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Types of Caves for Kids - How Caves are Formed for Children - FreeSchool

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Raised, flat-surfaced areas bounded on one or more sides by cliffs or steep slopes are known as plateaus. They are found on every continent, along continental shelves, and in most oceans. Continental plateaus, along with their enclosed basins, account for about 45 percent of Earth 's land surface.

The largest plateaus include the continent of Africa, the western plateau of Australia, and the Arabian Peninsula. In North America, plateaus occur along the western mountain system, in Alaska, and west of the Appalachian Highlands. South America has plateaus in five regions: Guiana Highlands, Brazilian Highlands, Paraná Basin, Patagonia region, and the Andes. In Europe, plateaus are found in Spain, north of the Alps, in Scandinavia, and in western Russia. There are spectacular plateaus in central Turkey, Iran, Central Asia, and the extensive region covered by the Tibetan Plateau.

Plateaus develop in many ways and occur in a variety of geologic settings. Lava plateaus are built by the accumulation of basalt released by volcanic activity. Typical examples of these formations can be seen in Ethiopia, Somalia, the western United States, the Paraná Basin, Iceland, and the Deccan Plateau of India. Intermontane plateaus-high areas of diverse topography enclosed within mountain systems-can be found in most of the world's major mountain chains. The Tibetan Plateau in the Himalayas is the largest of these landforms, covering 530,000 square miles (1,400,000 square kilometers). Intermontane plateaus are distributed worldwide and include much of Central Asia, part of China's Sichuan Province, Mongolia, Anatolia, Armenia, and Iran. In the United States and Mexico, intermontane plateaus are called basin and range topography and consist of closely spaced, parallel mountain ranges separated by elongated basins. These landforms are found chiefly between the Sierra Nevada and the Rockies in the United States and alongside the Sierra Madre Occidental mountains in Mexico.

Ice plateaus cover most of Greenland and Antarctica. In many places the sheets are more than 4,900 feet (1,500 meters) thick with bases that descend well below sea level. The ice is a relic of the Pleistocene ice age when continental glaciers covered much of the Arctic, sub-Arctic, and some temperate regions. Most oceanic plateaus occur along midocean rises, such as the Albatross Plateau of the eastern equatorial Pacific, or along continental slopes, as exemplified by the Blake Plateau off the southeastern coast of the United States. These plateaus rise at least 660 feet (200 meters) above the surrounding seafloor and are overlain by flat-lying marine sediments.

Continental plateaus are subject to erosion and dissection by wind, streams, and glaciers. In some cases dissection has been so thorough that little remains of the original plateau surface. In arid and semiarid regions, such as the southwestern United States, plateaus may be eroded into mesas and buttes. Plateaus, being elevated, often create local climates, which range from temperate to desertlike conditions.

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Raised, flat-surfaced areas bounded on one or more sides by cliffs or steep slopes are known as plateaus. They are found on every continent, along continental shelves, and in...

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Official name: United States Virgin Islands (USVI)

Type of territory: Organized, unincorporated territory of the United States

Capital: Charlotte Amalie, located on the island of St. Thomas

Population (2021 estimate): 105,870

Location: The USVI is located in the Lesser Antilles in the Caribbean Sea.

Official language: English

Experts think that the Arawak people were the first to settle on the three main islands that are now known as the U.S. Virgin Islands (USVI): St. Thomas, St. Croix, and St. John. They likely arrived on the islands around the year 1000, journeying from South America. (The Taíno people of Puerto Rico are a subgroup of the Arawak.) In the 15th century, a warring tribe called the Caribs invaded the islands.

Christopher Columbus arrived on the island that's now called St. Croix in 1493, and he named the cluster of islands the Virgin Islands in honor of a Roman Catholic saint. For more than 60 years, the Carib people fought off the explorers. But in 1555, the Spanish king sent forces to kill the natives and claim the territory. Over the next hundred years, European settlers arrived, forcing enslaved people from Africa to come, too. In that time the countries of Spain, France, Britain, Denmark, and the Netherlands fought over ownership so they could grow sugar, cotton, indigo, and other crops. In 1754, Denmark took control of the islands and renamed them the Danish West Indian Islands.

At this time, the islands served as a hub for the slave trade across the Americas. But in 1848, enslaved laborer John Gottlieb, known as General Buddhoe, lead a historic revolt on St. Croix. The uprising forced the governor to abolish slavery throughout the islands on July 3, 1848.

In 1917, the United States purchased the islands from the Danish government. The United States wanted to use the islands' location and deep bays for naval warships while the country was fighting in World War I.

