The Great Lakes system—Superior, Michigan, Huron, Erie, and Ontario—holds approximately 20% of the world’s surface freshwater. While the lakes have experienced significant periods of low water, such as the record lows seen in 2013, they have also recently rebounded to record high levels. Water levels are highly dynamic, driven by a balance of natural forces and increasingly influenced by human activity. The primary concern is not a continuous decline, but rather the rapid, dramatic swings between extreme high and extreme low water marks.
Understanding Water Level Cycles
Water levels in the Great Lakes naturally fluctuate over various timescales, ranging from hours to decades. The supply of water is determined by a simple equation: inflow (precipitation and runoff) minus outflow (evaporation and water leaving the system) equals the change in lake level. These changes are primarily driven by meteorological conditions.
Short-term fluctuations occur seasonally, typically peaking in the late summer or fall after spring snowmelt and summer rains, and reaching their lowest point in the winter. Long-term fluctuations represent multi-year trends, resulting from consecutive periods of cold, wet years (high levels) or warm, dry years (declines). For instance, Lake Michigan-Huron saw a decade of low water levels between the late 1990s and 2013, followed by a dramatic rise. The range between historical record high and low monthly averages can reach 6.2 feet for Lakes Michigan and Huron.
Natural Factors Driving Water Loss
The overall water balance is a zero-sum calculation defined by inflow and outflow. Precipitation, including rain, snow, and condensation directly onto the lake surface, is the sole source of new water. Runoff from the surrounding basin, particularly from spring snowmelt, also contributes significantly to the inflow.
The single largest factor in water loss is evaporation from the lake surface. Evaporation is greatest in the fall and early winter when the lake water is still relatively warm, but the air above it has become cold and dry. Ice cover plays a role in this balance; the formation of ice acts as a “cap” that drastically reduces evaporation during the cold months. A winter with less ice cover extends the period of maximum evaporation, leading to greater water loss.
Climate Change and Accelerated Water Loss
Climate change is not introducing new variables to the water cycle, but rather amplifying the extremes of existing natural factors. The Great Lakes have experienced some of the warmest water temperatures on record in recent decades. Warmer surface water increases the rate of evaporation, pushing the system toward lower levels, particularly during warmer, longer open-water seasons.
Rising temperatures have reduced winter ice cover by 71% across the Great Lakes since 1973. This loss extends the “evaporation window,” allowing for greater water loss throughout the winter months. This increased evaporation can lead to more intense “lake effect” snow over nearby land, but the net effect on the lake itself is a loss of water.
Shifts in precipitation patterns are becoming more pronounced, characterized by less reliable snowpack and more frequent, intense storms. While increased precipitation from these storms can cause rapid, record-high water levels, the overall trend is toward greater volatility. Climate change is pushing the system toward more rapid and dramatic fluctuations, resulting in “higher highs” and “lower lows.”
Human Influence: Diversion and Consumption
Direct human manipulation of the Great Lakes water system occurs through both diversion and consumption, though their impact is generally smaller compared to meteorological forces. Diversion refers to the transfer of water across the natural watershed boundaries via channels or pipelines. The most well-known example is the Chicago Diversion, which reverses the flow of the Chicago River, sending Lake Michigan water into the Mississippi River system.
This diversion, capped at 3,200 cubic feet per second by a 1967 Supreme Court decree, lowers the level of Lakes Michigan-Huron by an estimated 2.4 inches. Conversely, the Long Lac and Ogoki diversions in Ontario redirect water from the Hudson Bay watershed into Lake Superior, which adds water to the system. The combined effect of all major diversions is a relatively minor net change, but they are ecologically and politically significant.
Consumption involves the withdrawal of water for municipal, agricultural, and industrial use that is not returned to the Great Lakes basin. While cities and industries withdraw billions of gallons daily, only about 5.5% is consumed and permanently lost from the system through processes like evaporation from irrigation or incorporation into products. The majority of withdrawn water is returned to the lakes. This consumptive use is a small fraction of the total water budget, but it is a closely monitored factor.