Introduction
A lake is a body of standing water surrounded by land. Lakes occur on every continent and range from tiny mountain lakes to enormous inland water bodies that resemble seas. Some contain fresh water, while others are saline or even hypersaline.
Lakes are important parts of the water cycle. They store water, provide habitats for plants and animals, supply drinking and irrigation water, support fisheries and transportation, and influence local climates. The U.S. Geological Survey classifies lakes and reservoirs as major forms of surface-water storage. (US Geological Survey)
Lakes also preserve evidence of Earth's history. Their sediments can record changes in climate, vegetation, volcanic activity, and human settlement over thousands or even millions of years.
Learning Objectives
After studying this guide, you should be able to:
Explain what a lake is and distinguish lakes from ponds and reservoirs.
Describe major ways lakes form.
Distinguish freshwater, saline, open, and closed lakes.
Identify the main ecological and thermal zones of a lake.
Explain eutrophication and lake turnover.
Recognize major world lakes and important geographical records.
What Is a Lake?
A lake is a body of relatively still water occupying a depression or basin in the landscape.
Unlike a river, a lake does not continuously flow through a narrow channel. However, rivers and streams often feed lakes or carry water away from them.
Lakes receive water from sources such as:
Rain and snow
Rivers and streams
Groundwater
Melting snow and glaciers
Surface runoff
Water leaves lakes through evaporation, seepage into groundwater, or an outlet such as a river.
Lakes vs Ponds
There is no single worldwide scientific rule separating every lake from every pond.
Ponds are generally smaller and shallower. In many definitions, sunlight can reach most or all of a pond's bottom, allowing rooted vegetation to grow over a greater proportion of it.
Lakes are generally larger or deep enough to contain open-water areas where sunlight does not reach the bottom. The U.S. National Park Service uses this general ecological distinction while noting that both are standing-water, or lentic, ecosystems. (National Park Service)
Natural Lakes vs Reservoirs
A natural lake occupies a naturally formed basin.
A reservoir is usually a human-made body of water created by constructing a dam across a river or valley.
Reservoirs may provide:
Drinking water
Irrigation
Hydroelectric power
Flood control
Recreation
Reservoirs behave like lakes in many ecological respects, so limnologists often study the two together.
What Is Limnology?
Limnology is the scientific study of inland waters.
It includes the study of:
Lakes
Reservoirs
Rivers
Streams
Wetlands
Limnologists examine water chemistry, temperature, organisms, nutrients, circulation, sediments, and relationships between water bodies and their surrounding landscapes.
How Lakes Form
Lakes form whenever a depression becomes capable of holding water. The basin itself can be produced by many different geological processes. (National Park Service)
Glacial Lakes
Glacial lakes are formed by the movement or melting of glaciers.
Glaciers can:
Carve deep basins into rock.
Block valleys with sediment called moraine.
Leave depressions when buried blocks of ice melt.
Many lakes in northern North America and northern Europe have glacial origins.
The North American Great Lakes were strongly shaped by repeated glaciation. (National Park Service)
Tectonic Lakes
Tectonic lakes form when movements of Earth's crust create depressions.
Faulting, rifting, and other tectonic processes can create very deep basins.
Lake Baikal in Siberia lies within an active continental rift system and is an important example of a tectonic lake.
Volcanic and Caldera Lakes
Volcanic activity can create depressions capable of holding water.
A large explosive eruption may empty part of a magma chamber and cause the ground above it to collapse, forming a caldera.
Water can later accumulate in the depression.
Crater Lake in Oregon formed in the depression left when a large volcano collapsed about 7,700 years ago. (National Park Service)
Oxbow Lakes
An oxbow lake forms when a river abandons one of its looping bends.
Erosion on the outside of a river bend and deposition on the inside can eventually cause the river to cut through a narrow section of land. The abandoned loop becomes separated from the main channel.
Oxbow lakes often have a curved or horseshoe shape.
Landslide and Barrier Lakes
A landslide, lava flow, glacier, or other natural barrier can block a valley or river.
Water accumulates behind the blockage and creates a lake.
Some of these lakes are temporary because the natural dam may eventually erode or collapse.
Human-Made Lakes
People create lakes and reservoirs by damming waterways or excavating basins.
Although artificial in origin, these water bodies can eventually develop complex aquatic ecosystems.
Freshwater and Saline Lakes
Not every lake contains fresh water.