The USVI are tropical islands located on the Puerto Rico Trench, the deepest part of the Atlantic Ocean. The U.S. National Park Service manages five national parks in the USVI; four of them are dedicated to the islands' marine life.

Virgin Islands National Park covers most of St. John, and 40 percent of the park is underwater. The park houses over 500 species of fish including barracuda, nurse shark, stingray, yellow snapper, and kingfish, and it also includes Trunk Bay, a popular snorkeling site. Another national park, the Virgin Islands Coral Reef National Monument, is all underwater.

The islands' official bird is the bananaquit, nicknamed the sugar bird because it drinks nectar. The only native land mammals are bats. The great bulldog bat, which is one of six species that live on the islands, catches fish for dinner. Leatherback, hawksbill, and green sea turtles all live here.

PEOPLE AND CULTURE

Around 75 percent of the USVI population are Black people who are the descendants of the enslaved people forced over from Africa. People of European, Asian, and Latin descent make up make up the rest of the population.

Fungi (FOON-jee) is one of the best known dishes in the USVI-but it's not mushrooms! Made with salted cornmeal, water, and okra, fungi is usually served with whole fried fish. Pates (pah-TAYS) is the USVI version of the empanada, fried dough stuffed with beef or salted fish.

The festival of Carnival is celebrated across the Caribbean with steel drum bands, colorful costumes that can take months to prepare, and parades. Each island celebrates Carnival at a different time of the year: St. Thomas celebrates after Easter, St. John around the Fourth of July, and St. Croix during the week between Christmas and New Year's Day.

GOVERNMENT

The USVI is an organized, unincorporated territory of the United States, meaning it follows U.S. laws but also has its own local government. It also means that not all of the U.S. Constitution applies to the people living there. In 1932, the U.S. Congress extended citizenship to all residents born in the USVI, but citizens couldn't elect their own governor until 1970. Now, the USVI has executive, legislative, and judicial branches of government to make laws on the islands. People elect a representative who serves in the U.S. Congress and participates in congressional committees, but can't vote.

* Former NBA star Tim Duncan of the San Antonio Spurs was born and raised on St. Croix.

* Founding Father Alexander Hamilton was born on the island of Nevis but grew up in St. Croix. Today you can take a tour dedicated to Hamilton's life on the island.

* Pirates, including Captain Kidd and Black Sam Bellamy, raided the islands in the 16th and 17th centuries.

* This territory is the only place in the United States where people drive on the left side of the road.

* A small island near St. John was the setting for the novel Treasure Island by Robert Louis Stevenson.

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Official name: United States Virgin Islands (USVI) Type of territory: Organized, unincorporated territory of the United States Capital: Charlotte Amalie, located on the island of St. Thomas Population (2021 estimate): 105,870 Location: The USVI is located in the Lesser Antilles in the Caribbean Sea.

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Mountains are tall, rocky features that rise high above the surrounding land. They are usually found in groups called ranges, like the Rocky Mountains in North America. Mountains are formed by movements of the Earth's crust.

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Deltas form as rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river.

Subjects

Biology, Ecology, Geography, Physical Geography

Deltas are wetlands that form as rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river. Although very uncommon, deltas can also empty into land.

A river moves more slowly as it nears its mouth, or end. This causes sediment, solid material carried downstream by currents, to fall to the river bottom.

The slowing velocity of the river and the build-up of sediment allows the river to break from its single channel as it nears its mouth. Under the right conditions, a river forms a deltaic lobe. A mature deltaic lobe includes a distributary network-a series of smaller, shallower channels, called distributaries, that branch off from the mainstream of the river.

In a deltaic lobe, heavier, coarser material settles first. Smaller, finer sediment is carried farther downstream. The finest material is deposited beyond the river's mouth. This material is called alluvium or silt. Silt is rich in nutrients that help microbes and plants-the producers in the food web-grow.

As silt builds up, new land is formed. This is the delta. A delta extends a river's mouth into the body of water into which it is emptying.

A delta is sometimes divided into two parts: subaqueous and subaerial. The subaqueous part of a delta is underwater. This is the most steeply sloping part of the delta, and contains the finest silt. The newest part of the subaqueous delta, furthest from the mouth of the river, is called the prodelta.

The subaerial part of a delta is above water. The subaerial region most influenced by waves and tides is called the lower delta. The region most influenced by the river's flow is called the upper delta.

This nutrient-rich wetland of the upper and lower delta can be an extension of the river bank, or a series of narrow islands between the river's distributary network.

