Lake | National Geographic Society (2023)

A lake is a body of water that is surrounded by land. There are millions of

lakes

in the world. They are found on every continent and in every kind of environment—in mountains and deserts, on plains, and near seashores.

Lakes

vary greatly in size. Some measure only a few square meters and are small enough to fit in your backyard. Such small

lakes

are often referred to as ponds. Other

lakes

are so big that they are called seas. The Caspian

Sea

, in Europe and Asia, is the world’s largest

lake

, with an area of more than 370,000 square kilometers (143,000 square miles).

Lakes

also

vary

greatly in depth. The world’s deepest

lake

is

Lake

Baikal, in Russia. Its bottom is nearly 2 kilometers (m

ore

than 1 mile) below the surface in places. Although

Lake

Baikal covers less than half the surface area of

Lake

Superior—one of North America’s Great Lakes—it is about four times deeper and holds nearly as much water as all five of the

Great

Lakes

combined. Other

lakes

are so shallow that a person could easily wade across them.

Lakes

exist at many different elevations. One of the highest is

Lake

Titicaca, in the Andes

Mountains

between Bolivia and Peru. It is about 3,810 meters (12,500 feet) above sea level. The lowest

lake

is the Dead

Sea

, between Israel and Jordan. It is m

ore

than 395 meters (1,300 feet) below

sea

level

.

The water in

lakes

comes from rain, snow, melting ice, streams, and groundwater seepage. Most

lakes

contain freshwater.

All

lakes

are either open or closed. If water leaves a

lake

by a river or other outlet, it is said to be open. All

freshwater

lakes

are open. If water only leaves a

lake

by evaporation, the

lake

is closed. Closed lakes usually become saline, or salty. This is because as the water evaporates, it leaves behind solids—mostly

salts

. The Great

Salt

Lake

, in the U.S. state of Utah, is the largest

saline

lake

in North America. Its water is

saltier

than the ocean. Surrounding the Great

Salt

Lake

are salt flats, areas where the

lake

has

evaporated

, leaving only stretches of white

salt

.

How Lakes Are Formed

All

lakes

fill bowl-shaped depressions in the Earth’s surface, called basins. Lake basins are formed in several ways.

Many

lakes

, especially those in the Northern Hemisphere, were formed by glaciers that covered large areas of land during the most recent ice age, about 18,000 years ago.

The huge masses of

ice

carved out great pits and scrubbed the land as they moved slowly along. When the

glaciers

melted, water filled those depressions, forming

lakes

.

Glaciers

also carved deep valleys and deposited large quantities of

earth

, pebbles, and boulders as they melted. These materials sometimes formed dams that trapped water and created m

ore

lakes

.

Many areas of North America and Europe are dotted with glacial lakes. The U.S. state of Minnesota is nicknamed “The Land of 10,000

Lakes

” because of the number of

glacial

lakes

. Many

lakes

in North America, including the

Great

Lakes

, were created primarily by

glaciers

.

Some

lake

basins

form where plate tectonics changed the

Earth

’s crust, making it buckle and fold or break apart. When the

crust

breaks, deep cracks, called faults, may form. These

faults

make natural basins that may fill with water from

rainfall

or from

streams

flowing in the basin. When these movements occur near the

ocean

, part of the

ocean

may be trapped by a new block of land thrust up from below the

Earth

’s surface. The Caspian

Sea

was formed this way.

Lake

Baikal was also formed by the movement of tectonic plates.

Many

lakes

form as a result of volcanoes. After a

volcano

becomes inactive, its crater may fill with

rain

or melted

snow

. Sometimes the top of a

volcano

is blown off or collapses during an eruption, leaving a depression called a caldera. It, too, may fill with

rainwater

and become a

lake

.

Crater

Lake

, in the U.S. state of

Oregon

, one of the deepest

lakes

in the world, was created when ancient Mount Mazama’s volcanic cone

collapsed

.

Not all

lakes

are created by basins filling with water. Some

lakes

are formed by

rivers

. Mature

rivers

often wind back and forth across a

plain

in wide loops called meanders. During periods of flooding, a swollen, rushing

river

may create a shortcut and bypass a

meander

, leaving a body of standing water. This type of small

lake

is called an oxbow lake, because its shape resembles the U-shaped frame that fits over an ox’s neck when it is harnessed to pull a wagon or a plow.

Lakes

may also be created by landslides or mudslides that send soil, rock, or mud sliding down hills and

mountains

. The debris piles up in natural dams that can block the flow of a

stream

, forming a

lake

.

Dams

that beavers build out of tree branches can plug up

rivers

or

streams

and make large

ponds

or marshes.