The USGS defines freshwater broadly as water containing less than 1,000 milligrams per liter of dissolved solids, most commonly salts. (US Geological Survey)
Freshwater Lakes
Freshwater lakes contain relatively low concentrations of dissolved salts.
They are commonly supplied by:
Rivers
Rain
Snowmelt
Groundwater
Many freshwater lakes have outlets that carry dissolved minerals away.
Saline Lakes
A saline lake contains higher concentrations of dissolved salts.
Salinity often develops in lakes that have no outlet to the ocean. Rivers bring dissolved minerals into the basin, but when water evaporates, the salts remain behind.
The Caspian Sea and Dead Sea are famous saline inland water bodies.
Open and Closed Lakes
Lakes can also be classified according to drainage.
Open Lakes
An open, or exorheic, lake has an outlet carrying water toward another river, lake, or ocean.
Such lakes often remain relatively fresh because dissolved minerals can leave with the outflow.
Closed Lakes
A closed, or endorheic, lake has no natural surface-water outlet to the ocean.
Water leaves mainly by:
Evaporation
Groundwater seepage
Salts and minerals can therefore become concentrated.
The Caspian Sea is an example of an endorheic basin.
Parts of a Lake Ecosystem
A lake contains several ecological zones.
Littoral Zone
The littoral zone is the shallow area close to shore where sunlight reaches the bottom.
It often contains:
Rooted plants
Insects
Snails
Amphibians
Small fish
Because light and nutrients are available, littoral zones can be biologically productive.
Open-Water Zone
The open-water region away from shore is sometimes called the limnetic or pelagic zone.
Plankton and swimming fish are important inhabitants.
Profundal Zone
In deep lakes, the profundal zone is the deeper water where little or no sunlight reaches.
Photosynthesis is limited there.
Benthic Zone
The benthic zone is the lake bottom.
Organisms such as insect larvae, worms, microorganisms, and mollusks can live in or on lake sediments.
Thermal Stratification
Deep lakes often develop distinct temperature layers.
USGS identifies three important layers in thermally stratified lakes. (U.S. Geological Survey)
Epilimnion
The epilimnion is the warmer upper layer.
It interacts strongly with sunlight, wind, and the atmosphere.
Metalimnion
The metalimnion is the transition layer.
It contains the thermocline, where temperature changes rapidly with depth.
Hypolimnion
The hypolimnion is the colder deep-water layer.
During strong summer stratification, it may mix only weakly with surface water.
Lake Turnover
Seasonal temperature changes can cause lake water to mix vertically.
This process is called turnover or overturn.
During turnover, oxygen and nutrients can be redistributed through the water column.
In many temperate deep lakes, major mixing occurs during spring and autumn when surface and deep-water temperatures become more similar. (U.S. Geological Survey)
Nutrients and Trophic State
Lakes vary greatly in biological productivity.
Important nutrients include:
Phosphorus
Nitrogen
Too few nutrients limit plant and algae growth, while excessive nutrients can create serious ecological problems.
Oligotrophic Lakes
Oligotrophic lakes contain relatively low nutrient levels.
They commonly have:
Clear water
Low algal productivity
Relatively low biomass
Deep mountain lakes are often oligotrophic.
Mesotrophic Lakes
Mesotrophic lakes have intermediate levels of nutrients and biological productivity.
Eutrophic Lakes
Eutrophic lakes contain relatively high nutrient concentrations and support high biological productivity.
They may contain substantial algae and aquatic vegetation.
EPA classifies oligotrophic, mesotrophic, eutrophic, and hypereutrophic conditions according to measures of biological productivity such as chlorophyll concentrations. (US EPA)
Eutrophication
Eutrophication is the process by which a lake becomes increasingly enriched with nutrients.
It can occur naturally over thousands of years as lakes age.
Human activities can accelerate the process dramatically through nutrient inputs from:
Fertilizers
Sewage
Animal waste
Urban runoff
Excessive nutrient enrichment can cause algal blooms. When large amounts of algae die and decompose, microorganisms consume oxygen, potentially creating low-oxygen conditions and fish kills. (US EPA)
Why Lakes Matter
Lakes provide many ecological and economic benefits.
They can supply:
Drinking water
Irrigation
Fisheries
Transportation
Hydroelectric resources
Recreation
Wildlife habitat
Lake systems also influence local climate because large bodies of water heat and cool more slowly than surrounding land.