Like most wetlands, deltas are incredibly diverse and ecologically important ecosystems. Deltas absorb runoff from both floods (from rivers) and storms (from lakes or the ocean). Deltas also filter water as it slowly makes its way through the delta's distributary network. This can reduce the impact of pollution flowing from upstream.

Deltas are also important wetland habitats. Plants such as lilies and hibiscus grow in deltas, as well as herbs such as wort, which are used in traditional medicines.

Many, many animals are indigenous to the shallow, shifting waters of a delta. Fish, crustaceans such as oysters, birds, insects, and even apex predators such as tigers and bears can be part of a delta's ecosystem.

Not all rivers form deltas. For a delta to form, the flow of a river must be slow and steady enough for silt to be deposited and build up. The Ok Tedi, in Papua New Guinea is one of the fastest-flowing rivers in the world. This river becomes a tributary of the Fly River. (The Fly, on the other hand, does form a rich delta as it empties into the Gulf of Papua, part of the Pacific Ocean.)

A river will also not form a delta if exposed to powerful waves. The Columbia River in Canada and the United States, for instance, deposits enormous amounts of sediment into the Pacific Ocean, but strong waves and currents sweep the material away as soon as it is deposited.

Tides also limit where deltas can form. The Amazon, the largest river in the world, is without a delta. The tides of the Atlantic Ocean are too strong to allow silt to create a delta on the Amazon.

There are two major ways of classifying deltas. One considers the influences that create the landform, while the other considers its shape.

There are four main types of deltas classified by the processes that control the build-up of silt: wave-dominated, tide-dominated, Gilbert deltas, and estuarine deltas.

In a wave-dominated delta, the movement of waves controls a delta's size and shape. The Nile delta (shaped by waves from the Mediterranean Sea) and Senegal delta (shaped by waves from the Atlantic Ocean) are both wave-dominated deltas.

Tide-dominated deltas usually form in areas with a large tidal range, or area between high tide and low tide. The massive Ganges-Brahmaputra delta, in India and Bangladesh, is a tide-dominated delta, shaped by the rise and fall of tides in the Bay of Bengal.

Gilbert deltas are formed as rivers deposit large, coarse sediments. Gilbert deltas are usually confined to rivers emptying into freshwater lakes. They are usually steeper than the normal flat plain of a wave-dominated or tide-dominated delta. This type of delta was first identified by the geologist Grove Karl Gilbert, who described mountain streams feeding ancient Lake Bonneville. (Utah's Great Salt Lake is the only remnant of Lake Bonneville.)

Estuarine deltas form as a river does not empty directly into the ocean, but instead forms an estuary. An estuary is a partly enclosed wetland that features a brackish water (part-saltwater, part-freshwater) habitat. The Yellow River forms an estuary, for instance, as it reaches the Bohai Sea off the coast of northern China.

The term delta comes from the upper-case Greek letter delta (Δ), which is shaped like a triangle. Deltas with this triangular or fan shape are called arcuate (arc-like) deltas. The Nile River forms an arcuate delta as it empties into the Mediterranean Sea.

Stronger waves form a cuspate delta, which is more pointed than the arcuate delta, and is tooth-shaped. The Tiber River forms a cuspate delta as it empties into the Tyrrhenian Sea near Rome, Italy.

Not all deltas are triangle-shaped. A bird-foot delta has few, widely spaced distributaries, making it look like a bird's foot. The Mississippi River forms a bird-foot delta as it empties into the Gulf of Mexico.

Another untraditional looking delta is the inverted delta. The distributary network of an inverted delta is inland, while a single stream reaches the ocean or other body of water. The delta of the Sacramento-San Joaquin River in northern California is an inverted delta. The rivers and creeks of the Sacramento and San Joaquin distributary networks meet in Suisun Bay, before flowing to the Pacific Ocean through a single gap in the Coast Range, the Carquinez Strait.

Inland deltas, which empty into a plain, are extremely rare. The Okavango delta in Botswana is probably the most well-known-and so unusual it is recognized as one of the "Seven Natural Wonders of Africa." Water from the Okavango River never reaches another body of water. The delta spreads water and silt across a flat plain in the Kalahari Desert before being evaporated.

An abandoned delta forms as a river develops a new channel, leaving the other to dry up or stagnate. This process is called avulsion. Avulsion occurs when the slope of a channel decreases and the sediment build-up increases. These forces allow the channel to overflow its banks or levees and find a steeper, more direct route to the ocean or other body of water. The process of avulsion in deltaic lobes is called delta lobe switching. Over time, delta switching can create entirely new deltaic lobes. Delta switching has resulted in seven or eight distinct deltaic lobes of the Mississippi River over, at least, the past 5,000 years.

