What is the anatomy of an avalanche? See what makes a fatal slide
An avalanche Tuesday near Castle Peak in the Lake Tahoe area swept away 15 backcountry skiers and guides, killing nine, The Sacramento Bee reported. Six were rescued. A search continues for one missing skier, who is presumed dead.
It is the deadliest avalanche on record in California.
Here’s what to know about avalanches:
What is an avalanche?
An avalanche is a rapid flow of snow down a hill or mountainside, according to the National Weather Service.
Avalanches can reach speeds of up to 100 mph and vary in destructive potential, ranging from harmless slides to ones that can destroy forests or flatten villages, the American Avalanche Association said.
What causes avalanches?
Skiers, snowmobilers and hikers can set off an avalanche when a layer of snow collapses and starts to slide down the slope. In the U.S., avalanches are most common from December to April, but they can happen at any time if the conditions are right, National Geographic reported.
“Avalanches are based on the storms that come into the area and the existing snowpack structure,” Steve Reynaud, a forecaster at the Sierra Avalanche Center, told The Sacramento Bee.
The heavy amounts of snow can then roll down a nearby slope, such as a hill or mountain, Reynaud said.
“Avalanches can also be caused from existing snow, the snow that comes in, or avalanches that occur down deeper in the snow,” Reynaud said.
About 90% of avalanches involving humans are human-caused, the Utah Avalanche Center said.
Contrary to popular belief, loud noises don’t cause avalanches, unless they are extremely loud, like the sound of an explosion.
How deadly are avalanches?
About 40 people die in avalanches each year in North America, the American Avalanche Association said.
California averages one avalanche fatality per year, according to the Colorado Avalanche Information Center.
Before Tuesday’s avalanche, at least six people in the U.S. had died in avalanches this season, according to the center. That includes two earlier deaths in California: a snowmobiler who was killed near Castle Peak on Jan. 5 and a ski patroller killed near Mammoth Lakes on Dec. 26.
According to Wendy Antibus, education manager for the Sierra Avalanche Center, the risk of an avalanche being fatal comes down to a variety of factors, including the size of the snow slide, how dense the snow itself is and what kind of terrain the avalanche is in.
How do avalanches kill?
“The good news is that even dense avalanche debris is about 60-70 percent air, but that’s not the problem,” the Utah Avalanche Center said.
While people caught in a fast-moving avalanche also risk being battered to death, most avalanche victims die because carbon dioxide builds up in the snow around their mouth, the center said.
Statistics show that 93% of avalanche victims can be saved if they are dug out in the first 15 minutes, but the numbers drop “catastrophically” after that, the center said.
“You have about 20 minutes to live” before your chances for survival are basically cut in half, according to Simon Trautman, director of the U.S. Forest Service’s National Avalanche Center in Bellingham in Washington state.
After 45 minutes, only 20%-30% of avalanche victims are still alive and after two hours almost no one is alive, according to the Utah Avalanche Center.
“In other words, you don’t have much time,” the center said.
According to the Forest Service, factors that can affect your survival rate after being trapped in an avalanche include how long you are buried under the snow, how deep you are buried and the injuries you may suffer as you’re swept down the mountain.
Antibus said being dragged into trees or over cliffs are common ways people die from trauma injuries in avalanches.
“The best way to survive an avalanche is not to get in one in the first place,” Trautman said in a Forest Service news release in 2021.
Can you die in an avalanche if you aren’t in the backcountry?
According to Antibus, avalanche-prone terrain is not exclusive to deep, wooded areas of the mountain.
Across the country, some sections of highway roads cross avalanche paths in the mountains.
However, Antibus said highway officials frequently conduct avalanche mitigation efforts around parts of the highway known to be in avalanche terrain.
Antibus said actively traversing the backcountry is the most likely way to be hit by an avalanche, even while walking or snowshoeing. Still, she said she has never heard of any snowshoe-related backcountry avalanche deaths in the Lake Tahoe area.
“You have to be in terrain that can produce an avalanche in order to get caught in an avalanche,” Antibus said.
What’s it like to be caught in an avalanche?
Bruce Tremper, an ex-national circuit ski racer, recounted his story of being caught in an avalanche as a 24-year-old in 1978 to the Utah Avalanche Center.
He was caught up in an avalanche while tightening bolts on chairlift towers at a ski resort, working alone and without a beacon, when he failed to out ski an oncoming avalanche, Tremper said.
“Then the tumbling started, over and over like being stuck in a giant washing machine filled with snow,” he said. “My hat and mittens were quickly ripped off along with both my skis. Snow went everywhere, down my neck, up my sleeves, down my underwear, even under my eyelids, something I would have never imagined.”
Every time he opened his mouth to breathe, the avalanche pushed more snow into it, Tremper recalled.
“Just when I needed to breathe the most, I couldn’t. Drowning to death, high in the mountains, in the middle of winter and miles from the nearest water,” he said.
The avalanche finally slowed down and he was able to “swim” to the surface before the snow set “like cement.”
“Everything below the snow surface was like a body cast,” Tremper said. He luckily escaped serious injury, although both his skis were broken.
What do avalanche warnings mean?
Avalanche warnings are issued by the National Weather Service and the U.S. Forest Service through local avalanche centers.
Like other weather advisories, avalanches have two types of advisories based on level of severity. Avalanche watches are less severe and are considered a planning tool, issued when dangerous conditions are expected to develop in 24-48 hours.
When an avalanche warning is issued, it indicates dangerous conditions for avalanche formation actively exist. According to the National Avalanche Center, warnings are usually put in place when the avalanche danger is forecast at “high” or “extreme,” the two highest danger levels.
Traveling through terrain under an avalanche warning is highly discouraged.
This story was originally published February 22, 2026 at 5:00 AM with the headline "What is the anatomy of an avalanche? See what makes a fatal slide."
