• 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 (more 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 more 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 more 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 twice 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 more 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 industry 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 more 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 more than 680 kilograms (1,500 pounds).

    Plants growing along the lakeshore 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 shore, 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 shore are important to them.

    Many kinds of water birds live on lakes or gather there to breed 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 more 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 shore 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 store 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 more 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 more 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 more 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 more 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 unnoticed 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 disease that is deadly to birds that eat the mussels. Zebra mussels multiply so quickly that they clog pipes. This harms machinery at industrial 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 before leaving the water-access area. They should also drain 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 combat. 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 fluorescent 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.

    A cob, pen, and cygnets—male, female, and baby swans—enjoy a day on the lake.

    Lake Vostok
    Lake Vostok, in Antarctica, is one of the largest subglacial lakes in the world. Lake Vostok is about the same size as Lake Ontario, and even has an island in the middle of it. On top of the lake is an icecap 4 kilometers (2.5 miles) thick. The ice actually insulates the water, preventing it from freezing.

    A Lake by Any Other Name
    A mere is a large, shallow lake. Meres are common in the United Kingdom, while meers (the Dutch word for lake) are found in the Netherlands.

    Lochs are lakes or bays mostly found in Scotland.

    Lake District
    The Lake District is a famous wilderness area in northern England. Lake District National Park is one of the countrys most popular parks. Besides lakes, the Lake District is filled with mountains and hills, valleys and streams, bogs and plains. The Lake District was a favorite place of the so-called Lake Poets, a group of 19th-century English writers including William Wordsworth and Samuel Taylor Coleridge.

  • Term Part of Speech Definition Encyclopedic Entry
    abiotic Adjective

    lacking or absent of life.

    acid Noun

    chemical compound that reacts with a base to form a salt. Acids can corrode some natural materials. Acids have pH levels lower than 7.

    acid rain Noun

    precipitation with high levels of nitric and sulfuric acids. Acid rain can be manmade or occur naturally.

    agriculture Noun

    the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

    Encyclopedic Entry: agriculture
    air conditioning Noun

    system that cools the air.

    algae Plural Noun

    (singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

    algal bloom Noun

    the rapid increase of algae in an aquatic environment.

    ancient Adjective

    very old.

    aquatic Adjective

    having to do with water.

    archaeologist Noun

    person who studies artifacts and lifestyles of ancient cultures.

    asthma Noun

    disease that makes it difficult to breathe.

    bacteria Plural Noun

    (singular: bacterium) single-celled organisms found in every ecosystem on Earth.

    Encyclopedic Entry: Bacteria
    bait Noun

    object used to attract something.

    bald eagle Noun

    white-headed bird of prey native to North America.

    ballast Noun

    heavy material, usually water, used to provide stability for large ships or other oceangoing vessels.

    base Noun

    chemical compound that reacts with acid to form a salt. Bases have pH levels higher than 7.

    bass Noun

    type of popular game fish, found in both ocean and freshwater environments.

    bat Noun

    flying mammal.

    bay Noun

    body of water partially surrounded by land, usually with a wide mouth to a larger body of water.

    Encyclopedic Entry: bay
    beaver Noun

    large, semiaquatic rodent with sharp teeth, a flat tail, and the ability to build dams.

    biological pollutant Noun

    non-native species that changes an ecosystem's native habitat.

    biological process Noun

    method of work or functionality of a living thing or series of living things.

    blue-green algae Noun

    type of aquatic bacteria (not algae) that can photosynthesize light to create energy. Also called cyanobacteria and (in freshwater habitats) pond scum.

    bog Noun

    wetland of soft ground made mostly of decaying plant matter.

    boulder Noun

    large rock.

    breeding ground Noun

    place where animals mate, give birth, and sometimes raise young.

    bronchitis Noun

    irritation of the main air passages to the lungs.

    buffer Noun

    a cushion or shield.

    buoy Noun

    floating object anchored to the bottom of a body of water. Buoys are often equipped with signals.

    caldera Noun

    large depression resulting from the collapse of the center of a volcano.

    Encyclopedic Entry: Types of Calderas
    catfish Noun

    freshwater fish with wiry organs that look like whiskers on its upper jaw.

    cattail Noun

    aquatic plant.

    celery Noun

    edible plant.

    century Noun

    100 years.

    choke Verb

    to cut off the air supply of a living organism.

    cichlid Noun

    spiny-finned freshwater fish.

    classify Verb

    to identify or arrange by specific type or characteristic.

    Clean Air Act Noun

    (1955) set of rules and regulations to control air pollution in the United States. Originally called the Air Pollution Control Act, most recently updated in 1990.

    climate Noun

    all weather conditions for a given location over a period of time.

