For this task, we must find an about volume for Garibaldi Lake. We know the surface area which is 9.94km2 and the average depth is 119m2. You want to start of with conversions. We can make everything in meters.
9.94km x 1000m = 9940m
9940 m x 1000 L = 9940 000 L
9940 000 L x 119 m = 1 182 860 000 L
The volume will be 1 182 860 000 L
1 L of water = 1 Kg of water = 1 m3
The lake is 15.7 m deeper than the barrier. So 1 182 860 000 Kg = 75 341 401.27 Kg is about how much 15.7
water is behind/against the barrier, and nothing bellow.
This is definitely an over estimate as the lake is not consistently 119 m deep, but that is the average depth. The lake is also not a perfect shape so it will not be exact either.
If the barrier was to fall, I think that the water behind it would rush out at great force. If the water is being held behind it and it were to fall, the water would escape. Water is very heavy, so I believe with the amount of water that was being held back it would take out trees and anything that would be in the path of the barrier.