You were a lot closer than I was. I had to look it up, lol. And I'm Canadian.

I never remember learning about the watershed origins of the name. In school, I only remember them teaching that Upper and Lower Canada names are the opposite of the way they appear on a map and that's how you remember them.

AFAIK, there's no major "Laurentian" river in the area. There are Laurentian mountains, a Laurentian University and Quebec elites like Trudeau are referred to as Laurentian elites somehow.