A landslide, tsunami and then a flood: the massive hazard cascade that shook the world

New research reveals the intensity of British Columbia’s 2020 hazard cascade as members of the Homalco First Nation continue to pick up the pieces.
 

By Lauren A. Koenig, Ph.D., Science Writer (@Lauren_A_Koenig)
 

Citation: Koenig, L., 2022, A landslide, tsunami and then a flood: the massive hazard cascade that shook the world, Temblor, http://doi.org/10.32858/temblor.261
 

In late November, 2020 a geological mystery appeared on seismographs around the world. A signal comparable to a magnitude-5.0 earthquake emanated from deep within the southern Coast Mountains of British Columbia (B.C.), Canada.

The cause of this ground-shaking event remained unknown for two weeks, until forestry workers passing through traditional territory of the Homalco First Nation happened upon its aftermath in the Elliot Creek watershed. The glacier-carved valley, narrowly framed by mile-high rocky walls, was decimated by a massive hazard cascade — a chain reaction of geological events — involving a landslide, tsunami, outburst flood and sediment plume. What was once a verdant environment for the region’s famed salmon is now an ashen alley that fans out into a sea of debris.

The sheer scale of the cascade can be hard to comprehend even when viewing the valley from a helicopter, said Marten Geertsema, a research geomorphologist with the B.C. Ministry of Forests and the lead author of a new study that describes the events.
 

 

“It’s staggering when you just stand there,” said Geertsema. “It’s kind of hard to wrap your head around how powerful that all was.”

Homalco First Nation and researchers from the B.C.-based Hakai Institute are assessing the long-term ecological impacts on the region, especially for fisheries. Ongoing unstable conditions in the valley suggest that recovery could take decades. Moreover, Elliot Creek has erratically changed course numerous times in the past year, which can make restoration plans irrelevant essentially overnight.

“If we get a massive rain event like last year, the whole river could change again and it’s not money well spent,” said Erik Blaney, an environmental technical of the Tla’amin Nation who was contracted by the Homalco Nation to lead assessment and recovery efforts. “You’re playing with mother nature.”
 

A cascade of unfortunate events

The hazard cascade began with the fifth largest landslide on record in British Columbia, involving, according to study co-author Göran Ekström, the equivalent of the combined mass of Canada’s 25 million cars. Ekström is a seismologist at Columbia University. Nearly half of the debris crashed onto the toe of West Grenville glacier, near the base of the valley. The rest ran up the opposite wall of the valley before gravity carried it down once again. Traveling at more than 100 miles per hour (170 kilometers per hour), the landslide plunged into an alpine lake left behind by the glacier during its retreat over the last century.

Like the splash after a jump off a high-dive, the landslide’s impact was fast and violent: the rockfall catapulted enough water out of the lake to reduce its area by nearly 20%, creating islands in its newly shallow depths. In just over a minute, a tsunami wave towering more than 330 feet (100 meters) high sped across the lake before cresting the opposite shore, creating what is known as a glacial lake outburst flood.
 

 

The water was then forcefully channeled down the confines of the valley like a marble in a Rube Goldberg machine. Though it generally takes millennia for water to steadily erode deep ravines, the flood gouged out a groove 160 feet (50 meters) deep in the streambed within minutes.

As the creekbank gave way and trees were mowed down, the flood became a thick soup of debris that left an enormous fan of sand, mud and wood extending from the mouth of the valley. It contaminated local fresh and marine waterways, creating a sediment plume — suspended organic materials — that destroyed water quality.

“You need certain elements in place to create these massive domino effects,” said Geerstema. “This goes to show us the damaging footprint of these events when you have water in the right place.”
 

Looking with LiDAR

The landslide’s remote location meant that fortunately no one was around when the hazard cascade took place. To map out what happened, Geertsema, who regularly scours satellite imagery for evidence of landslides in high-mountain areas, worked with members of Canada’s First Nations, the Hakai Institute and other institutions around the world to simulate the events using numerical modeling and LiDAR — a survey method that pulses lasers from an airplane to create 3D representations of the surface.

Geertsema, who compared post-landslide images with those taken only one year prior, said the team was very lucky to have such detailed imagery. “We wouldn’t have been able to produce these models without that input data,” he said.
 

 

Fewer glaciers, more hazards

British Columbia’s mountainous terrain is no stranger to landslides, floods and tsunamis. Climate change, however, has exacerbated the impacts and frequency of these hazards — especially as warming temperatures cause ground-stabilizing permafrost and glaciers to melt away.

As glaciers retreat, weak bedrock loses the support that prevents its collapse, said Tom Millard, a research geomorphologist with the B.C. Ministry of Forests and co-author of the study. The meltwater lakes left in their wake, such as at Elliot Creek, also tend to get larger, which ratchets up the hazard of a potential tsunami or outburst flood.
 

Living with the consequences

The chain reaction of geological events created a cascade of ecological effects that will linger for decades. The flood destroyed most of the salmon population, as well as the spawning habitat that they return to each year. The fish are unable to survive current turbidity levels, which remain more than 25 times higher than normal (especially after a rainstorm), said Blaney.

More than food, salmon are an important part of the Homalco First Nation’s culture and livelihood. Grizzly bears’ annual feasting on salmon draws in tourism that helps the community thrive. But this past year, low salmon numbers meant the bears went hungry.

As recovery effort coordinator, Blaney has ideas for sustainable ways to help the ecosystem return to some semblance of normal. One solution is to prune crab apple trees as another source of food for the bears.

“It’s something that our people did before,” said Blaney.

Blaney is also considering installing a platform that would provide a safer way for researchers to monitor the salmon population, diverting the creek through a more stable area with remaining trees, and planting native vegetation to control for erosion.

Finding funding for these projects, however, is only one obstacle that is part of an even greater challenge: living with the increasingly stark effects of climate change. Severe wildfires in summer 2021 burned across B.C., and the Coast Mountains are experiencing some of the highest rates of glacier loss on earth, meaning hazard cascades like the one at Elliot Creek could become more frequent.

“I don’t think the average person living in a city can really understand or see the changes that we’re seeing and the devastation that they’re having on salmon and other important pieces of our survival and our culture,” said Blaney. “We’re seeing change, and it’s happening fast and it’s beyond any scope we could have imagined.”
 

Further Reading

For the full multimedia feature by the Hakai Institute — which includes video, interactive maps, and more — click here.

Geertsema, M., Menounos, B., Bullard, G., Carrivick, J. L., Clague, J. J., Dai, C., … & Sharp, M. A. (2022). The 28 November 2020 landslide, tsunami, and outburst flood–a hazard cascade associated with rapid deglaciation at Elliot Creek, British Columbia, Canada. Geophysical research letters, 49(6), e2021GL096716.

Menounos, B., Hugonnet, R., Shean, D., Gardner, A., Howat, I., Berthier, E., … & Dehecq, A. (2019). Heterogeneous changes in western North American glaciers linked to decadal variability in zonal wind strength. Geophysical Research Letters, 46(1), 200-209.