[cmrdporcupine]

I believe a lot of it is ultimately fed by hundreds of thousands of square kilometers of smaller lakes upstream. The Canadian Shield is dotted with gazillions of lakes, it's crazy to fly over it. And I think it's safe to assume most of those lakes are glacial remnant water, not sure if they're fully refreshed by rain and snow.

[bregma]

I know it's the internet and facts are as real as birds, but just in case you want some: [0].

[0] https://www.lre.usace.army.mil/Missions/Great-Lakes-Informat...

[garbagetime]

That's a pretty unecsscesary footnote

[maxerickson]

The watershed(s) aren't crazy huge: https://project.geo.msu.edu/geogmich/watershed.html

(Like just as a gut check, much of the border of Wisconsin and Minnesota is a tributary of the Mississippi River)

[throwaway894345]

Then I would expect these upstream lakes to be shrinking as of the last 10-15kya.