My Estimate on how much water the Barrier contains behind in the lake is about 3/4 of the water in Garibaldi lake which is about 4,500,000,000 cubic meters since the volume is 6,099,609,059 cubic meters.
If the Berrier faulted, what do you think would happen. It would wipe out a lot. The weight of the water after the berrier would be broken would be 2,766,764.519 pounds per cubic meter, which could kill someone 35,000 times.
I would think that since the lake is 258.7 meters deep so if the berrier were to break i would imagine that only 3/4 of the water out.