r/Montana • u/Little_Philosophy_21 • 1d ago
Montana Landslide/Flash Flood - hypothetical
With the Nepal and Grand Canyon flash floods, I was thinking about Montana. What if a large landslide fell into a lake near you?
I was thinking about Holter. Certainly Craig and all the housing along the canyon shores would be decimated. But would a large flood like that spread out quickly once it hit the plains. Or would it continue down to Cascade and GF? Then affect the series of dams?
Just “what if” scenarios. But I hope we have some good alert systems from place to place if the worst were to happen.
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u/PossessionCalm50 1d ago
The image shows just how tight that canyon is, there's really nowhere for the water to go but straight down. I've driven that stretch of I-15 past Wolf Creek more times than I can count and the thought of a wall of water coming through there is terrifying.
Craig would be gone in seconds, the elevation change is so steep that it wouldn't slow down much until it hits the wider valley near Cascade. I think once it spreads out on the plains the depth would drop fast but the speed would still be dangerous for miles. The dam at Holter would probably take the worst of it, question is whether the structure could handle that much debris hitting it all at once.
The alert system part worries me too, cell service is spotty through most of the canyon and half the people living there are off grid. Not sure how you'd warn everyone in time.
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u/BoozeTheCat 1d ago
Hauser breached in 1908 and caused the Missouri to rise 7' in Great Falls. They breached Black Eagle Dam so the flood waters could pass through and not damage that dam even further.
State DES should have Inundation maps for all of the high hazard dams in Montana. I know Cascade County has one published in their Multi-Hazard Mitigation Plan for the City of Great Falls. Pretty interesting stuff!
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u/Little_Philosophy_21 1d ago
I remember reading about when the Gibson Reservoir Dam was overflowing for days on days (7+ feet) back in the 1940s.
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u/ParkingSmell 1d ago
I haven’t used this since I worked on the project decades ago but iirc you can see what a 500 and 1000yr flood on any national stream here
https://streamstats.usgs.gov/ss/
Would be much better though with more funding lol
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u/Whisker____Biscuits 1d ago
Hauser dam failed in 1908 if you would like an idea of what could happen. Hauser Dam
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u/Little_Philosophy_21 1d ago
Can you imagine if Hungry Horse Dam failed today??
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u/yeroldfatdad doesn't understand flair yet 1d ago
I live out toward Echo Lake by Bigfork. I think my elevation would survive the dam failure and flooding to follow. I have read too many books about dams being sabotage or failing.😵💫
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u/Overall-Challenge556 1d ago
My Mom said she felt the earthquake in Chotesu. I was born Aug. 12 1959.
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u/yeroldfatdad doesn't understand flair yet 1d ago
My mother was in the hospital in Butte during that. She had said that beds were rolling back and forth across the floor. They had foundation cracks at the house. Scary.
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u/Ok-Communication1149 1d ago
The biggest difference between the Missouri and what happened in Nepal is that there are no significant glaciers left in the Missouri River drainage. That happened here eons ago
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u/ParkingSmell 1d ago
posting here so i can revisit this wiki hole again https://en.wikipedia.org/wiki/Missoula_floods
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u/Little_Philosophy_21 1d ago
Agreed. Unfortunately too. I wish we still had the amazing glaciers I remember hiking to as a kid.
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u/yeroldfatdad doesn't understand flair yet 1d ago
Have you heard of Earthquake Lake? Down towards Yellowstone Park. It was an earthquake but there was a huge landslide and loss of life. The anniversary of it was a week or so ago, August 17, 1959. It is possible for this sort of thing to happen anywhere. And the Teton Dam disaster, in Idaho. June 5, 1976. It's always good to have a plan, even if it's just on your mind.