Deltas are incredibly important to the human geography of a region. They are important places for trade and commerce, for instance.

The booming city of Vancouver, British Columbia, Canada, sits on the delta of the Fraser River as it empties into the Strait of Georgia, part of the Pacific Ocean. The Fraser delta helps make Vancouver one of the busiest, most cosmopolitan ports in the world, where goods from the interior of Canada are exported, and goods from around the world are imported.

The Pearl River Delta, sometimes called the Delta of Guangdong, is another heavily urbanized river delta. The Pearl River delta is one of the fastest-growing centers of China's economy. The Pearl River delta includes both of China's two special administrative regions, the former British colony of Hong Kong and the former Portuguese colony of Macau. Hong Kong and Macau are welcoming to western business, and provide an entryway to the Chinese market. The Pearl River delta region is growing so quickly, it frequently experiences labor shortages as immigrants from the Chinese interior settle in the area, seeking a better life and higher wages.

Deltas have a rich accumulation of silt, so they are usually fertile agricultural areas. The world's largest delta is the Ganges-Brahmaputra delta in India and Bangladesh, which empties into the Bay of Bengal. Bangladesh sits almost entirely on this delta. Fish, other seafood, and crops such as rice and tea are the leading agricultural products of the delta.

Similarly, the inverted delta of the Sacramento and San Joaquin Rivers in northern California is one of the most agriculturally rich areas in the U.S. The soil supports crops from asparagus to zucchini, wine grapes to rice.

Extensive river management threatens deltas. River management involves monitoring and administering a river's flow (often through the use of dams). River management increases the amount of land available for agricultural or industrial development, and controls access to water for drinking, industry, and irrigation.

Engineers and government officials must consistently debate the interests of agriculture, industry, the environment, and citizen safety and health when putting delta wetlands at risk.

River management in Egypt has radically altered the way land is farmed around the Nile delta, for instance. Construction of the Aswan Dam in the 1960s reduced annual flooding of the delta. This flooding had distributed silt and nutrients along the banks of the Nile. Today, Egypt is much more reliant on fertilizers and irrigation. The Nile delta is also shrinking as a result of the Aswan Dam and other river management techniques. Without silt and other sediments to fortify it in a prodelta, the waves of the Mediterranean Sea are eroding the delta faster than the Nile can replace it.

In the United States, dams on the Colorado River nearly prevent it from reaching its delta on the Sea of Cortez, Mexico. The ecosystem (what was once the world's largest desert estuary) has been reduced to a fraction of its former area, and many indigenous species are vulnerable, threatened, or endangered.

Finally, decades of river management prevent the Mississippi River from naturally flowing through its delta wetlands. Like the Nile delta, the Mississippi delta is also eroding. According to Drawing Louisiana's New Map 62 square kilometers (24 square miles) of wetland was lost each year between 1990 and 2000-that's about one football field of mud washed into the Gulf of Mexico every 38 minutes. This situation contributed to the devastation caused by Hurricane Katrina in 2005.

Fast Fact

Delta Blues
Delta blues is a style of music developed by African American artists living and performing in the Mississippi Delta region of the southern United States. The Mississippi Delta is actually a floodplain between two rivers in northwestern Mississippi, the Mississippi and the Yazoo, and it is sometimes referred to as the Yazoo-Mississippi Delta.

Slide guitar is one of the standard instruments used by Delta blues musicians, while familiar topics include poverty and injustice. Robert Johnson, widely recognized as one of the greatest guitarists of all time, played the Delta blues. Listen to Robert Johnson here.

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Editor

Jeannie Evers, Emdash Editing, Emdash Editing

Producer

National Geographic Society

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Last Updated

October 19, 2023

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Deltas form as rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river.

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Rivers are large, flowing bodies of water that move across the land. Rivers form valleys and change the shape of the land. The Mississippi River is one of the longest in the USA.

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Landforms are natural features on the surface of the Earth. There are many kinds, such as mountains, valleys, plains, and hills. Let's learn about them!

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Landforms for Kids | Learn about the 4 Types of Landforms

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Try this interactive game to recognize and name common landforms.

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Deserts are dry places with little rain. The Sahara in Africa and the Mojave in the USA are both deserts. Deserts form in places with very low rainfall.

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Watch this video to see how volcanoes, earthquakes, wind, and water help create and shape landforms.

