What part of the Mississippi River becomes unnavigable (either freezes or accumulates too much ice to navigate) during an average winter? Perplexingly I could not find this through Google. I suppose no part could become unnavigable all year-round? But that seems unlikely to me as the Mississippi reaches well into Minnesota.
Upper Mississippi river near St Paul Minnesota is closed December - March. US Army Corp of Engineers is responsible river navigation in the United States. From winonapost.com, (from 2015) the article states that all vessels need to be south of Lock and Dam 9, near Lynxville, Wisconsin. Sounds like the upper river is closed so locks can be dewatered and maintanence performed.
My guess is most of the Mississippi river remains open to navigation south of Wisconsin during the winter.
While working in southern Minnesota for several months in the winter and spring of 2022, I noticed the river channel was frozen solid at La Crosse throughout January and February. When I first arrived in the area in early December ‘21, there was ice, but it was broken ice covering the river channel. Being from the south, I was fascinated by this and would always observe the River as I crossed it going into La Crosse each week. There was a lot of ice fishing on the tributaries/marshes just off of the river in the area, and I’m pretty sure I spotted some ice fishing in the distance on the Mississippi itself just north of the dam and not too far from the river bank in La Crosse (La Crosse side of the river). All beautiful and fascinating.