Erosion at the bottom of Lake St. Clair is causing a drop-off in water levels on Lake Huron and Lake Michigan.
A privately funded report says the drop-off is apparently caused by years of dredging and other human activities on the river.
The volume of water they have lost during since 1860 would fill Lake St. Clair 28 times.
Several environmental groups say the report illustrates the unintended consequences of large-scale activities that alter natural geology of the lake system.
The $200,000 study was commissioned by a group representing about 17,000 people who live on the on the eastern side of Lake Huron.