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Types of Landforms  | Landforms | Video for Kids

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Try this simple science experiment at home or in class to see how water can change the shape of land.

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How Volcanoes Form

In 1980 in Washington, after 123 years of hibernation, Mount St. Helens erupted. The blast destroyed and scorched 230 square miles (370 square kilometers) of forest within minutes. The eruption released an avalanche of hot ash, gas, steam, and rocks that mowed down giant trees up to 15 miles (24 kilometers) away.

When magma finds a way to escape from beneath the earth's surface, it creates a volcano.

Volcanoes erupt in different ways. Some, like Mount St. Helens, explode. Explosive eruptions are so powerful, they can shoot particles 20 miles up (32 kilometers), hurl 8-ton boulders more than a half mile (0.8 kilometers) away, and cause massive landslides. Explosive eruptions also create an avalanche of hot volcanic debris, ash, and gas that bulldozes everything in its path. Explosive volcanoes cause most of the volcano-related fatalities.

Volcanoes, like Mauna Loa in Hawaii, are effusive. Rather than a violent explosion, lava pours or flows out. Fatalities from effusive volcanoes are rare because people can usually outrun the lava. However, some people get too close or become trapped with no escape. The flowing lava burns, melts, and destroys everything it touches including farms, houses, and roads.

A volcanic eruption forever changes the landscape. Though volcanoes destroy, they also create mountains, islands, and, eventually, incredibly fertile land.

Carpet of Ash

Volcanic eruptions can cause damage hundreds of miles away. Volcanic ash causes airplane engines to fail, destroys crops, contaminates water, and damages electronics and machinery. The ash carpets the ground, burying everything, sometimes even causing buildings to collapse. Mount St. Helens produced more than 490 tons of ash that fell over a 22,000 square mile (56,980 square kilometer) area and caused problems in cities 370 miles (600 kilometers) away.

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When magma finds a way to escape from beneath the earth's surface, it creates a volcano.

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Valleys are low areas between hills or mountains, often with a river running through them. They are shaped by water or moving ice.

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This animated science movie introduces landforms such as plains, prairies, hills, mountains, plateaus, canyons, valleys, coasts, islands, and peninsulas.

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The Five Oceans | Jack Hartmann | Oceans of the World

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Epic reaches 75 million kids in class and after school.

Built on a collection of 40,000+ popular, high-quality books from 250+ of the world's best publishers, Epic safely fuels curiosity and reading confidence for kids 12 and under.

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Caves | Weird But True! | S1 E14 | Full Episode | @natgeokids

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A waterfall is a river or other body of water's steep fall over a rocky ledge into a plunge pool below. Waterfalls are also called cascades.

The process of erosion, the wearing away of earth, plays an important part in the formation of waterfalls. Waterfalls themselves also contribute to erosion.

Often, waterfalls form as streams flow from soft rock to hard rock. This happens both laterally (as a stream flows across the earth) and vertically (as the stream drops in a waterfall). In both cases, the soft rock erodes, leaving a hard ledge over which the stream falls.

A fall line is the imaginary line along which parallel rivers plunge as they flow from uplands to lowlands. Many waterfalls in an area help geologists and hydrologists determine a region's fall line and underlying rock structure.

As a stream flows, it carries sediment. The sediment can be microscopic silt, pebbles, or even boulders. Sediment can erode stream beds made of soft rock, such as sandstone or limestone. Eventually, the stream's channel cuts so deep into the stream bed that only a harder rock, such as granite, remains. Waterfalls develop as these granite formations form cliffs and ledges.

A stream's velocity increases as it nears a waterfall, increasing the amount of erosion taking place. The movement of water at the top of a waterfall can erode rocks to be very flat and smooth. Rushing water and sediment topple over the waterfall, eroding the plunge pool at the base. The crashing flow of the water may also create powerful whirlpools that erode the rock of the plunge pool beneath them.

The resulting erosion at the base of a waterfall can be very dramatic, and cause the waterfall to "recede." The area behind the waterfall is worn away, creating a hollow, cave-like structure called a "rock shelter." Eventually, the rocky ledge (called the outcropping) may tumble down, sending boulders into the stream bed and plunge pool below. This causes the waterfall to "recede" many meters upstream. The waterfall erosion process starts again, breaking down the boulders of the former outcropping.

Erosion is just one process that can form waterfalls. A waterfall may form across a fault, or crack in the Earth's surface. An earthquake, landslide, glacier, or volcano may also disrupt stream beds and help create waterfalls.

