The Great Lakes Nature

The foundation for the present Great Lakes basin was set about 3 billion years ago, during the Precambrian Era. This era occupies about five-sixths of all geological time and was a period of great volcanic activity and tremendous stresses, which formed great mountain systems. Early sedimentary and volcanic rocks were folded and heated into complex structures. These were later eroded and, today, appear as the gently rolling hills and small mountain remnants of the Canadian Shield, which forms the northern and northwestern portions of the Great Lakes basin. Granitic rocks of the shield extend southward beneath the Paleozoic, sedimentary rocks where they form the 'basement' structure of the southern and eastern portions of the basin.

With the coming of the Paleozoic Era, most of central North America was flooded again and again by marine seas, which were inhabited by a multitude of life forms, including corals, crinoids, brachiopods and mollusks. The seas deposited lime silts, clays, sand and salts, which eventually consolidated into limestone, shales, sandstone, halite and gypsum.

During the Pleistocene Epoch, the continental glaciers repeatedly advanced over the Great Lakes region from the north. The first glacier began to advance more than a million years ago. As they inched forward, the glaciers, up to 2,000 metres (6,500 feet) thick, scoured the surface of the earth, leveled hills, and altered forever the previous ecosystem. Valleys created by the river systems of the previous era were deepened and enlarged to form the basins for the Great Lakes. Thousands of years later, the climate began to warm, melting and slowly shrinking the glacier. This was followed by an interglacial period during which vegetation and wildlife returned. The whole cycle was repeated several times.

Sand, silt, clay and boulders deposited by the glaciers occur in various mixtures and forms. These deposits are collectively referred to as 'glacial drift' and include features such as moraines, which are linear mounds of poorly sorted material or 'till', flat till plains, till drumlins, and eskers formed of well-sorted sands and gravels deposited from meltwater. Areas having substantial deposits of well-sorted sands and gravels (eskers, kames and outwash) are usually significant groundwater storage and transmission areas called 'aquifers'. These also serve as excellent sources of sand and gravel for commercial extraction.

As the glacier retreated, large volumes of meltwater occurred along the front of the ice. Because the land was greatly depressed at this time from the weight of the glacier, large glacial lakes formed. These lakes were much larger than the present Great Lakes. Their legacy can still be seen in the form of beach ridges, eroded bluffs and flat plains located hundreds of metres above present lake levels. Glacial lake plains known as 'lacustrine plains' occur around Saginaw Bay and west and north of Lake Erie.

As the glacier receded, the land began to rise. This uplift (at times relatively rapid) and the shifting ice fronts caused dramatic changes in the depth, size and drainage patterns of the glacial lakes. Drainage from the lakes occurred variously through the Illinois River Valley (towards the Mississippi River), the Hudson River Valley, the Kawartha Lakes (Trent River) and the Ottawa River Valley before entering their present outlet through the St. Lawrence River Valley. Although the uplift has slowed considerably, it is still occurring in the northern portion of the basin. This, along with changing long-term weather patterns, suggests that the lakes are not static and will continue to evolve.


The weather in the Great Lakes basin is affected by three factors: air masses from other regions, the location of the basin within a large continental landmass, and the moderating influence of the lakes themselves. The prevailing movement of air is from the west. The characteristically changeable weather of the region is the result of alternating flows of warm, humid air from the Gulf of Mexico and cold, dry air from the Arctic.

In summer, the northern region around Lake Superior generally receives cool, dry air masses from the Canadian northwest. In the south, tropical air masses originating in the Gulf of Mexico are most influential. As the Gulf air crosses the lakes, the bottom layers remain cool while the top layers are warmed. Occasionally, the upper layer traps the cooler air below, which in turn traps moisture and airborne pollutants, and prevents them from rising and dispersing. This is called a temperature inversion and can result in dank, humid days in areas in the midst of the basin, such as Michigan and Southern Ontario, and can also cause smog in low-lying industrial areas.

Increased summer sunshine warms the surface layer of water in the lakes, making it lighter than the colder water below. In the fall and winter months, release of the heat stored in the lakes moderates the climate near the shores of the lakes. Parts of Southern Ontario, Michigan and western New York enjoy milder winters than similar mid-continental areas at lower latitudes.

In the autumn, the rapid movement and occasional clash of warm and cold air masses through the region produce strong winds. Air temperatures begin to drop gradually and less sunlight, combined with increased cloudiness, signal more storms and precipitation. Late autumn storms are often the most perilous for navigation and shipping on the lakes.