    Encyclopedic Entry: climate
    clog Verb

    to obstruct or prevent travel.

    closed lake Noun

    inland body of fresh water that does not have a river or other outlet flowing from it, losing water only through evaporation.

    coal Noun

    dark, solid fossil fuel mined from the earth.

    collapse Verb

    to fall apart completely.

    compact fluorescent light bulb (CLF) Noun

    source of electric light that uses less power and lasts longer than an incandescent light bulb or lamp. Also called compact fluorescent lamp.

    cone Noun

    hill created by a volcanic eruption.

    Congress Noun

    legislative branch of the government, responsible for making laws. The U.S. Congress has two bodies, the House of Representatives and the Senate.

    contaminate Verb

    to poison or make hazardous.

    continent Noun

    one of the seven main land masses on Earth.

    Encyclopedic Entry: continent
    crappie Noun

    freshwater fish.

    crater Noun

    bowl-shaped depression formed by a volcanic eruption or impact of a meteorite.

    Encyclopedic Entry: crater
    crayfish Noun

    crustacean resembling a small lobster. Also called a crawdad.

    crop Noun

    agricultural produce.

    Encyclopedic Entry: crop
    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    curb Verb

    to restrain or control.

    cyanobacteria Noun

    type of aquatic bacteria that can photosynthesize light to create energy. Also called blue-green algae (even though it is not algae) and (in freshwater habitats) pond scum.

    dam Verb

    to block a flow of water.

    dam Noun

    structure built across a river or other waterway to control the flow of water.

    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    decompose Verb

    to decay or break down.

    dense Adjective

    having parts or molecules that are packed closely together.

    desert Noun

    area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

    Encyclopedic Entry: desert
    destination Noun

    place where a person or thing is going.

    dimictic lake Noun

    lake whose waters mix twice a year, usually in the fall and spring.

    dinosaur Noun

    very large, extinct reptile chiefly from the Mesozoic Era, 251 million to 65 million years ago.

    dissolve Verb

    to break up or disintegrate.

    dragonfly Noun

    insect that preys on mosquitoes and other insects.

    drought Noun

    period of greatly reduced precipitation.

    Encyclopedic Entry: drought
    duck Noun

    aquatic bird.

    dust Noun

    tiny, dry particles of material solid enough for wind to carry.

    Encyclopedic Entry: dust
    earth Noun

    soil or dirt.

    earthquake Noun

    the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

    ecosystem Noun

    community and interactions of living and nonliving things in an area.

    Encyclopedic Entry: ecosystem
    eel Noun

    fish with a long body and no fins on the lower surface of the body.

    electricity Noun

    set of physical phenomena associated with the presence and flow of electric charge.

    electronics Noun

    devices or tools that use electricity to work.

    elevation Noun

    height above or below sea level.

    Encyclopedic Entry: elevation
    emission Noun

    discharge or release.

    environment Noun

    conditions that surround and influence an organism or community.

    epilimnion Noun

    surface and upper part of a lake.

    erode Verb

    to wear away.

    eruption Noun

    release of material from an opening in the Earth's crust.

    eutrophication Noun

    build-up of sediment and organic matter in bodies of water, which may cause a change in the productivity of the ecosystem.

    evaporate Verb

    to change from a liquid to a gas or vapor.

    evaporation Noun

    process by which liquid water becomes water vapor.

    Encyclopedic Entry: evaporation
    excavate Verb

    to expose by digging.

    excess Noun

    extra or surplus.

    exhaust Noun

    gases and particles expelled from an engine.

    exotic Adjective

    foreign or strange.

    exotic species Noun

    non-native species.

    factory Noun

    one or more buildings used for the manufacture of a product.

    farm Noun

    land cultivated for crops, livestock, or both.

    farmer Noun

    person who cultivates land and raises crops.

    fault Noun

    a crack in the Earth's crust where there has been movement.

    fern Noun

    flowerless plant.

    fertilizer Noun

    nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.

    filter Verb

    to remove particles from a substance by passing the substance through a screen or other material that catches larger particles and lets the rest of the substance pass through.

    flood Verb

    to overflow or cover in water or another liquid.

    food chain Noun

    group of organisms linked in order of the food they eat, from producers to consumers, and from prey, predators, scavengers, and decomposers.

    Encyclopedic Entry: food chain
    food web Noun

    all related food chains in an ecosystem. Also called a food cycle.

    Encyclopedic Entry: food web
    fossil Noun

    remnant, impression, or trace of an ancient organism.

