Great Lakes waters are composed of numerous aquifers (groundwater) that have filled with water over the centuries, waters that flow in the tributaries of the Great Lakes, and waters that fill the lakes themselves. Although the total volume in the lakes is vast, on average less than 1 percent of the waters of the Great Lakes is renewed annually by precipitation, surface water runoff, and inflow from groundwater sources.

Lake levels are determined by the combined influence of precipitation (the primary source of natural water supply to the Great Lakes), upstream inflows, groundwater, surface water runoff, evaporation, diversions into and out of the system, and water level regulation. Because of the vast water surface area, water levels of the Great Lakes, even with large variations in precipitation and runoff, remain remarkably steady, with a normal fluctuation ranging from 0.3 to 0.6 meters (12–24 in.) in a single year, shown in the table above. Climatic conditions control precipitation (and thus groundwater recharge), runoff, and direct supply to the lakes as well as the rate of evaporation. These are the primary driving factors in determining water levels.

The U.S. Army Corps of Engineers  has water level records for the period from 1918 to 1999, during which there were several periods of extremely high and extremely low water levels and flows. Exceptionally low levels were experienced in the mid-1920s, mid-1930s, and early 1960s. High levels occurred in 1929–30, 1952, 1973–74, 1985–86, and 1997–98. Studies of water level fluctuations have shown that the Great Lakes naturally respond to periods of above average, below-average, or extreme precipitation, water supply, and temperature conditions. The effects of existing control structures, diversions, and dredging on levels are minor in comparison.

Great Lakes levels respond to climatic variability, as illustrated by the impact of high water levels in the early 1950s and mid-1980s and of low water levels in the 1930s and mid-1960s. Significant variability will continue whether or not human-induced climate change is superimposed on natural fluctuations. An example of how quickly water levels can change in response to extreme climatic conditions is the period 1998–99, when the water levels of Lakes Michigan and Huron dropped 0.64 meters (25 in.) in 12 months.

Part of the reason for the dramatic drop in Lakes Michigan and Huron, and all of the Great Lakes overall, was the low precipitation in the Lake Superior region during the winter months of 1998-99, causing less water to runoff into the lake. Additionally, warmer air temperatures throughout the Great Lakes during 1998-99 caused warmer water temperatures that increased evaporation rates, contributing to lower lake levels. While this may be a positive change for coastal areas such as wetlands, algal blooms were also more common during this time period due to the higher water temperatures. Boaters experienced high maintenance due to shallow waters, which caused marinas to extend their docks or dredge or risk losing business. As a result of the present low water levels beaches expanded to widths not seen in over 30 years.

Source:

Great Lakes Trends: Into the New Millennium, May 2000 prepared by Office of the Great Lakes, Michigan Department of Environmental Quality.

International Joint Commission:  Protection of the Waters of the Great Lakes - Interim Report to the Governments of Canada and the United States. 1999.