Lake Michigan's depths: red shallow, dark blue the deepest part. Lake Michigan is divided into two basins. Where you see a change from green to dark blue shows the two basins. It takes 99 years for the water to circulate out of Lake Michigan from the south to the north. That's why it is important to keep pollutants out of the Lake. Read more about the circulation of water and more in my book, The Dynamic Great Lakes available in bookstores and on line at Amazon.com and Barnes & Noble as well as many other venues.