People make

lakes

by digging basins or by

damming

rivers

or springs. These artificial

lakes

can become reservoirs, storing water for irrigation, hygiene, and industrial use. Artificial

lakes

also provide recreational use for boating, swimming, or fishing.

Artificial

lakes

can provide electricity through hydroelectric power plants at the

dam

.

Lake

Mead, in the U.S. states of Arizona and Nevada, was formed when the Hoover Dam was built during the Great Depression. The

dam

was built to control the unpredictable Colorado

River

and provides

electricity

to the western United States.

Chemical and Physical Aspects of Lakes

Temperature, light, and

wind

are three of the main factors that affect the physical characteristics of a

lake

.

Temperature

and light

vary

from

lake

to

lake

. Depth,

plant

growth, dissolved materials, time of day, season, and latitude can all affect light’s ability to pass through the

lake

’s water.

Light and

wind

affect the

temperature

in

lakes

. Sunlight warms the water, and

wind

cools it down. Most

lakes

go through a process called thermal stratification.

Thermal stratification

refers to a

lake

’s three main layers, each with a different

temperature

range. A

lake

’s shallowest layer is the epilimnion. Its middle layer is the metalimnion, or thermocline. The deepest layer is the hypolimnion.

The most important chemicals in a

lake

are nitrogen and phosphorus. These chemicals allow nutrient-rich

plants

and algae to grow. Other organisms feed off these

plants

and

algae

, creating a complex, healthy ecosystem.

The chemistry of a

lake

is affected by biological, geological, and human processes. The balance of

nutrients

may be altered by biological phenomena such as “algal blooms,” when

algae

reproduces so rapidly it prevents any

nutrients

from reaching below the

lake

’s surface. Natural processes such as the

eruption

of a nearby

volcano

can alter the chemical aspect of a

lake

by introducing new gases or minerals. Pollution, such as the introduction of toxic chemicals from industry or agriculture, can also affect a

lake

’s chemistry.

The amount of oxygen and the

pH

level can also affect a

lake

’s chemistry. A

lake

must have a healthy amount of

oxygen

to sustain life.

Lakes

that do not have enough

oxygen

to sustain life are abiotic.

The

pH

level is a chemical property of all substances. A substance’s

pH

level indicates whether it is an acid or a base. Substances with a

pH

of less than 7 are

acidic

; substances with a

pH

greater than 7 are basic.

Lakes

have different

pH

levels, with life adapting to different chemical

environments.

Lake

Tanganyika, one of the African

Great

Lakes

, has an extremely high

pH

. It is full of

dissolved

minerals

. Fish such as cichlids thrive in

Lake

Tanganyika. Tilapia, a variety of

cichlid

, can also thrive in

lakes

with very low

pH

.

The Life Cycle of Lakes

Once formed,

lakes

do not stay the same. Like people, they go through different life stages—youth, maturity, old age, and death. All

lakes

, even the largest, slowly disappear as their basins fill with sediment and

plant

material. The natural aging of a

lake

happens very slowly, over the course of hundreds and even thousands of years. But with human influence, it can take only decades.

A

lake

’s

plants

and

algae

slowly die. The warm, shallow water of the upper layer of the

lake

causes

plants

and

algae

to decompose, and eventually they sink to the basin. Dust and

mineral

deposits on the bottom of the

lake

combine with the

plants

to form

sediment

.

Rain

washes

soil

and

pebbles

into the basin. The remains of fish and other animals pile up on the

lake

’s bottom. The

lake

becomes smaller, starting at the edges and working toward the middle. Eventually, the

lake

becomes a

marsh

, bog, or swamp. At this point, the drying-up process slows down dramatically; limnologists, people who study

lakes

and

ponds

, aren’t sure why. Eventually, the

lake

becomes dry land.

Dry

lake

beds are a perfect place to find and study fossils. Archaeologists often excavate

ancient

lake

beds, such as

Fossil

Butte in the U.S. state of Wyoming. The

remains

of organisms, from single-celled bacteria to dinosaurs, were preserved over time as

sediment

on the

lake

bed built up around and on top of them. In fact, some scientists believe the first living organisms on

Earth

developed in

lakes

.

Lake Classification

There are three basic ways that

limnologists

classify

lakes

: how many

nutrients

lakes

have, how their water mixes, and what kinds of fish live in them.

When

lakes

are classified by the amount of

nutrients

they have,

limnologists

are using the trophic system. Generally, the clearer the water in the

lake

, the fewer

nutrients

it has.