Lake Biodiversity
Lakes support microorganisms, aquatic plants, insects, crustaceans, fish, birds, mammals, and amphibians.
Ancient isolated lakes can contain unusually high numbers of species found nowhere else.
Lake Baikal is especially notable for its endemic wildlife. UNESCO describes its long isolation and great age as having produced an exceptionally rich and unusual freshwater fauna. (UNESCO World Heritage Centre)
Major Lakes and World Records
Lake or System | Important Fact |
Caspian Sea | World's largest inland water body/lake by surface area |
Lake Baikal | Deepest lake and largest freshwater lake by volume |
Lake Superior | Largest freshwater lake by surface area when the Great Lakes are counted conventionally as separate lakes |
Great Lakes | Largest freshwater lake system on Earth |
Michigan–Huron | Hydrologically connected at the same level and sometimes treated scientifically as one lake |
Lake Baikal
Lake Baikal, in southeastern Siberia, is one of the world's most remarkable lakes.
UNESCO describes it as:
The world's deepest lake
About 25 million years old
The largest freshwater lake by volume
A reservoir holding roughly one-fifth of the world's unfrozen freshwater
Its depth reaches roughly 1.6–1.7 kilometres. (UNESCO World Heritage Centre)
Quiz Connection
Baikal → deepest + greatest freshwater volume + exceptionally old
Lake Superior
Lake Superior is the largest of the five Great Lakes and, under the conventional treatment of the Great Lakes as separate named lakes, the world's largest freshwater lake by surface area.
Its surface area is roughly 82,000 square kilometres, and its maximum depth is about 406 metres. (apps.glerl.noaa.gov)
Important Quiz Trap: Michigan–Huron
Lakes Michigan and Huron are connected through the Straits of Mackinac and stand at essentially the same water level.
NOAA notes that hydrologically they can be considered one continuous lake, Lake Michigan–Huron. When combined this way, its surface area exceeds Lake Superior's. (GLERL)
Most general-knowledge quizzes, however, follow the traditional five-lake naming system and identify Lake Superior as the largest freshwater lake by surface area.
The Great Lakes
The North American Great Lakes are:
Superior → Michigan → Huron → Erie → Ontario
Together they form the world's largest freshwater lake system and contain roughly one-fifth of Earth's surface freshwater. (NOAA Fisheries)
The system ultimately drains through the St. Lawrence River toward the Atlantic Ocean.
Threats to Lakes
Lakes are vulnerable because pollutants and nutrients can accumulate in their basins.
Major threats include:
Nutrient pollution
Harmful algal blooms
Sewage and chemical contamination
Invasive species
Habitat destruction
Sedimentation
Excessive water withdrawal
Climate-related changes in water temperature and ice cover
EPA's National Lakes Assessment found nutrient pollution to be one of the most widespread stresses affecting surveyed U.S. lakes. (US EPA)
Large lakes can also respond slowly to pollution. Lake Superior, for example, has a very long water-retention time, meaning some contaminants can remain within the system for many decades. (apps.glerl.noaa.gov)
Common Mistakes
The Caspian Sea Is Geographically a Lake
Despite its name, the Caspian Sea is a landlocked inland water body and is commonly classified as the world's largest lake by surface area.
Lake Superior Is Not the Deepest Lake
Lake Superior is exceptionally large, but Lake Baikal is far deeper.
The Largest Lake Depends on the Measurement
Be specific:
Largest lake by surface area → Caspian Sea
Largest freshwater lake by conventional surface-area ranking → Lake Superior
Largest freshwater lake by volume → Lake Baikal
Deepest lake → Lake Baikal
Every Lake Is Not Freshwater
Many lakes are saline because dissolved minerals accumulate faster than they can leave.
Reservoir and Natural Lake Are Not Synonyms
A reservoir is normally created or enlarged by human engineering.
Algae Are Not Always Harmful
Algae form an important part of aquatic food webs. Problems arise when certain species or excessive nutrient enrichment produce harmful or unusually large blooms.
Memory Tips
For formation:
Ice → glacial lake
Fault → tectonic lake
Volcano → crater or caldera lake
River bend → oxbow lake
For famous lakes:
Caspian → area
Baikal → depth and volume
Superior → freshwater surface area
For lake layers:
Epilimnion → top
Metalimnion → middle
Hypolimnion → bottom
For nutrients:
Oligotrophic → low nutrients
Eutrophic → high nutrients
Summary
Lakes are standing bodies of water occupying depressions in the land. They form through processes such as glaciation, tectonic movement, volcanic collapse, river erosion, natural barriers, and human dam construction.
