What would happen if the Barrier at Garibaldi Lake were to collapse?

Bad things. Terrible things.

Garibaldi Lake is a lake located just north of Squamish, BC, and lies between mountains and volcanoes. Surprisingly enough, the sheer amount of water the lake holds is more dangerous than the active volcanoes.

There is one thing separating the lovely town people of Squamish and approximately 1.2 x tons of water, and that is the Barrier. If it’s important enough to have a Wikipedia page, you better believe it’s pretty vital. The Barrier is about 300 metres thick and 2 kilometres wide, which seems like a lot, but if for some reason it were to break, all the water in Garibaldi Lake would come rushing out and probably swamp any surrounding towns.

There actually used to be a town next to the lake called (you guessed it) Garibaldi, but they had to relocate in 1981 because the dangerous tectonic plates and heavy rainfall made it an incredibly dangerous place to be.

The water in Garibaldi Lake is equivalent to ten times the amount of tap water Americans drink every day.

Calculations: 

Interesting note: while trying to find the equivalent, I discovered MYSELF that when Mount Vesuvius erupted in Pompeii in 79 AD, it erupted close to 1.35 x 10^10 tons of debris, which is a lot. About ten times more than Garibaldi. Source.

   Latex coding does not support exponents larger than 10. So I apologize for the not-so-fancy format. 

Sources:

https://en.wikipedia.org/wiki/Garibaldi_Lake

https://en.wikipedia.org/wiki/The_Barrier

http://www.baycountyfl.gov/water/facts.php

https://www.tripadvisor.ca/LocationPhotoDirectLink-g154922-d310715-i37635573-Garibaldi_Provincial_Park-British_Columbia.html (Photo)