Lakes

that are very

nutrient

-rich are cloudy and hard to see through; this includes

lakes

that are unhealthy because they have too many

nutrients

.

Lakes

need to have a balance of

nutrients

.

Lakes

can also be classified by how the water mixes, or turns over from top (

epilimnion

) to bottom (

hypolimnion

). This is called lake turnover. Water in some

lakes

, mostly shallow ones, mixes all year long. These

lakes

have very little

lake

turnover

.

Deep

lakes

experience

lake

turnover

on a large scale. The middle layer, the

thermocline

, mixes and turns over throughout the year. It turns over due to climate,

nutrient

variations, and geologic activity such as earthquakes. However, major

lake

turnover

happens during the fall and spring, when the

lake

’s cold and warm waters mix and readjust. Most

lakes

that experience

lake

turnover

are dimictic lakes, meaning their waters mix tw

ice

a year, usually in fall and spring.

Lake

turnover

changes with the

seasons

. During the summer, the

epilimnion

, or surface layer, is the warmest. It is heated by the sun. The deepest layer, the

hypolimnion

, is the coldest. The sun’s radiation does not reach this cold, dark layer.

During the fall, the warm surface water begins to cool. As water cools, it becomes m

ore

dense, causing it to sink. This cold,

dense

water sinks to the bottom of the

lake

. It forces the water of the

hypolimnion

to rise.

During the winter, the

epilimnion

is coldest because it is exposed to

wind

,

snow

, and low air

temperatures

. The

hypolimnion

is the warmest. It is insulated by the

earth

. This is why there is

ice

on

lakes

during the winter, while fish swim in slightly warmer, liquid water beneath.

During the spring, the

lake

turns over again. The cold surface water sinks to the bottom, forcing the warmer, less

dense

water upward.

The final way to

classify

lakes

is by the kinds of fish they have. This helps people in the fishing

in

dustry

identify what kinds of fish they might be able to catch in that

lake

. For example, calling a

lake

a cold-water

lake

tells a fisherman that he can probably expect to find trout, a cold-water fish. A

lake

that has thick, muddy

sediment

is m

ore

likely to have catfish.

There are other ways of

classifying

a

lake

, such as by whether it is closed or fed by a

river

or

stream

. States also divide

lakes

into ones that are available for public use and ones that are not. Many people refer to

lakes

by size.

How Animals and Plants Use Lakes

Lakes

are important in preserving wildlife. They serve as migration stops and breeding grounds for many birds and as refuges for a wide variety of other animals. They provide homes for a diversity of organisms, from microscopic

plants

and animals to fish that may weigh hundreds of kilograms. The largest fish found in

lakes

is the sturgeon, which can grow to 6 meters (20 feet) and weigh m

ore

than 680 kilograms (1,500 pounds).

Plants

growing along the

lakesh

ore

may include mosses, ferns, reeds,

rushes

, and cattails. Small animals such as snails, shrimp, crayfish, worms, frogs, and dragonflies live among the

plants

and lay their eggs on them both above and below the waterline. Farther from the sh

ore

, floating

plants

such as water lilies and water hyacinths often thrive. They have air-filled bladders, or sacs, that help keep them afloat. These

plants

shelter small fish that dart in and out under their leaves. Waterbugs, beetles, and spiders glide and skitter across the surface or just below it. Small islands, floating

plants

, or fallen logs provide sunny spots for turtles to warm themselves.

Other animals live near the

lake

, such as bats and semi-aquatic animals, such as mink, salamanders,

beavers

, and

turtles

.

Semi-aquatic animals need both water and land to survive, so both the

lake

and the sh

ore

are important to them.

Many kinds of water birds live on

lakes

or gather there to b

reed

and raise their young. Ducks are the most common

lake

birds. Others include swans, geese, loons, kingfishers, herons, and bald eagles.

Many people think of fish when they think of

lakes

. Some of the most common fish found in

lakes

are tiny shiners, sunfish, perch, bass, crappie, muskie, walleye,

perch

, lake trout, pike, eels,

catfish

, salmon, and

sturgeon

. Many of these provide food for people.

How People Use Lakes

Lakes

are an important part of the water cycle; they are where all the water in an area collects. Water filters down through the watershed, which is all the

streams

and

rivers

that flow into a specific

lake

.

Lakes

are valuable resources for people in a variety of ways. Through the centuries,

lakes

have provided routes for travel and trade. The

Great

Lakes

of North America, for example, are major inland

routes

for ships carrying grain and raw materials such as

iron

ore

and coal.

