic compound hazard chain – landslide, displacement wave, debris flow, debris flood and possibly even a submarine density flow in Bute Inlet.
Detailed analysis of this event may need to wait until the Spring, but good data should be available from the regional seismic network, which should yield data on the timing, volume and velocity of the event. It is very interesting that this large failure occurred very close to the snout of a retreating glacier – there are parallels to the Barry Arm landslide in Alaska.