It has been several decades since my studies of isostatic rebound, but as I remember, what is happening is that the rebound further south is pretty much done in comparison to further north. The result is the land is slowly tilting with areas to the north getting higher which tends to drain Huron more. Not saying that dredging has not had an impact, but if it did have a huge impact then Erie would be much fuller than it is.
Finished the circumnavigation in early February in Grenada. Have to work on a book project for the next several months so the boat will be waiting for next year.