    Encyclopedic Entry: fossil
    freshwater Noun

    water that is not salty.

    frog Noun

    animal (amphibian) with smooth skin and long hind legs for jumping.

    frost Noun

    thin coat of ice covering objects when the dew point is below freezing.

    Encyclopedic Entry: frost
    furnace Noun

    device used for heating by burning a fuel, such as wood or coal.

    generator Noun

    machine that converts one type of energy to another, such as mechanical energy to electricity.

    geological process Noun

    method by which the Earth changes.

    glacial lake Noun

    body of water created by a melting glacier.

    glacier Noun

    mass of ice that moves slowly over land.

    Encyclopedic Entry: glacier
    goose Noun

    aquatic bird with a long neck.

    grain Noun

    harvested seed of such grasses as wheat, oats, and rice.

    Encyclopedic Entry: grain
    Great Depression Noun

    (1929-1941) period of very low economic activity in the U.S. and throughout the world.

    Great Lakes Noun

    largest freshwater bodies in the world, located in the United States and Canada. Lake Huron, Lake Ontario, Lake Michigan, Lake Erie, and Lake Superior make up the Great Lakes.

    groundwater seepage Noun

    process by which water from an aquifer seeps up to form a lake or pond.

    habitat Noun

    environment where an organism lives throughout the year or for shorter periods of time.

    Encyclopedic Entry: habitat
    harness Verb

    to control or guide for a specific purpose.

    heron Noun

    long-legged wading bird.

    Hoover Dam Noun

    dam on the Colorado River between the U.S. states of Arizona and Nevada. Also called the Boulder Dam.

    human process Noun

    system that produces, maintains, and changes human activities, such as migration or diffusion.

    hydroelectric energy Noun

    energy generated by moving water converted to electricity. Also known as hydroelectricity.

    Encyclopedic Entry: hydroelectric energy
    hydroelectric power Noun

    usable energy generated by moving water converted to electricity.

    hygiene Noun

    science and methods of keeping clean and healthy.

    hypolimnion Noun

    dense bottom layer of a lake.

    ice Noun

    water in its solid form.

    Encyclopedic Entry: ice
    ice age Noun

    long period of cold climate where glaciers cover large parts of the Earth. The last ice age peaked about 20,000 years ago. Also called glacial age.

    inadvertent Adjective

    accidental or not on purpose.

    indigenous Adjective

    characteristic to or of a specific place.

    Encyclopedic Entry: indigenous
    industrial Adjective

    having to do with factories or mechanical production.

    industry Noun

    activity that produces goods and services.

    inland Adjective

    area not near the ocean.

    insulate Verb

    to cover with material to prevent the escape of energy (such as heat) or sound.

    invasive species Noun

    type of plant or animal that is not indigenous to a particular area and causes economic or environmental harm.

    Encyclopedic Entry: invasive species
    iron Noun

    chemical element with the symbol Fe.

    irrigate Verb

    to water.

    irrigation Noun

    watering land, usually for agriculture, by artificial means.

    Encyclopedic Entry: irrigation
    island Noun

    body of land surrounded by water.

    Encyclopedic Entry: island
    kingfisher Noun

    bird that preys on fish and insects.

    lake Noun

    body of water surrounded by land.

    Encyclopedic Entry: lake
    lake basin Noun

    dip or depression in the surface of the Earth that used to be the site of a lake.

    lakebed Noun

    bottom of a lake.

    Lake District Noun

    mountainous region in northwest England.

    lake effect Noun

    process where cold winds blowing over a relatively warm lake cause rapid cloud formation and precipitation.

    Lake Poets Noun

    group of 19th century writers who lived and worked in the Lake District in northern England: William Wordsworth, Samuel Taylor Coleridge, and Robert Southey.

    lake trout Noun

    freshwater fish often caught for food.

    lake turnover Noun

    process of the dense lower layer of a lake rising to become the upper, less-dense layer.

    landslide Noun

    the fall of rocks, soil, and other materials from a mountain, hill, or slope.

    Encyclopedic Entry: landslide
    latitude Noun

    distance north or south of the Equator, measured in degrees.

    Encyclopedic Entry: latitude
    limnologist Noun

    person who studies lakes and ponds.

    loch Noun

    lake or bay.

    loon Noun

    aquatic bird.

    lung Noun

    organ in an animal that is necessary for breathing.

    machinery Noun

    mechanical appliances or tools used in manufacturing.

    marsh Noun

    wetland area usually covered by a shallow layer of seawater or freshwater.

