If my geography lessons haven't failed me, an estuary is where a river meets the sea, right? So I would look at the closest sea and look where the rivers are meeting it. Not sure Michigan is a good place for estuaries...
For general mud, you can look at the 'mud maps' on quicksandfans and mudboyuk (second one requires an account but that's easy to make). I also look at websites from local nature services and AllTrails to find sites or trails that are described as muddy. Usually if the path is muddy, there's more mud off-trail.
Love you, too
