Doesn’t the St. Lawrence Seaway close down about the end of November every year for precisely this reason? And reopen sometime in March?
That is correct.
The locks at Sault St. Marie also close down because the St. Mary’s River freezes over. Coast Guard ice breakers try to keep it open for part of the winter. I have worked on lake freighters, actually called ore boats, on what is called the winter run. Usually hauling coal or iron ore between ports on Lake Michigan and Lakes Huron and Erie. But as noted sometime Erie freezes over.