What Makes the Great Lakes So Great?

Sitting on the shores of Lake Michigan, with the Chicago skyline to my left and endless blue ahead of me, I find myself glad to be alive in this moment. The Great Lakes are sources of beauty and inspiration and part of what defines home. For me, they are also a source of questions and a reminder of the incredible, slow moving, natural processes that shape our world.

To us, the Great Lakes are permanent. They are one of the most easily recognizable landmarks from space, and Chicago would not be the great city it is without Lake Michigan. But how (and when) did they form? Why do they matter now? 

How the Great Lakes Started

While the Great Lakes are a defining feature of the upper Midwest, they are relatively young, on a geologic time scale. Until about 10,000 years ago, the area around what is now the Great Lakes had been covered by glaciers. This was the time of the Wisconsin Ice Age – lasting from 25,000 to 13,000 years before present – when the Laurentide Ice Sheet extended from the North Pole to the top of Illinois, and glaciers, up to a mile thick, extended down to lower Illinois.

Ice sheets and glaciers of this size are heavy and push the surface of the Earth down. When they retreat and melt, these depressions stay behind, much like how the legs of a heavy table leave imprints in carpeting. The depression left behind by the weight of the ice can be deepened even more by erosion. As it retreats, ice can scraping some of the Earth’s surface away from its source location. These depressions, since they are at lower elevations, ultimately get filled in by melting ice and become drainage basins for rainwater, becoming lakes.

As the glaciers go through patterns of advance and retreat, some of this eroded material gets deposited, forming moraines — basically, piles of debris rustled up by a glacier. A complex system of moraines now forms the southern boundary of Lake Michigan. Over time, since the weight of the ice has been removed, the sunken land is (very) slowly rising back to flat levels in a process known as isostatic rebound. But don’t expect to find a mountain in Chicago anytime soon.

Lake Effect Snow and Wind

Perhaps the most striking example of the impact of the Great Lakes on this region is how these large inland bodies of water affect local climate and weather. Those of us who live near one of the Great Lakes are all too familiar with the lake effect: while we are glad for the breeze off the lake that cools us down in the summer, the same processes cause increased winter snowfall, making cold winters extra harsh. 

Water takes more energy to warm or cool water than it does land. That’s why even as summer approaches, the lake water still remains quite cold for some time and makes springtime splashing in the lake rather unpleasant. The opposite is also true – as it gets colder in the winter, the lake water doesn’t immediately get freezing cold, even after a couple days of winter weather.

We can also see the slowness with which water changes temperature in how it doesn’t change from day to night: the water will be the same temperature over a 24-hour period, even though the air and land temperature may change drastically.

As a result, the water temperature will still be (relatively) warm in the late fall or early winter, even as the temperature plunges. As cold air moves south from Canada or northern areas of the US, it contacts the relatively warm water of the lake. This warms the air above the water, bringing moisture with it. The warm air is less dense than the cold air, so it rises. As it does, the moisture in the air condenses, forming clouds, which can grow and shed snow as they move closer to shore. This is known as lake effect snow. 

Similarly, lake effect wind happens because of temperature differences between air and water. But this tends to happen when the land is warmer than the water, which tends to happen in the summer, especially in the mornings. Overnight, the land cools down to a temperature similar to that of the water. but as the sun rises, it heats the land and the air above it while the water stays cool. The warm air rises, causing the cooler, denser air from above the lake to move into its place. Hence, wind. 

How the Great Lakes Have Grown and Changed Over Time

The boundaries of the lakes continuously shift through time in response to a lot of different factors, including changes in weather patterns, water flow, and the local ecosystem. These shifts happen so slowly we can’t watch it happen directly. Instead, one way we can track these changes is by looking at patterns in how ecosystems fill in newly-dry land near the shores over the course of several decades.

Certain plants are the first to grow in after a disturbance or formation of new land. Then, other plants grow in to form a “climax community,” in which the plant has found equilibrium and will not change significantly without a significant change in its surroundings. To see how Lake Michigan has shifted over time, we can follow these different ecological boundaries at the Indiana Dunes.

