There are lots of fascinating landslides along the Sea to Sky corridor, each with their own geological history and characteristics, and it’s a great place to see them. Landslides have always been part of life in the Coast Mountains, and are included in First Nations oral history and place names, such as Mount Currie near Pemberton, known as Ts’zil, or “slides on the mountain” in the Lillooet language. There are literally hundreds of documented landslides sometimes causing devastating property damage and loss of life, with more than a hundred fatalities in the past 170 years.
Landslides are not an abstract to Sea to Sky residents; there seems to be a large one every few years. Landslides usually occur due to a combination of factors such as steep, unstable mountain slopes or weak rocks which are then triggered by an event like an earthquake or heavy rainfall that pushes the slope over the edge causing it to fail, like the proverbial “straw that broke the camel’s back.” This knowledge is not very reassuring as you drive up Highway 99 past Porteau Cove, as rain cascades in torrents over massive granite blocks held in place by tiny steel anchors.
In southern B.C. some of the factors causing landslides can be directly attributed to the Cascadia Subduction Zone, a dynamic and destructive plate boundary located just west of Vancouver Island, that produces lots of ground-shaking earthquakes that can initiate landslides. This plate boundary has also forced the Coast Mountain rocks upwards, creating a steep, mountainous terrain prone to landslides. There are also about 30 volcanoes, including Mount Garibaldi, Cayley and Meager, which occasionally blow their tops, and have steep slopes formed from weak rocks that have historically produced massive, dangerous landslides.
Glaciation has dramatically changed the Coast Mountains, leaving us with very steep mountain bowls and U-shaped valleys prone to failure and thick piles of sediment ground out by Ice Age glaciers. Southern B.C. is also wet, with about 1,500 millimetres of precipitation annually—which can promote slippage, loosen or dislodge rocks, and saturate the earth triggering soils or debris flows.
Humans can also cause landslides as we change the landscape to fit our needs. This human intervention is responsible for a spate of landslides, especially around Porteau Cove and Lions Bay during the building booms in the 1950s, ’80s and ’90s. Thankfully much of the risk is now largely mitigated. Logging activities, which remove forest cover and increase water runoff, also commonly produce mud/debris slides.
In recent years there is also a growing concern about the effects of climate change on mountain stability. In the Whistler area the permafrost level has historically been about 1,800m above sea level, but due to global warming the permafrost is starting to melt, reducing cohesion, weakening outcrops, and releasing water promoting failure. There have been multiple examples of this in recent years, including the Joffre Peak rock avalanche in 2019 that sent 6 million cubic metres of debris into the valley.
Sir Isaac Newton famously commented whilst sitting under an apple tree, “what goes up must come down.” Whether it’s an apple or a pile of rocks that could fill BC Place, gravity prevails, and the Coast Mountains are collapsing. I wonder if global warming will increase the frequency and rate of collapse? I wouldn't be surprised.
Written by Stephen Carney
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