See: https://en.m.wikipedia.org/wiki/Selkirk_Concession

And in Montana it's Triple Divide Peak https://en.m.wikipedia.org/wiki/Triple_Divide_Peak_(Montana)

Northeastern North Dakota. The website just shows it stopping at Canada since it's based on USGS data.

I think it must be northern?

Yes, anywhere that drains to the Red River, which flows north. The Louisiana Purchase you'll recall only covers land that drains into the Mississippi.