To estimate the volume of this lake we would need to take the surface area that is given and then multiply it by the average depth of the lake.

Now to find out how much water the barrier contains behind it, we need to look at how tall the barrier is and it is taller than the average water depth therefore it holds back all the water. So we need to multiply the surface area by the average depth.

The surface area= 9.94 $km^2$

The average depth = 119m

To multiply them I first converted them to the same units. So I converted 9.94  $km^2$ into  $m^2$

so 9.94 $km^2$  would convert into  9 940 000 $m^2$

Now you multiply 119m X 9940000 $m^2$ And it equals 1 182 860 000 $m^3$

Now we take 1 182 860 000 $m^3$ and turn it into Litres and therefore we know that the volume of the Garibaldi Lake is approximately 1 trillion litres.

Now if the barrier was to explode or fall apart most of the water will be released but some of the water would not because there are  some holes on the ground which will be filled with water.

The barrier is an unstable damn for this big lake. The potential energy is 200 times the energy released by the bomb on Hiroshima,

The barrier is 1 400 metres tall, it is a ticking time bomb, “if it were to explode or fall apart in an earthquake, the wave that would be produced by all that water flowing out of Garibaldi Lake down into Squamish would be 120 metres high” -Steve Quane from Quest University

My sources:

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

http://www.squamishchief.com/news/garibaldi-lake-a-ticking-time-bomb-1.1753732