The Dunes provide a beautiful example of this ecological effect. New dunes are forming as the water recedes, giving us a map of where the Lake is receding northwards. The first plants to take root on newly formed land in this region is marram grass. Eventually, oak savanna forests form a climax community. These communities are also gradually moving northwards as the lake moves and time passes. 

The coastline of the lakes constantly changes as moving water erodes shorelines or carries sediment to create new beaches. The water levels also rise and fall over periods of drought or flooding. The water level changes based on shifts in rainfall and evaporation.

With such large bodies of water, a single rainfall is unlikely to change the lake level. But while current lake levels are still within historically normal levels, flooding in recent years has led to a rapid rise, resulting in waves overtaking roads and paths along the shoreline. Scientists predict that increasing climate change will cause more extreme fluctuations between low lows and high highs in the coming decades. 

Why the Great Lakes Aren’t Salty

In many ways, the Great Lakes seem to resemble seas or oceans more than lakes. They not only affect the weather, but also have currents and tides and even natural sandy beaches! The key characteristic that differentiates them from oceans, however, is that they are not salty. But why? 

Bodies of water become salty if more dissolved salt ions enter the water than leave it over time. The oceans, for example, are salty because rivers carry sediment filled with dissolved sodium, chlorine, calcium, and magnesium ions. Water evaporates from the oceans (some of which is then precipitated over land), but the ions remain in the water. As a result, over billions of years, the oceans have gotten saltier.

The Great Lakes, on the other hand, still receive ions from the rivers that feed them, but the same ions are also carried out by rivers that drain the lake, so the ions never accumulate. 

What’s Living Inside Our Lakes 

Although each of the Great Lakes is naturally connected to the others as part of the Great Lakes Drainage Basin, they each have unique physical and ecological features. In recent years, Lake Michigan has become known for becoming clearer and bluer. That may sound like the result of an effective anti-pollution campaign, but actually, it’s the result of an invasive mussel population that’s feeding on phytoplankton, the microscopic green organisms that occupy our lakes.

Lake Erie is the shallowest of the Great Lakes, and frequently sees cyanobacteria (blue-green algae) blooms that turn the water green and murky when nutrient concentrations grow, sometimes as the result of increased agricultural runoff. While certain amounts of algae are normal and critical to the Great Lakes, large blooms can be dangerous. Some species can actually release toxic compounds, killing people and animals who drink the water or consume fish from the lake. These are known as Harmful Algal Blooms (HABs) and are regularly monitored by NASA, NOAA, and local groups. They can also create “dead zones,” as the decomposition process of dying algae consumes oxygen in the lake, killing off other organisms. 

While certain amounts of algae are normal and critical to the Great Lakes, large blooms can be dangerous. Some species can actually release toxic compounds, killing people and animals who drink the water or consume fish from the lake. These are known as Harmful Algal Blooms (HABs) and are regularly monitored by NASA, NOAA, and local groups. They can also create “dead zones,” as the decomposition process of dying algae consumes oxygen in the lake, killing off other organisms. 

The Great Lakes are also thriving with macroscopic life and many people use them for fishing. The lakes themselves are full of numerous species of fish, bivalves, and plants, and are also important feeding and resting grounds for birds. Yellow perch, walleye, and lake sturgeon are all common native fish, while loons, common terns, and double-crested cormorants are a few of the birds that call the lakes home.

Invasive species (those that are not native and cause problems for the lake) are unfortunately abundant as well. Particularly problematic organisms include the zebra mussel, Eurasian milfoil, and spiny water flea.

Non-native species can take hold because they lack natural predators in the area and have abundant resources that help them thrive. They dramatically reduce the numbers of native organisms by consuming native populations or out-competing them, using up available resources. 

Some of the wonderful places we call home, including Chicago, are drastically impacted by the presence of the Great Lakes. From having an abundant source of clean drinking water to providing a cool breeze on sweltering summer days, Lake Michigan is integral to my life. The Great Lakes, more broadly, have a fascinating geologic history that helps us understand our natural world, and they continue to serve as sources of scientific and creative inspiration for all people who get to experience them.