In winter, the Great Lakes region is affected by two major air masses. Arctic air from the northwest is very cold and dry when it enters the basin, but is warmed and picks up moisture traveling over the comparatively warmer lakes. When it reaches the land, the moisture condenses as snow, creating heavy snowfalls on the lee side of the lakes in areas frequently referred to as snowbelts. For part of the winter, the region is affected by Pacific air masses that have lost much of their moisture crossing the western mountains. Less frequently, air masses enter the basin from the southwest, bringing in moisture from the Gulf of Mexico. This air is slightly warmer and more humid. During the winter, the temperature of the lakes continues to drop. Ice frequently covers Lake Erie but seldom fully covers the other lakes.

Spring in the Great Lakes region, like autumn, is characterized by variable weather. Alternating air masses move through rapidly, resulting in frequent cloud cover and thunderstorms. By early spring, the warmer air and increased sunshine begin to melt the snow and lake ice, starting again the thermal layering of the lakes. The lakes are slower to warm than the land and tend to keep adjacent land areas cool, thus prolonging cool conditions sometimes well into April. Most years, this delays the leafing and blossoming of plants, protecting tender plants, such as fruit trees, from late frosts. This extended state of dormancy allows plants from somewhat warmer climates to survive in the western shadow of the lakes. It is also the reason for the presence of vineyards in those areas.

Climate Change And The Great Lakes

At various times throughout its history, the Great Lakes basin has been covered by thick glaciers and tropical forests, but these changes occurred before humans occupied the basin. Present-day concern about the atmosphere is premised on the belief that society at large, through its means of production and modes of daily activity, especially by ever increasing carbon dioxide emissions, may be modifying the climate at a rate unprecedented in history.

The very prevalent 'greenhouse effect' is actually a natural phenomenon. It is a process by which water vapor and carbon dioxide in the atmosphere absorb heat given off by the earth and radiate it back to the surface. Consequently the earth remains warm and habitable (16°C average world temperature rather than -18°C without the greenhouse effect). However, humans have increased the carbon dioxide present in the atmosphere since the industrial revolution from 280 parts per million to the present 350 ppm, and some predict that the concentration will reach twice its pre-industrial levels by the middle of the next century.

Climatologists, using the General Circulation Model (GCM), have been able to determine the manner in which the increase of carbon dioxide emissions will affect the climate in the Great Lakes basin. Several of these models exist and show that at twice the carbon dioxide level, the climate of the basin will be warmer by 2-4°C and slightly damper than at present. For example, Toronto's climate would resemble the present climate of southern Ohio. Warmer climates mean increased evaporation from the lake surfaces and evapotranspiration from the land surface of the basin. This in turn will augment the percentage of precipitation that is returned to the atmosphere. Studies have shown that the resulting net basin supply, the amount of water contributed by each lake basin to the overall hydrologic system, will be decreased by 23 to 50 percent. The resulting decreases in average lake levels will be from half a metre to two metres, depending on the GCM used.

Large declines in lake levels would create large-scale economic concern for the commercial users of the water system. Shipping companies and hydroelectric power companies would suffer economic repercussions, and harbors and marinas would be adversely affected. While the precision of such projections remains uncertain, the possibility of their accuracy embraces important long-term implications for the Great Lakes.

The potential effects of climate change on human health in the Great Lakes region are also of concern, and researchers can only speculate as to what might occur. For example, weather disturbances, drought, and changes in temperature and growing season could affect crops and food production in the basin. Changes in air pollution patterns as a result of climate change could affect respiratory health, causing asthma, and new disease vectors and agents could migrate into the region.

The Hydrologic Cycle

Water is a renewable resource. It is continually replenished in ecosystems through the hydrologic cycle. Water evaporates in contact with dry air, forming water vapor. The vapor can remain as a gas, contributing to the humidity of the atmosphere; or it can condense and form water droplets, which, if they remain in the air, form fog and clouds. In the Great Lakes basin, much of the moisture in the region evaporates from the surface of the lakes. Other sources of moisture include the surface of small lakes and tributaries, moisture on the land mass and water released by plants. Global movements of air also carry moisture into the basin, especially from the tropics.

Moisture-bearing air masses move through the basin and deposit their moisture as rain, snow, hail or sleet. Some of this precipitation returns to the atmosphere and some falls on the surfaces of the Great Lakes to become part of the vast quantity of stored fresh water once again. Precipitation that falls on the land returns to the lakes as surface runoff or infiltrates the soil and becomes groundwater.