    Encyclopedic Entry: marsh
    meander Noun

    large curve in a lake or stream.

    mere Noun

    shallow lake with large surface area.

    metalimnion Noun

    middle layer of a lake where temperature change occurs most frequently.

    methylmercury Noun

    toxic chemical that builds up in living tissue and enters the food chain.

    microscopic Adjective

    very small.

    migration Noun

    movement of a group of people or animals from one place to another.

    migration stop Noun

    point in a migration route where animals rest and eat.

    mineral Noun

    inorganic material that has a characteristic chemical composition and specific crystal structure.

    mink Noun

    semi-aquatic animal related to the weasel.

    mollusk Noun

    large phylum of invertebrate animal, all possessing a mantle with a significant cavity used for breathing and excretion, a radula (except for bivalves), and the structure of the nervous system. 

    moss Noun

    small, flowerless plant that grows in tufts or mats.

    mountain Noun

    landmass that forms as tectonic plates interact with each other.

    Mount Mazama Noun

    extinct volcano in the U.S. state of Oregon whose caldera holds Crater Lake.

    mudslide Noun

    rapid, downhill flow of soil and water. Also called a mudflow.

    muskie Noun

    large freshwater fish. Also called a muskellunge.

    native species Noun

    species that occur naturally in an area or habitat. Also called indigenous species.

    neutralize Verb

    to counteract and make ineffective.

    nitrogen Noun

    chemical element with the symbol N, whose gas form is 78% of the Earth's atmosphere.

    non-native species Noun

    a type of plant or animal that is not indigenous to a particular area. Non-native species can sometimes cause economic or environmental harm as an invasive species.

    nutrient Noun

    substance an organism needs for energy, growth, and life.

    Encyclopedic Entry: nutrient
    ocean Noun

    large body of salt water that covers most of the Earth.

    Encyclopedic Entry: ocean
    open lake Noun

    inland body of fresh water with a river or other body of water flowing out of it.

    ore Noun

    deposit in the Earth of minerals containing valuable metal.

    Encyclopedic Entry: ore
    oxbow lake Noun

    lake formed from an abandoned bend in a river.

    Encyclopedic Entry: oxbow lake
    oxygen Noun

    chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

    park Noun

    area of land set aside for recreational use.

    pebble Noun

    very small, rounded rock.

    penetrate Verb

    to push through.

    perch Verb

    to sit or rest on a tree branch or other elevated position.

    pH Noun

    measure of a substance's acid or basic composition. Distilled water is neutral, a 7 on the pH scale. Acids are below 7, and bases are above.

    phenomena Plural Noun

    (singular: phenomenon) any observable occurrence or feature.

    phosphorus Noun

    chemical element with the symbol P.

    physical characteristic Noun

    physical feature of an organism or object.

    pike Noun

    large freshwater fish.

    plain Noun

    flat, smooth area at a low elevation.

    Encyclopedic Entry: plain
    plant Noun

    organism that produces its own food through photosynthesis and whose cells have walls.

    plate tectonics Noun

    movement and interaction of the Earth's plates.

    plow noun, verb

    tool used for cutting, lifting, and turning the soil in preparation for planting.

    pollutant Noun

    chemical or other substance that harms a natural resource.

    pollution Noun

    introduction of harmful materials into the environment.

    Encyclopedic Entry: pollution
    pond Noun

    small body of water surrounded by land.

    pond scum Noun

    type of aquatic bacteria that can photosynthesize light to create energy. Also called cyanobacteria and blue-green algae (even though it is not an algae).

    pregnant Noun

    status of females carrying offspring in their bodies before birth.

    prevent Verb

    to keep something from happening.

    primarily Adverb

    first or most important.

    prohibit Verb

    to disallow or prevent.

    radiation Noun

    energy, emitted as waves or particles, radiating outward from a source.

    rain Noun

    liquid precipitation.

    Encyclopedic Entry: rain
    rapidly Adverb


    raw material Noun

    matter that needs to be processed into a product to use or sell.

    recreational Adjective

    having to do with activities done for enjoyment.

    reduce Verb

    to lower or lessen.

    reed Noun

    grass with tall, strong stalks that grows in marshy ecosystems.

    refuge Noun

    shelter or protection from danger.

    remains Noun

    materials left from a dead or absent organism.

    reservoir Noun

    natural or man-made lake.

    Encyclopedic Entry: reservoir
    resource Noun

    available supply of materials, goods, or services. Resources can be natural or human.

    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    rock Noun

    natural substance composed of solid mineral matter.

    route Noun

    path or way.

    runoff Noun

    overflow of fluid from a farm or industrial factory.