Farmers use

lake

water to irrigate crops. The effect of very large

lakes

on

climate

also helps farmers. Because water does not heat or cool as

rapidly

as land does,

winds

blowing from

lakes

help keep the

climate

m

ore

even. This is the “lake effect.” The city of Chicago, in the U.S. state of Illinois, benefits from the

lake

effect

. Chicago sits on the sh

ore

of

Lake

Michigan. When the western part of Illinois is

snowing

, Chicago often

remains

slightly warmer.

The

lake

effect

can help

farmers

. In autumn,

lakes

blow warmer air over the land, helping the

season

last longer so

farmers

can continue to grow their

crops

. In spring, cool

lake

winds

help

plants

not to grow too soon and avoid the danger of early-spring frosts, which can kill the young

crops

.

Lakes

supply many communities with water. Artificial

lakes

are used to st

ore

water for times of drought.

Lakes

formed by

dams

also provide hydroelectric energy. The water is channeled from the

lake

to drive generators that produce

electricity

.

Because they are often very beautiful,

lakes

are popular recreation and vacation spots. People seek out their sparkling waters to enjoy boating, swimming, water-skiing, fishing, sailing, and, in winter,

ice

skating,

ice

boating, and

ice

fishing. Many public

parks

are built near

lakes

, allowing people to picnic, camp, hike, bike, and enjoy the

wildlife

and scenery the

lake

provides.

For some people,

lakes

are permanent homes. For example, indigenous people called the Uros have lived on

Lake

Titicaca in the Andes

Mountains

for centuries. The

lake

supplies almost everything the

Uros

need. They catch fish from the

lake

and hunt water birds.

The

Uros

also use the

reeds

that grow in

Lake

Titicaca to build floating “

islands

” to live on. The

islands

are about 2 meters (6.5 feet) thick. On them, the

Uros

build

reed

houses and make

reed

sleeping mats, baskets, fishing boats, and sails. They also eat the roots and the celery-like stalks of the

reeds

.

Lake Health: Blue-Green Algae

Although

lakes

naturally age and die, people have sped up the process by polluting the water. A major problem that threatens many

lakes

is

blue-green

algae

.

Blue-green

algae

is sometimes referred to as “pond scum” and can be blue-green, blue, green, reddish-purple, or brown. It stays on the surface of the water and forms a sort of mat. When the conditions are just right, the

algae

multiplies quickly. This is called an

algal bloom

and is harmful to

lakes

, animals,

plants

, and people.

Blue-green

algae

is different from true

algae

because it is not eaten by other organisms. True

algae

is an important part of the food web because it supplies energy for tiny animals, which are then eaten by fish, which are then eaten by other fish, birds, animals, or people.

Blue-green

algae

, also called cyanobacteria, is not a part of the

food web

. It uses up important

nutrients

without contributing to the

lake

ecosystem

. Instead, the

algal bloom

chokes up a

lake

and uses up the

oxygen

that fish and other living things depend on for survival.

Plants

die m

ore

quickly, sinking to the bottom and filling up the

lake

basin

.

Blue-green

algae

also can become so

dense

that it

prevents

light from penetrating the water, changing the chemistry and affecting species living below the surface.

When an

algal bloom

happens, water becomes contaminated. The

toxic

water can kill animals and make humans sick.

Blue-green

algae

is not a new problem. Scientists have found evidence of it from hundreds of years ago. The problem has increased, though, as humans pollute

lakes

.

Eutrophication is when a

lake

gets too many

nutrients

, causing

blue-green

algae

growth. How do the excess

nutrients

get into

lakes

? Sewage from towns and cities causes explosive growth of

blue-green

algae

, and waste from factories can wash into the

lakes

and pollute them.

Phosphorus

-

based

fertilizers from

farms

, golf courses,

parks

, and even neighborhood lawns can wash into

lakes

and pollute them. The

phosphorus

seeps into the ground and eventually reaches the

lake

.

Phosphorus

is an important

nutrient

for a

lake

, but too much of it is not a good thing because it encourages

blue-green

algae

.

How can

blue-green

algae

be

prevented

or reduced? At home, people can help by using

phosphorus

-free

fertilizer

and by fertilizing only where it’s needed.

Preventing

lawn clippings and leaves from washing into the gutter and maintaining a buffer of native

plants

help

filter

water and stop

debris

from washing away. Making sure septic systems don’t have leaks, safely disposing of household chemicals (like paint), and minimizing activities that erode

soil

also help

prevent

the spread of

blue-green

algae

.

Controlling

phosphorous

and chemicals from factories and

farms

is much m

ore

complicated. Citizens need to work with businesses and elected leaders to help

reduce

the amount of runoff and water

pollution

.