Whether it becomes surface runoff or groundwater depends upon a number of factors. Sandy soils, gravels and some rock types contribute to groundwater flows, whereas clays and impermeable rocks contribute to surface runoff. Water falling on sloped areas tends to run off rapidly, while water falling on flat areas tends to be absorbed or stored on the surface. Vegetation also tends to decrease surface runoff; root systems hold moisture-laden soil readily, and water remains on plants.

Surface Runoff

Surface runoff is a major factor in the character of the Great Lakes basin. Rain falling on exposed soil tilled for agriculture or cleared for construction accelerates erosion and the transport of soil particles and pollutants into tributaries. Suspended soil particles in water are deposited as sediment in the lakes and often collect near the mouths of tributaries and connecting channels. Much of the sediment deposited in nearshore areas is resuspended and carried farther into the lake during storms. The finest particles (clays and silts) may remain in suspension long enough to reach the mid-lake areas.

Before settlement of the basin, streams typically ran clear year-round because natural vegetation prevented soil loss. Clearing of the original forest for agriculture and logging has resulted in both more erosion and runoff into the streams and lakes. This accelerated runoff aggravates flooding problems.


Wetlands are areas where the water table occurs above or near the land surface for at least part of the year. When open water is present, it must be less than two metres deep (seven feet), and stagnant or slow moving. The presence of excessive amounts of water in wetland regions has given rise to hydric soils, as well as encouraged the predominance of water tolerant (hydrophytic) plants and similar biological activity.

Four basic types of wetland are encountered in the Great Lakes basin: swamps, marshes, bogs and fens. Swamps are areas where trees and shrubs live on wet, organically rich mineral soils that are flooded for part or all of the year. Marshes develop in shallow standing water such as ponds and protected bays. Aquatic plants (such as species of rushes) form thick stands, which are rooted in sediments or become floating mats where the water is deeper. Swamps and marshes occur most frequently in the southern and eastern portions of the basin.

Bogs form in shallow stagnant water. The most characteristic plant species are the sphagnum mosses, which tolerate conditions that are too acidic for most other organisms. Dead sphagnum decomposes very slowly, accumulating in mats that may eventually become many metres thick and form a dome well above the original surface of the water. It is this material that is excavated and sold as peat moss. Peat also accumulates in fens. Fens develop in shallow, slowly moving water. They are less acidic than bogs and are usually fed by groundwater. Fens are dominated by sedges and grasses, but may include shrubs and stunted trees. Fens and bogs are commonly referred to as 'peatlands' and occur most frequently in the cooler northern and northwestern portions of the Great Lakes basin.

Wetlands serve important roles ecologically, economically and socially to the overall health and maintenance of the Great Lakes ecosystem. They provide habitats for many kinds of plants and animals, some of which are found nowhere else. For ducks, geese and other migratory birds, wetlands are the most important part of the migratory cycle, providing food, resting places and seasonal habitats. Economically, wetlands play an essential role in sustaining a productive fishery. At least 32 of the 36 species of Great Lakes fish studied depend on coastal wetlands for their successful reproduction. In addition to providing a desirable habitat for aquatic life, wetlands prevent damage from erosion and flooding, as well as controlling point and nonpoint source pollution.

Coastal wetlands along the Great Lakes include some sites that are recognized internationally for their outstanding biological significance. Examples included the Long Point complex and Point Pelee on the north shore of Lake Erie and the National Wildlife Area on Lake St. Clair. Long Point also was designated a UNESCO Biosphere Reserve. Wetlands of the lower Great Lakes region have also been identified as a priority of the Eastern Habitat Joint Venture of the North American Waterfowl Management Plan, an international agreement between governments and non-government organizations (NGOs) to conserve highly significant wetlands.

Although wetlands are a fundamentally important element of the Great Lakes ecosystem and are of obvious merit, their numbers continue to decline at an alarming rate. Over two-thirds of the Great Lakes wetlands have already been lost and many of those remaining are threatened by development, drainage or pollution.


Groundwater is important to the Great Lakes ecosystem because it provides a reservoir for storing water and slowly replenishing the lakes in the form of base flow in the tributaries. It is also a source of drinking water for many communities in the Great Lakes basin. Shallow groundwater also provides moisture to plants.

As water passes through subsurface areas, some substances are filtered out, but some materials in the soils become dissolved or suspended in the water. Salts and minerals in the soil and bedrock are the source of what is referred to as 'hard' water, a common feature of well water in the lower Great Lakes basin.