Some lakes are freshwater, while closed drainage basins can become saline as evaporation concentrates dissolved minerals. Lakes also contain distinct ecological and temperature zones, particularly in deep systems that undergo thermal stratification and seasonal turnover.
Nutrient availability strongly affects lake ecosystems. Oligotrophic lakes have relatively low productivity, while eutrophic lakes contain more nutrients and biological growth. Human-caused nutrient enrichment can accelerate eutrophication and trigger harmful algal blooms or oxygen depletion.
Several famous lakes hold different geographical records. The Caspian Sea is the largest lake by surface area, Lake Baikal is the deepest and largest freshwater lake by volume, and Lake Superior is normally recognized as the largest freshwater lake by surface area under the conventional five-Great-Lakes classification.
FAQ
What is a lake?
A lake is a body of relatively still water occupying a basin and surrounded by land.
What is the difference between a lake and a pond?
Ponds are generally smaller and shallower, often allowing sunlight to reach most of the bottom. There is no single universal size boundary separating all ponds from lakes. (National Park Service)
What is the world's largest lake?
The Caspian Sea is the world's largest inland lake by surface area.
What is the world's deepest lake?
Lake Baikal is the deepest lake in the world. (UNESCO World Heritage Centre)
What is the largest freshwater lake by volume?
Lake Baikal holds the greatest volume of freshwater of any individual lake. (UNESCO World Heritage Centre)
What is the largest freshwater lake by surface area?
Under the conventional separate-lake classification, Lake Superior is the largest freshwater lake by surface area. (GLERL)
What is an oxbow lake?
An oxbow lake is an abandoned river meander that becomes separated from the river's main channel.
What is lake turnover?
Lake turnover is seasonal mixing of surface and deeper water caused largely by changes in water temperature and density.
What is eutrophication?
Eutrophication is increasing nutrient enrichment of a lake, which can increase algae and aquatic-plant growth. Human activity can greatly accelerate the process. (US EPA)
What are the five Great Lakes?
They are Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario.
Key Takeaways
Lakes are standing bodies of water formed when water collects in natural or artificial basins.
Major lake origins include glacial, tectonic, volcanic, river, barrier, and human-made processes.
Lakes can be freshwater or saline and may have external drainage or form closed endorheic basins.
Lake Baikal is the world's deepest lake and the largest freshwater lake by volume, while Lake Superior leads the conventional freshwater surface-area ranking.
Nutrients, temperature, oxygen, circulation, pollution, and surrounding land use all influence lake ecosystems.
References
U.S. Geological Survey — Freshwater Lakes and Rivers and the Water Cycle Authoritative introduction to freshwater, lakes, reservoirs, and their role in the water cycle.
USGS — Freshwater Lakes and RiversU.S. National Park Service — Lakes and Ponds Educational overview of lake ecosystems and glacial, tectonic, river, and volcanic lake formation.
National Park Service — Lakes and PondsU.S. Geological Survey — Lakes and Reservoirs: Basic Limnology Technical reference explaining thermal stratification, the epilimnion, metalimnion, hypolimnion, and seasonal mixing.
USGS — Lakes and Reservoirs GuidelinesU.S. Geological Survey — Oxbow Lakes Explanation and satellite examples showing how abandoned river meanders develop into oxbow lakes.
USGS — Oxbow LakesU.S. Environmental Protection Agency — National Lakes Assessment Scientific information on lake trophic states, eutrophication, nutrients, algae, oxygen, and ecological condition.
EPA — National Lakes AssessmentUNESCO World Heritage Centre — Lake Baikal Authoritative reference on Lake Baikal's depth, age, freshwater volume, biodiversity, and World Heritage status.
UNESCO — Lake BaikalNOAA Great Lakes Environmental Research Laboratory — Great Lakes Physical Characteristics Official measurements for the Great Lakes, including surface area, depth, volume, drainage area, and retention time.
NOAA — Great Lakes Physical CharacteristicsNOAA Fisheries — Great Lakes Regional Ecosystem Overview of the Great Lakes as the world's largest freshwater system and their ecological and economic importance.
NOAA — Great Lakes Regional Ecosystem