Lake Health: Invasive Species

When a

plant

or animal species is moved to a location where it’s not originally from, the species is called an exotic species. When that species harms the natural balance in an

ecosystem

, the species is called invasive.

Invasive species

can harm life in a

lake

by competing for the same

resources

that native species do. When introduced to new food sources,

invasive species

multiply quickly, crowding out the helpful

native species

until there are m

ore

invasive than

native species

.

Invasive species

can change the natural habitat of the

lake

and are known as biological pollutants when this happens. Once non-native species have been introduced into a

lake

, they are almost impossible to get rid of.

How do

invasive species

invade in the first place? Non-native

plants

and animals are almost always introduced by people. As people use waterways m

ore

frequently, they may inadvertently move organisms from one area to another.

Plants

such as Eurasian watermilfoil, an invasive

aquatic

plant

in the U.S., may cling to boats, clothing, pets, equipment, and vehicles. Small animals such as the spiny water flea can

travel

unnot

iced

by hopping onto a kayak or other

recreational

equipment.

Species are also carried by large ships bringing goods from one country to another. These ships take in ballast water, which helps stabilize the ship as it crosses the

ocean

. When the ship reaches its destination, it releases the

ballast

water. The water may be full of

non-

native species

accidentally captured as the ship took on

ballast

.

The most famous

invasive species

in

lakes

is probably the zebra mussel, a small mollusk native to the Black

Sea

and the Caspian

Sea

in Europe and Asia. In the late 1980s,

zebra mussels

were found in several of North America’s

Great

Lakes

. Since then,

zebra mussels

have spread to

lakes

from the U.S. state of Louisiana to the Canadian province of Quebec.

Zebra mussels

devastate native

plants

and animals. Some scientists say they carry a type of di

sease

that is deadly to birds that eat the mussels.

Zebra mussels

multiply so quickly that they clog pipes. This harms machinery at

in

dustrial

plants

that use water, including hydroelectric

dams

and water filtration

plants

. Ships, docks, anchors, and buoys have also been destroyed by the invasive

zebra mussel

.

Communities have worked to

reduce

the impact of

invasive species

. Many states have laws prohibiting the sale or transport of

non-

native species

. People are encouraged to inspect their boats and other equipment for

wildlife

. Boaters should remove

plants

, animals, and mud bef

ore

leaving the water-access area. They should also d

rain

any water from their boat. Rinsing boats, equipment, and even people can help

reduce

the transfer of harmful species. People should also get rid of leftover bait and report any species they see that look like they might not be native. These steps can make a big difference in keeping the

habitat

of a

lake

healthy.

Lake Health: Acid Rain

Another major threat to

lakes

today is

acid

rain

. Some

acid

is natural, even in pure

rain

. This slightly

toxic

chemical slowly weathers

rocks

and

soil

.

Acid

rain

, however, is caused by human activities and is harmful. It is caused by

toxic

gases from factories,

coal

-fired power

plants

, vehicle exhaust, and home furnaces.

Nitrogen

and sulfur, the main ingredients of

acid

rain

, rise in the air and may be carried hundreds of kilometers by

wind

. When these gases mix with the moisture in clouds, they form strong

acids

, which kill fish,

plants

, and other organisms when the

acids

fall as

rain

or

snow

on

lakes

.

Acid

rain

can also affect humans, causing asthma and bronchitis, and

damaging

lung tissue. Methylmercury, a

toxic

form of mercury, has been linked to

acid

rain

. Eating fish containing high levels of this mercury is particularly harmful for pregnant women, the elderly, and children.

Lakes

and

soil

can neutralize normal levels of

acid

, but

acid

rain

is too strong for

lakes

to com

bat

. Eventually,

acid

rain

leaves

lakes

sterile and lifeless. There are many

lakes

today in the United States, Canada, and parts of Europe dead or drying up because of

acid

rain

.

Some steps have been taken to curb

acid

rain

. The Clean Air Act was passed by the United States Congress in 1990. It required all utility companies to

reduce

the amount of

toxic

emissions by 40 percent by the year 2000. At home, people can help the problem by replacing old

furnaces

, turning off electronics when they’re not being used, and using fans or opening

windows

in the summer instead of air conditioning. Using compact flu

orescent

light bulbs (CFLs) and energy-efficient vehicles also help

reduce

the amount of

pollution

going into the air.

Lakes

are among the most valuable and most beautiful of the

Earth

’s

resources

. Most experts agree that

lakes

must be kept clean and free from

pollution

if they are to continue to provide the many benefits that we receive from them today.

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