    Encyclopedic Entry: runoff
    rush Noun

    grasslike aquatic plant.

    salamander Noun

    lizard-like amphibian.

    saline Adjective


    salmon Noun

    cold-water fish hunted for food and game.

    salt Noun

    (sodium chloride, NaCl) crystalline mineral often used as a seasoning or preservative for food.

    salt flat Noun

    large, flat expanse of earth covered by a thick layer of salt left by an evaporated saline lake or pond. Also called a playa, sink, or salt pan.

    Samuel Taylor Coleridge Noun

    (1772-1834) English poet.

    sea Noun

    large part of the ocean enclosed or partly enclosed by land.

    Encyclopedic Entry: sea
    sea level Noun

    base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

    Encyclopedic Entry: sea level
    seashore Noun

    beach or coast.

    season Noun

    period of the year distinguished by special climatic conditions.

    Encyclopedic Entry: season
    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    Encyclopedic Entry: sediment
    seep Verb

    to slowly flow through a border.

    semi-aquatic Adjective

    needing both a water and land environment to live and reproduce.

    septic system Noun

    individual sewage treatment system, usually for a single residence or place of business.

    sewage Noun

    liquid and solid waste material from homes and businesses.

    shiner Noun

    small freshwater fish or minnow.

    shrimp Noun

    animal that lives near the bottom of oceans and lakes.

    skitter Verb

    to move quickly across a surface in no set pattern.

    smog Noun

    type of air pollution common in manufacturing areas or areas with high traffic.

    Encyclopedic Entry: smog
    snail Noun

    marine or terrestrial animal (mollusk) with a shell and one foot on which it glides.

    snow Noun

    precipitation made of ice crystals.

    soil Noun

    top layer of the Earth's surface where plants can grow.

    specific Adjective

    exact or precise.

    spider Noun

    eight-legged animal (arachnid) that usually spins webs to catch food.

    spiny water flea Noun

    small crustacean native to northern Europe.

    square meter Noun

    space equal to a square measuring one meter on each side.

    stalk Noun

    stem of a plant.

    sterile Adjective

    not able to reproduce.

    stream Noun

    body of flowing water.

    Encyclopedic Entry: stream
    sturgeon Noun

    type of marine or freshwater large, long, bony fish.

    subglacial lake Noun

    inland body of fresh water that exists under a glacier or ice cap.

    sulfur Noun

    chemical element with the symbol S.

    sunfish Noun

    small freshwater fish.

    surface area Noun

    amount of exposed land, water, or other material.

    survival Noun

    ability to live.

    swamp Noun

    land permanently saturated with water and sometimes covered with it.

    Encyclopedic Entry: swamp
    swan Noun

    large aquatic bird.

    tectonic plate Noun

    massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    Encyclopedic Entry: temperature
    thermal stratification Noun

    thermal layering of a lake, including the top layer (epilimnion), middle layer (metalimnion), and lowest layer (hypolimnion).

    thermocline Noun

    level or layer of a fluid depth where temperature changes more rapidly than the fluid either above or below it.

    toxic Adjective


    trade Noun

    buying, selling, or exchanging of goods and services.

    travel Noun

    movement from one place to another.

    trophic system Noun

    way of classifying lakes based on the amount of nutrients the lakes possess.

    turtle Noun

    type of reptile with a shell encasing most of its body.

    Uros Noun

    people and culture native to Lake Titicaca in South America.

    utility Noun

    company or organization that distributes electricity, water, or gas to residents and businesses.

    utility company Noun

    business that organizes and delivers public services such as electricity, water, or natural gas.

    valley Noun

    depression in the Earth between hills.

    vary Verb

    to change.

    visibility Noun

    the ability to see or be seen with the unaided eye. Also called visual range.

    volcanic cone Noun

    hill created by a volcanic eruption.

    Encyclopedic Entry: Types of Volcanic Cones
    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: Plate Tectonics and Volcanic Activity
    wade Verb

    to walk through shallow water.

    wagon Noun

    wheeled vehicle pulled by horses or other beasts of burden.

    walleye Noun

    freshwater fish native to North American rivers.

    water bug Noun

    insect that lives near a body of water.

    water hyacinth Noun

    aquatic plant native to South America.

    water lily Noun

    flowering plant that lives in bodies of fresh water.

    watershed Noun

    entire river system or an area drained by a river and its tributaries.

    Encyclopedic Entry: watershed
    weather Verb

    to change as a result of exposure to wind, rain, or other atmospheric conditions.

    wildlife Noun

    organisms living in a natural environment.

    William Wordsworth Noun

    (1770-1850) English poet.

    wind Noun

    movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.

    worm Noun

    animal with a long, limbless body.

    zebra mussel Noun

    aquatic animal (mollusk) native to Europe.