During a hike last week on Tricouni Peak with family and friends, Whistler's Christy Craig had just finished explaining the history of the Barrier — an ancient wall of hardened lava that holds back Garibaldi Lake — when the group witnessed a small rockslide on the Barrier itself.
"Literally two minutes later, we were heading down the road and my 10-year-old son says 'Look, it's sliding!' so we stopped to watch it," Craig said.
"It was pretty incredible to see and I'm pretty glad it stopped when it did."
Reports of the slide prompted an assessment from the provincial Ministry of Forests, Lands, Natural Resource Operations and Rural Development, which found no obvious, large accumulation of rock at the bottom of the slope, and no fresh indication of movement of the barrier.
"We had one of our qualified professionals attend the next day, look at it from the air, and he quickly came to the conclusion that there was no immediate threat to the public," said Dave Southam, district manager for the Sea to Sky natural resources district.
"Just being abundantly cautious, we always make sure that we have our professional team look at it and make an assessment on whether or not there is any downstream or downhill threats to the infrastructure or public safety."
Steve Quane, a physical sciences tutor with a focus on geology at Quest University, has been studying the Barrier and the surrounding Garibaldi Lake system for years.
Though there's been plenty of documented rockfall on the Barrier, including a very large landslide in the mid 19th century that dammed the Cheakamus River, there's been no recorded movement of the Barrier in recent memory, he said.
"We have these rockfalls all the time, some are seen by people and it's kinda fun, and it sounds like this was pretty big, and dry enough that the rock flower was making quite a big plume and got a lot of attention from people," Quane said.
Judging from the photo Craig took of the slide, Quane estimated that this particular area has probably collapsed quite a bit in the past.
"What we can see are textures that are indicative of the lava interacting with the ice... the big wall of the Barrier doesn't appear to have a lot of its primary textures left, so we think that they've collapsed off over time," he said.
"These events happen a lot on a smaller scale. If you go up there, especially on a spring day, and sit at the Barrier and look out for an hour, you'll see and hear many rock falls, especially in the spring — because of the freeze/thaw cycles, things are loosening up and collapsing."
But in terms of a catastrophic Barrier collapse, sending Garibaldi Lake crashing into the valley below, there's likely no need to worry, Quane said.
"It's an interesting system because you do have this unstable lava flow that's holding back a very large lake, and the lake is draining from below that dam... (but) I don't think we're really concerned about a catastrophic collapse of the entire barrier system because you're looking at like a kilometre of lava flow," he said, adding that the concern is small-to-midsize rock fall events.
"Geologically, in geologic time, it's a different story, (but) in terms of our lifetimes and human timescales, I think we're more worried about the parking lot for the Rubble Creek trail there and then potentially the highway."