Groundwater can also pick up materials of human origin that have been buried in dumps and landfill sites. Groundwater contamination problems can occur in both urban-industrial and agricultural areas. Protection and inspection of groundwater is essential to protect the quality of the entire water supply consumed by basin populations, because the underground movement of water is believed to be a major pathway for the transport of pollution to the Great Lakes. Groundwater may discharge directly to the lakes or indirectly as base flow to the tributaries.

Lake Levels

The Great Lakes are part of the global hydrologic system. Prevailing westerly winds continuously carry moisture into the basin in air masses from other parts of the continent. At the same time, the basin loses moisture in departing air masses by evaporation and transpiration, and through the outflow of the St. Lawrence River. Over time, the quantity lost equals what is gained, but lake levels can vary substantially over short-term, seasonal and long-term periods.

Day-to-day changes are caused by winds that push water on shore. This is called 'wind set-up' and is usually associated with a major lake storm, which may last for hours or days. Another extreme form of oscillation, known as a 'seiche', occurs with rapid changes in winds and barometric pressure.

Annual or seasonal variations in water levels are based mainly on changes in precipitation and runoff to the Great Lakes. Generally, the lowest levels occur in winter when much of the precipitation is locked up in ice and snow on land, and dry winter air masses pass over the lakes enhancing evaporation. Levels are highest in summer after the spring thaw when runoff increases.

The irregular long-term cycles correspond to long-term trends in precipitation and temperature, the causes of which have yet to be adequately explained. Highest levels occur during periods of abundant precipitation and lower temperatures that decrease evaporation. During periods of high lake levels, storms cause considerable flooding and shoreline erosion, which often result in property damage. Much of the damage is attributable to intensive shore development, which alters protective dunes and wetlands, removes stabilizing vegetation, and generally reduces the ability of the shoreline to withstand the damaging effects of wind and waves.

The International Joint Commission, the binational agency established under the Boundary Waters Treaty of 1909 between Canada and the U.S., has the responsibility for regulation of flows on the St. Marys and the St. Lawrence Rivers. These channels have been altered by enlargement and placement of control works associated with deep-draft shipping. Agreements between the U.S. and Canada govern the flow through the control works on these connecting channels.

The water from Lake Michigan flows to Lake Huron through the Straits of Mackinac. These straits are deep and wide, resulting in Lakes Michigan and Huron standing at the same elevation. There are no artificial controls on the St. Clair and Detroit Rivers that could change the flow from the Michigan-Huron Lakes system into Lake Erie. The outflow of Lake Erie via the Niagara River is also uncontrolled, except for some diversion of water through the Welland Canal. A large percentage of the Niagara River flow is diverted through hydroelectric power plants at Niagara Falls, but this diversion has no effect on lake levels.

Studies of possible further regulation of flows and lake levels have concluded that natural fluctuation is huge compared with the influence of existing control works. Further regulation by engineering systems could not be justified in light of the cost and other impacts. Just one inch (two and a half centimetres) of water on the surface of Lakes Michigan and Huron amounts to more than 36 billion cubic metres of water (about 1,260 billion cubic feet).

Lake Processes: Stratification And Turnover

The Great Lakes are not simply large containers of uniformly mixed water. They are, in fact, highly dynamic systems with complex processes and a variety of subsystems that change seasonally and on longer cycles.

The stratification or layering of water in the lakes is due to density changes caused by changes in temperature. The density of water increases as temperature decreases until it reaches its maximum density at about 4° Celsius (39° Fahrenheit). This causes thermal stratification, or the tendency of deep lakes to form distinct layers in the summer months. Deep water is insulated from the sun and stays cool and more dense, forming a lower layer called the 'hypolimnion'. Surface and nearshore waters are warmed by the sun, making them less dense so that they form a surface layer called the 'epilimnion'. As the summer progresses, temperature differences increase between the layers. A thin middle layer, or 'thermocline', develops in which a rapid transition in temperature occurs.

The warm epilimnion supports most of the life in the lake. Algal production is greatest near the surface where the sun readily penetrates. The surface layer is also rich in oxygen, which is mixed into the water from the atmosphere. A second zone of high productivity exists just above the hypolimnion, due to upward diffusion of nutrients. The hypolimnion is less productive because it receives less sunlight. In some areas, such as the central basin of Lake Erie, it may lack oxygen because of decomposition of organic matter.

In late fall, surface waters cool, become denser and descend, displacing deep waters and causing a mixing or turnover of the entire lake. In winter, the temperature of the lower parts of the lake approaches 4° Celsius (39° Fahrenheit), while surface waters are cooled to the freezing point and ice can form. As temperatures and densities of deep and shallow waters change with the warming of spring, another turnover may occur. However, in most cases the lakes remain mixed throughout the winter.

