“The extreme low levels earlier this year left many asking, “Where did the water go?” The answer is that it simply evaporated. The surface of the Great Lakes acts like an enormous evaporating pan under the right conditions. As explained in a previous post, the lack of ice cover in 2011-12 and record-breaking warm temperatures created ideal conditions for high rates of evaporation on Lakes Superior, Huron, and Michigan. These lakes had already been fluctuating below average levels for 15 years. A severe drought prevented the lakes from replenishing themselves, and water levels reached record lows.
Another reason the water levels on Lakes Michigan and Huron are lower than normal is the past dredging and erosion in the St. Clair River that resulted in a 10- to 15-inch (25- to 38-centimeter) lowering of water levels. These historic losses were never offset with mitigation measures. The only dredging that occurs today is to keep rivers at authorized depths for navigation. Recent studies show this is not the cause of low water.
Even though this is well documented by the agencies that have been monitoring water levels, going back to 1918, various theories about possible causes abound, especially online. Some of these theories and misleading facts get repeated so often, they become mythic. As the lakes begin their seasonal decline, a little myth-busting is in order.”
Read more: National Geographic