I am probably sticking my foot in my mouth.... When I read your post I started searching for lake tides from the great lakes and found a few references including the one below. I thought your numbers were a little large though.
"Some people are curious whether we have tides on Lake Superior. Yes, we do, but they are so small that they are not observable. About an inch of rise or fall because of the moon and sun's gravity is all that can be expected. However, we have another water lever fluctuation called a seiche (pronounced saysh), which can amount to about one foot on this lake. It is caused by atmospheric pressure (high at one end of the lake and low at the other), and by wind blowing in the right direction, which piles water up at one end of the lake, lowering it at the other. " http://bobscabins.duluth.com/geology.html
Open Seasonally
May 1st - Mid October