Great Lakes

The Great Lakes represent the largest freshwater lake complex on Earth. Created by continental glaciers over the past 18,000 years, these five major lakes (Ontario, Erie, Huron, Michigan, and Superior) provide significant resources for Canadians and Americans occupying the surrounding basin.

Geological Development of the Region

The Great Lakes are superlative features of the North American landscape. They make up the largest freshwater lake complex on Earth. According to the National Oceanic and Atmospheric Administration, as of 2024, approximately 21 percent of the global surface freshwater supply comes from the Great Lakes. Covering a total area of 245,000 square kilometers, the Great Lakes have a shoreline length of 17,000 kilometers. Lake Superior (82,100 square kilometers), Lake Huron (59,600 square kilometers), and Lake Michigan (57,800 square kilometers) are among the ten largest lakes on Earth.

The rocks forming the foundation of the Great Lakes date back some 600 million years. On the northern and northwestern shore of Lake Superior are remnants of the Canadian Shield, composed of igneous rocks of the Precambrian era, more than 1 billion years ago. Following volcanic activity and mountain building during the Precambrian era, the central region of North America was repeatedly covered by shallow tropical seas. At this time, during the Paleozoic era (600 million to 230 million years ago), sediments transported by rivers from adjacent eroding uplands were deposited in a shallow marine environment, and lime, salt, and gypsum precipitated from the seawater. All these soft materials were eventually hardened into sedimentary rock layers such as sandstone, shale, limestone, and halite. A multitude of fauna colonized the submarine environment, including corals, brachiopods, crinoids, and several species of mollusks.

As the layers of sediment accumulated over millions of years, the basin began to subside at its center. The Great Lakes basin structure may be compared to a series of bowls, one stacked on top of another. As viewed from above, only the top bowl is completely visible; however, the rims of the progressively deeper bowls are visible as several thin concentric rims along the basin's perimeter.

The Paleozoic era was followed by the Mesozoic era (230 to 63 million years ago), a time of little deposition. Despite the great age of the rocks making up the foundation of the Great Lakes, the lakes themselves were created in the relatively recent Pleistocene epoch. Between the 220 million years when the basin’s bedrock was deposited and the onset of Pleistocene glaciers, the landscape now occupied by the lakes was occupied by streams. The streams eroded the softer bedrock to form channels and valleys. The divides between and parallel to the eroded valleys were more erosion-resistant and are represented by higher elevations.

The streams, excavated shales, and weaker limestones now occupied the Lake Michigan and Lake Huron basins. An arc, composed of hard dolomitic rock and known as the Niagara Escarpment, extends in a northwesterly direction from Niagara, forming the Bruce Peninsula that separates the east side of Lake Huron from Georgian Bay. The same structure continues across Michigan’s upper peninsula, separating Lake Michigan from Green Bay as the Door Peninsula. The ancient stream channels were favored by the glaciers because they were at lower elevations and composed of more erodible bedrock. The linear shape of the lower Great Lakes is clearly related to the initial erosion by streams followed by continental ice.

Lake Superior is partly located on the Canadian Shield, and its geologic origin is less obvious. East-west faults underlie Lake Superior, and the rocks form a structural sag, or syncline, oriented along the lake's long axis in an approximately east-west direction.

The glacial origin and development of the Great Lakes is complex for several reasons. Each lake has a unique history and time of formation, making generalizations difficult. For example, the glaciers repeatedly advanced and retreated from many directions, covering and exposing each lake basin. There is abundant evidence regarding each ancestral lake's size, elevation, and precise geographical distribution; this historical information is documented by coastal landforms such as higher ancient shorelines and relict wave-cut features. Yet the changing of the lakes’ outlets to the ocean and reversals of drainage patterns complicate the sequence of events. Furthermore, because of the weight of the ice, the Paleozoic bedrock subsided to a lower level as the continental mass sought to “float” more deeply in the underlying mantle layer. As the glaciers receded, exposing different segments of the basin, the land began to recover its previous level and rise. This process, called isostatic rebound, is active today, causing elevation changes in many fossil shorelines. Although uplift has slowed since the ice exposed the newly created Great Lakes, the process continues.

As the ice began to retreat from the region, glacial landforms were deposited. Along many shorelines, moraines—composed of fragments of rock, sand, and silt—form spectacular bluffs. Along Lake Superior, the ice scraped and removed much of the soil, exposing the bedrock, which now forms high cliffs. Sand eroded from glacial sediment was transported by rivers to the lakes and deposited as beaches. The exposed beach sand was then transported inland by the wind to form coastal dunes.

