I don’t have an answer (or knowledgeable opinion) but I wonder if someone at the Minnesota Valley National Wildlife Refuge might have some thoughts. (although the staff might all be on furlough now).
https://www.fws.gov/refuge/Minnesota_Valley/
I did find an Minnesota Public Radio article about reintroducing beavers to wild areas but it was a national scope and didn’t discuss Minnesota in particular.
https://www.mprnews.org/story/2018/06/25/npr-bountiful-benefits-bring-back-beavers
oh, and welcome to the forum… from another person who visits the Minnesota River with some regularity. I had to look where your observations were since the Minnesota River covers a bit of territory. But I think we’re in the same general section of the river. :-)
I’ve seen dams (and gnawed trees) but no beavers, yet.