99% of the electricity in Quebec is from hydroelectric dams. This is how the costs are kept so low, and you don't really have to worry about resulting greenhouse gas emissions.
Don't expect much rotting that far north. It will take centuries to even start to rot. Also don't expect underwater logging either. This is the Boreal forest, not the rockies one. It's even past the northern limit for Boreal Cedar, the only mildly valuable tree you might find. After that, you get young evergreen and pine. They burn before they get old and grow slowly due to the climate.
Further, tiny dams produce very little power. So, if you play with the numbers you can get extremely different results.
On the other hand if you look at the annual 13,100 GWh from https://en.wikipedia.org/wiki/W._A._C._Bennett_Dam times the 49 years it's been in operation that's the equivalent of (1,000,00 kg / GWH from coal) * 13,100 * 49 = 707,573,630 short tons of CO2 which is vasly larger than all the biomass in the lake to start with. Further, you can log the land before you start minimizing the total biomass flooded.