The Great Lakes, an interconnected system of freshwater spanning the border between the United States and Canada, represent one of North America’s most significant natural resources. These five lakes—Superior, Michigan, Huron, Erie, and Ontario—are a geologic marvel that profoundly shaped the geography, economy, and climate of the region. The sheer magnitude of the lakes, combined with their complex history, holds many surprising facts often overlooked in a simple glance at a map.
The Immense Physical Scale of the Lakes
The scale of the Great Lakes system is difficult to fully grasp. The immense volume of water held within the basins accounts for an estimated 20 to 21 percent of the planet’s entire surface fresh water supply. This reservoir is so vast that, if its water were spread across the 48 contiguous U.S. states, it would submerge the entire area under nearly 10 feet of water.
The combined shoreline of the lakes rivals the entire coastlines of the United States. The total perimeter, including their islands, stretches for approximately 10,500 miles. Traversing the full length of this shoreline is roughly equivalent to traveling halfway around the equator of the Earth.
Lake Superior stands out for its exceptional size and depth. It is recognized as the largest freshwater lake in the world by surface area. Superior is also the deepest of the five, plunging to a maximum depth of 1,332 feet. This single lake holds a volume of water greater than all the other four Great Lakes combined.
Glacial Origins and Water Dynamics
The existence of these massive basins is a direct result of geological forces that occurred at the end of the last Ice Age. The Great Lakes were carved out by the Laurentide Ice Sheet, a massive continental glacier that began its final retreat approximately 14,000 years ago. The weight and scraping action of this ice sheet scoured out deep depressions in the earth’s surface. These depressions were then filled with enormous volumes of meltwater as the glacier receded northward.
The movement of water through this system is measured by water retention time, which varies dramatically among the individual lakes. Retention time is the average amount of time it takes for a drop of water to enter a lake and then exit through its outflow. For Lake Superior, with its immense volume and depth, the retention time is exceptionally long, estimated at 191 years.
In contrast, the water in Lake Erie cycles much faster, with a retention time of only about 2.6 years. This rapid flush rate is due to its relatively small volume and high rate of inflow and outflow, which has significant implications for how pollutants are managed within the system. Four of the five lakes are shared between the United States and Canada. Lake Michigan is the single exception, as it is the only Great Lake lying entirely within the borders of the United States.
Economic Engines and Unique Features
The Great Lakes are linked to the Atlantic Ocean by the St. Lawrence Seaway, a system of canals, channels, and locks that allows oceangoing vessels to travel as far inland as Duluth, Minnesota. This maritime trade corridor facilitates the movement of bulk goods. Cargoes such as iron ore, grain, steel, and potash are routinely transported along this waterway. The operation of the Seaway and the commerce it enables supports billions of dollars in economic activity and a large number of jobs across the region.
Beyond the surface water, the Great Lakes basin contains an equally significant, yet unseen, reservoir of fresh water. Scientists have informally dubbed the extensive network of groundwater aquifers beneath the region as the “Sixth Great Lake.” The volume of this groundwater is comparable to the water held in Lake Huron. This unseen resource is interconnected with the surface water, feeding rivers and streams, sustaining wetlands, and providing drinking water for millions of residents.
Lake Erie possesses characteristics that make it vulnerable to environmental stress. It is the shallowest of the Great Lakes, with an average depth of only 62 feet, and it is the smallest by volume. Because of its shallowness, Erie warms more quickly in the summer months than its deeper counterparts. This warmth, combined with the high population density and extensive agricultural activity in its watershed, makes it susceptible to harmful algal blooms and pollution.
The expanse of the lakes is also responsible for creating a localized weather phenomenon known as “lake effect snow.” This occurs when extremely cold air masses, often originating from Canada, move across the relatively warmer, unfrozen water surface. The open water transfers heat and moisture into the cold air, which then rises, condenses, and forms narrow but heavy bands of snow. These snow bands deposit massive amounts of snow on the downwind shores, typically the southern and eastern coasts, creating distinct “snowbelts” that experience drastically higher annual snowfall than nearby communities.