To estimate the volume of Garibaldi Lake, I would take it real simple and assume that the lake is as close to as a rectangle as possible. Just because it’s an estimate. That would be 
Next to the lake, there is the Barrier, a massive lava wall that acts as a natural dam keeps the lake in place and prevents it from gushing down to the villages. Right behind the Barrier is a portion of the lake called Rubble Creek. This creek drains the lake into the Cheakamus River. However, because the Barrier is truly the only object that keeps Garibaldi Lake in place, it has to sustain every milliliter of water pressure that the lake has to offer. So basically, the Barrier is sustaining 2,570,000,000,000 L of water. How? Well we know that 1 km=1000000 cm. So if we convert 
If the Barrier were to fail its job, then the lava dam explodes and all of the water comes gushing down the mountain, flooding the township of Garibaldi and Squamish, then drain into Howe Sound and raise the water level drastically. Almost 1 trillion L of water would escape the lake. All that water gushing down from 1400 m of elevation would have the energy 200 times larger than the one of the atomic bomb dropped at Hiroshima. Any wave that occurs during the fall will be 120 m high. Scientists say the collapse is inevitable; however, they also say it will not happen in our lifetime.
Garibaldi Lake in the top corner; the Barrier in the bottom center
Resources:
http://www.squamishchief.com/news/garibaldi-lake-a-ticking-time-bomb-1.1753732
https://www.unitconverters.net/volume/cubic-kilometer-to-liter.htm
https://en.wikipedia.org/wiki/Garibaldi_Lake
https://en.wikipedia.org/wiki/The_Barrier
https://en.wikipedia.org/wiki/Rubble_Creek