The layering and turnover of water annually are important for water quality. Turnover is the main way in which oxygen-poor water in the deeper areas of the lakes can be mixed with surface water containing more dissolved oxygen. This prevents anoxia, or complete oxygen depletion, of the lower levels of most of the lakes. However, the process of stratification during the summer also tends to restrict dilution of pollutants from effluents and land runoff.

During the spring warming period, the rapidly warming nearshore waters are inhibited from moving to the open lake by a thermal bar, a sharp temperature gradient that prevents mixing until the sun warms the open lake surface waters or until the waters are mixed by storms. Because the thermal bar holds pollutants nearshore, they are not dispersed to the open waters and can become more concentrated within the nearshore areas.

Living Resources

As an ecosystem, the Great Lakes basin is a unit of nature in which living organisms and nonliving things interact adaptively. An ecosystem is fueled by the sun, which provides energy in the form of light and heat. This energy warms the earth, the water and the air, causing winds, currents, evaporation and precipitation. The light energy of the sun is essential for the photosynthesis of green plants in water and on land. Plants grow when essential nutrients such as phosphorus and nitrogen are present with oxygen, inorganic carbon and adequate water.

Plant material is consumed in the water by zooplankton, which graze the waters for algae, and on land by plant-eating animals (herbivores). Next in the chain of energy transfer through the ecosystem are organisms that feed on other animals (carnivores) and those that feed on both animals and plants (omnivores). Together these levels of consumption constitute the food chain, or web, a system of energy transfers through which an ecological community consisting of a complex of species is sustained. The population of each species is determined by a system of checks and balances based on factors such as the availability of food and the presence of predators, including disease organisms.

Every ecosystem also includes numerous processes to break down accumulated biomass (plants, animals and their wastes) into the constituent materials and nutrients from which they originated. Decomposition involves micro-organisms that are essential to the ecosystem because they recycle matter that can be used again.

Stable ecosystems are sustained by the interactions that cycle nutrients and energy in a balance between available resources and the life that depends on those resources. In ecosystems, including the Great Lakes basin, everything depends on everything else and nothing is ever really wasted.

The ecosystem of the Great Lakes and the life supported within it have continuously altered with time. Through periods of climate change and glaciation, species moved in and out of the region; some perished and others pioneered under changed circumstances. None of the changes, however, has been as rapid as that which occurred with the arrival of European settlers.

When the first Europeans arrived in the basin nearly 400 years ago, it was a lush, thickly vegetated area. Vast timber stands, consisting of oaks, maples and other hardwoods dominated the southern areas. Only a very few small vestiges of the original forest remain today. Between the wooded areas were rich grasslands with growth as high as 2 or 3 metres (7 to 10 feet). In the north, coniferous forests occupied the shallow, sandy soils, interspersed by bogs and other wetlands.

The forest and grasslands supported a wide variety of life, such as moose in the wetlands and coniferous woods, and deer in the grasslands and brush forests of the south. The many waterways and wetlands were home to beaver and muskrat which, with the fox, wolf and other fur-bearing species, inhabited the mature forest lands. These were trapped and traded as commodities by the native people and the Europeans. Abundant bird populations thrived on the various terrains, some migrating to the south in winter, others making permanent homes in the basin.

It is estimated that there were as many as 180 species of fish indigenous to the Great Lakes. Those inhabiting the nearshore areas included smallmouth and largemouth bass, muskellunge, northern pike and channel catfish. In the open water were lake herring, blue pike, lake whitefish, walleye, sauger, freshwater drum, lake trout and white bass. Because of the differences in the characteristics of the lakes, the species composition varied for each of the Great Lakes. Warm, shallow Lake Erie was the most productive, while deep Superior was the least productive.

Changes in the species composition of the Great Lakes basin in the last 200 years have been the result of human activities. Many native fish species have been lost by overfishing, habitat destruction or the arrival of exotic or non-indigenous species, such as the lamprey and the alewife. Pollution, especially in the form of nutrient loading and toxic contaminants, has placed additional stresses on fish populations. Other human-made stresses have altered reproductive conditions and habitats, causing some varieties to migrate or perish. Still other effects on lake life result from damming, canal building, altering or polluting tributaries to the lakes in which spawning takes place and where distinct ecosystems once thrived and contributed to the larger basin ecosystem.

Introduction: The Great Lakes
Natural Processes in the Great Lakes
People & The Great Lakes
The Great Lakes Today - Concerns
Joint Management of the Great Lakes
New Directions for the Great Lakes Community

U.S. EPA Great Lakes National Program Office