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.
In South Africa doing lots of boat stuff. Departure north from Cape Town around December 15th. Arrival in the Caribbean around the beginning of February, after 5300 miles or so.