yeah, canada has a crapload of lakes, mostly in the norther regions.
These lakes (in the band you mention above) are generally caused by glacial advance and retreat... the glaciers essentially scrape off the land as they advance, and as they retreat, they dump everything they scraped up and move backwards... this creates large holes where the softer rocks wear away faster... but also the deposits the glacier makes can create lakes as well. heres an example of what we mean by a lot of lakes, this is near winnipeg, but areas all along canada have similar lakes.