It's a bit late to bemoan something that was done in 1913. The decline that you watched for 30 yrs must have been from other factors than the dam e.g. changes to other areas of the watershed.
The Eugenia plant was 1 of the first in Ont and supplied a big area at the time. But that was before central AC, TV, computers etc. It's our insistence on those things that's degraded many lakes. Pretty well all the S Ont lakes are dam controlled---Muskoka, Haliburton etc.---as well as many in N ont.