Weather and Climate

Weather and climate influence several processes occurring in the Great Lakes, including changing lake levels, storm surges and related seiches, and lake stratification and turnover. Through the hydrologic cycle, moisture is evaporated from the lake surfaces and is then returned as precipitation over the water and as runoff from the land. During cooler and wetter years, evaporation is retarded, and more water is contributed to the lakes by excess precipitation, causing lake levels to rise. In warmer and drier years, evaporation increases, and precipitation is retarded, causing lower lake levels. Such changes in water levels are not cyclic and occur over several years. In 1988, Lake Huron and Lake Michigan had record levels of 177.4 meters above sea level. In 1995, the average water level was 176.3 meters above sea level, a difference of 1.1 meters. In 2017 the US Army Corps of Engineers reported that Lake Ontario’s water level was at its highest monthly average since 1918. In 2020, water levels in Lake Michigan and Lake Huron reached record highs, but between 2020 and 2022, steadily declined. Still, at the end of 2023 and into 2024, the water levels had once again returned to average.

Unstable weather conditions generate storms that pass over the region, generally from west to east. When strong winds persist for several hours from a constant direction over a lake, the water level is “pushed” from one side of the lake to the other. This storm surge, accompanied by low atmospheric pressure, may elevate the water level as much as 2 meters along a shoreline in a matter of a few hours. Gale-force winds on October 30 and 31, 1996, over Lake Erie raised the lake level by 1.25 meters at Buffalo, New York. Concurrently, as the water rose at Buffalo, it was lowered in Toledo, Ohio, at the opposite end of the lake, by 2.25 meters. The total difference in water level was 3.50 meters. Following a storm, the level of a lake rocks back and forth as a “seiche” before settling to its normal level.

In turn, the lake waters dramatically affect the local weather. As winter approaches, lake-effect snows commonly occur. The effect is most common in the fall, before the lakes cool and freeze. Cold winds from the north or west pass over the basin, picking up moisture from the relatively warm lakes. The water vapor is then condensed, forming clouds that, in turn, dump heavy snow in coastal zones, especially along eastern Lake Michigan and southern Lake Superior, Lake Huron, Lake Erie, and Lake Ontario. From November 9 to November 12, 1996, 1.2 meters of snow fell along Lake Erie’s south shore, paralyzing local communities. A December 2022 snowstorm produced over 51 inches of snow in the Buffalo, New York area.

Except Lake Erie, the lake bottoms were scoured by glaciers to depths below sea level. The lakes, thus, have variable temperatures from the surface to the bottom. Water density is altered as the water temperature changes from season to season. During winter, as ice forms over the lakes, the water beneath the ice remains warmer. As the ice cover breaks up in spring, the deeper, warmer water rises, or “turns over,” to the surface. It heats up through the summer months, causing stratification of warmer water (called the epilimnion) above colder, denser water. The contrasting water layers are separated by a thermocline, demarcating a rapid temperature transition between the warmer epilimnion and cooler subsurface water.

An issue of concern to scientists regarding the Great Lakes is the impact of the intensification of the greenhouse effect on the lakes’ water levels. The increase of greenhouse gases in the atmosphere, especially carbon dioxide, appears to be causing warming at unprecedented rates. Although climatologists differ in their opinions on the impact of a warmer atmosphere over the Great Lakes, there is general agreement that both evaporation and precipitation will increase, and stream runoff will decrease over many years. Based on general circulation models, lake levels will be from 0.5 meters to 2 meters lower than present levels if the climate continues to warm. In 2009, research presented by the Earth Institute at Columbia University suggested that in the next one hundred years, the water levels in the Great Lakes will not only decline, but they may fall by 0.23 to 2.5 meters.

Wetlands along the shorelines of the Great Lakes are significant ecological zones located at the meeting of land and lake. Although many wetlands around the basin have been lost or degraded, the remaining habitat has multidimensional functions as part of both upland and aquatic ecosystems. The wetlands are exposed to both short-term (storm surges and seiches) and long-term changes in water levels that constantly alter the biogeography of these habitats. Because of the state of constant water-level change, or “pulse stability,” the distribution and types of wetland plants shift dramatically. Thus, a constant renewal of the flora is occurring. Furthermore, because of the flushing action of the rise and fall of lake levels, peat accumulation in Great Lakes wetlands does not commonly occur as it does in marine settings.

