By 8,300 years ago, it had drowned oak trees about 9 miles north of the modern coast. And by 6,300 years ago, the elevation of the lake was near its current position. The water continued to rise more than 20 feet higher than today by 5,500 years ago. The rising water was probably caused by two concurrent phenomena. One is a process called isostatic rebound.
Isostatic rebound is an uplift of the earth's crust after a weight has been removed.
In the Great Lakes area, the weight was the glacial ice. But now that the ice is gone, the crust is popping back up. The rebound more than 6,000 years ago elevated the outlets draining across Canada so that flow had to be redirected southward, following the current pattern of Great Lakes drainage. Lake level in Lake Michigan is a mirror image of temperature. Lake level falls.