The coastal wetlands serve significant ecological, economic, and social functions. They provide spawning habitats and nursery and resting areas for many species, including fishes, amphibians, reptiles, ducks, geese and other water birds, and mammals. Largely because of the sport fishing industry, these habitats contribute significant revenue to the surrounding states and the province of Ontario. Furthermore, pollution control and coastal erosion protection are additional benefits provided by these habitats.

Study of the Great Lakes

The creation and development of the Great Lakes have occurred relatively recently in terms of geologic time. Also, modifications such as erosion and deposition of coastal features are continual, active processes. Scientists use techniques that include varve analysis and radiocarbon dating to unravel the events leading to changes in the Great Lakes. A common theme of both techniques is that they express time in numbers of years within a reasonable range of accuracy rather than in a relative or comparative way. Varves consist of alternating light and dark sediment layers deposited in a lake. A light-colored mud is deposited during spring runoff; a dark-colored mud is deposited atop the lighter-colored layer during the following winter, as ice forms and there is less agitation of the lake water. One light and one dark band together represent one year of deposition. Numerous layers can be counted, like tree rings, and the number of years required for a sequence to be deposited can be determined. To obtain numerous undisturbed varve layers, researchers use a piston-coring device consisting of a hollow pipe attached to a cable released vertically into the lake. As it falls freely to the bottom, it plunges into the soft sediment. A piston allows the sediment to remain in the pipe as an attached cable raises it. The mud can then be extracted from the tube, cut open, and analyzed.

Radiocarbon dating of carbon-rich material such as peat, lime, coral, and even bone material is useful for absolute dating back to about 50,000 years ago. Carbon’s abundance in nature, coupled with the youthfulness of the Great Lakes, makes this tool very useful because many glacial, coastal, and sand dune landforms frequently contain some form of carbon suitable for absolute dating.

Old maps, navigation charts, and aerial photographs are used to map and detect recent changes in the landscape, such as coastal erosion or the rate of dune migration. Charts and maps of the coastal zone have been available for over a century, and aerial photographs of the region have been taken since the 1930s. By observing the position of a shoreline on historical sets of detailed aerial photographs over ten years, for example, changes in the shoreline can be detected, and the erosion rate per year can be determined.

Geographic positioning systems can accurately locate the latitude and longitude of a point on a shoreline, store the information, and compare the shoreline position with the position at some future time. Satellite pictures help to detect wetland types and determine acreage; this information can then be compared to a later environmental condition, such as a period of higher lake level, to see if species' habitat or acreage has changed.

Because of a geological process known as isostatic rebound, fossil shorelines become uplifted and exposed. Elevations retrieved from older topographic maps reveal how much uplift has occurred. If the age of a relict shoreline can be determined with radiocarbon analysis, the rate of glacial rebound in millimeters per century can then be assessed.

In the twenty-first century, scientists continue studying the Great Lakes to better understand its past and future. For example, in the mid-2020s, scientists discovered the remnants of complex hunting materials in the now-submerged Alpena-Amberley Ridge in Lake Huron. Such discoveries allow historians insight into the early peoples who lived in the Great Lakes region. Further, efforts are being made by those who study the Great Lakes to map its entire lakebed by 2030. This project is ongoing.

Principal Terms

epilimnion: a warmer surface layer of water that occurs in a lake during summer stratification; during spring, warmer water rises from great depths, and it heats up through the summer season

greenhouse effect: a natural process by which water vapor, carbon dioxide, and other gases in the atmosphere absorb heat and reradiate it back to Earth

isostatic rebound: a tendency of Earth’s continental surfaces to rise after being depressed by continental glaciers, without faulting

Pleistocene: a geologic era spanning about 2 million years that ended about 10,000 years ago, often considered synonymous with the “Ice Age”

seiche: rocking motion of lake level from one end of the lake to the other following high winds and low barometric pressure; frequently, a seiche will follow a storm event

storm surge: a rapid rise in lake level associated with low barometric pressure; the water level is frequently “pushed” above a shoreline on one end of the lake and depressed on the opposite end

thermocline: a well-defined layer of water in a lake separating the warmer and shallower epilimnion from the cooler and deeper hypolimnion

wetlands: areas along a coast where the water table is near or above the ground surface for at least part of the year; wetlands are characterized by wet soils, water-tolerant plants, and high biological production

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