Classifying Waterfalls

There is not a standard way to classify waterfalls. Some scientists classify waterfalls based on the average volume of water in the waterfall. A Class 10 waterfall using this scale is Inga Falls, Democratic Republic of Congo, where the Congo River twists in a series of rapids. The estimated volume of water discharged from Inga Falls is 25,768 cubic meters per second (910,000 cubic feet per second).

Another popular way of classifying waterfalls is by width. One of the widest waterfalls is Khone Phapheng Falls, Laos. At the Khone Phapheng Falls, the Mekong River flows through a succession of relatively shallow rapids. The width of the Khone Phapheng Falls is about 10,783 meters (35,376 feet).

Waterfalls are also classified by height. Angel Falls, the world's tallest waterfall, plummets 979 meters (3,212 feet) into a remote canyon in a rain forest in Venezuela. The water, from the Gauja River, often does not reach the bottom. The fall is so long, and so steep, that air pressure is stronger often than the water pressure of the falls. The water is turned to mist before it reaches the small tributary below.
Types of Waterfalls
One of the most popular, if least scientific, ways to classify waterfalls is by type. A waterfall's type is simply the way the descends. Most waterfalls fit more than one category.

A block waterfall descends from a wide stream. Niagara Falls, in the United States and Canada, is a block waterfall on the Niagara River.

A cascade is a waterfall that descends over a series of rock steps. Monkey Falls, in the Indira Gandhi Wildlife Sanctuary and National Park in Tamil Nadu, India, is a gently sloping cascade. The waterfall is safe enough for children to play in the water.

A cataract is a powerful, even dangerous, waterfall. Among the widest and wildest of cataracts are the thundering waters of the Iguazu River on the border between Brazil and Argentina.

A chute is a waterfall in which the stream passage is very narrow, forcing water through at unusually high pressure. Three Chute Falls is named for the three "chutes" through which the Tenaya Creek falls in Yosemite National Park, California, U.S.

Fan waterfalls are named for their shape. Water spreads out horizontally as it descends. Virgin Falls is a striking fan waterfall on Tofino Creek, on Vancouver Island, British Columbia, Canada.

Frozen waterfall s are just what they sound like. For at least part of the year, the waterfall freezes. Mountaineers often climb frozen waterfalls as a challenging test of their skill. The Fang is a single pillar of ice in Vail, Colorado, U.S., that vertically plunges more than 30 meters (100 feet).

Horsetail waterfalls maintain contact with the hard rock that underlies them. Reichenbach Falls, a fall on the Reichenbach Stream in Switzerland, is a horsetail waterfall where legendary detective Sherlock Holmes allegedly fell to his doom.

Multi-step waterfalls are a series of connected waterfalls, each with their own plunge pool. The breathtaking "falling lakes" of Plitvice Lakes National Park, Croatia, are a series of multi-step waterfalls.

Plunge waterfalls, unlike horsetail falls, lose contact with the hard rock. The tallest waterfall in Japan, Hannoki Falls, is a plunge waterfall that stands 497 meters (1,640 feet). Hannoki Falls is seasonally fed by snowmelt from the Tateyama Mountains.

Punchbowl waterfalls are characterized by wide pools at their base. Wailua Falls is a punchbowl waterfall on the island of Kauai, Hawai'i, U.S. Although the plunge pool is tranquil and popular for swimming, the area around Wailua Falls itself is dangerous.

The water flowing over segmented waterfalls separate as distinct streams. Huge outcroppings of hard rock separate the streams of Nigretta Falls, a segmented waterfall in Victoria, Australia, before they meet in a large plunge pool.

Case Study: Niagara Falls

The Niagara River has two falls, one in the U.S. state of New York and one in the province of Ontario, Canada. Each waterfall is less than 60 meters (200 feet) tall, but together they are more than a kilometer (0.62 miles) wide.

Niagara and many other falls with large volumes of water are used to generate hydroelectric power. A tremendous volume of water flows over Niagara Falls, as much as 5,525 cubic meters (195,000 cubic feet) per second. Power stations upstream from the falls convert hydroelectric energy into electricity for residential and commercial use.

The U.S. and Canadian governments manage the Niagara River so carefully that it is possible for either country to "turn off" the falls. This is done at night, so as not to disturb the tourism industry, and the falls are never actually turned off, just slowed down. Water is diverted to canals and reservoirs, and the decreased flow allows engineers to check for erosion and other damage on the falls. U.S. and Canadian authorities also work together to ensure Niagara Falls doesn't freeze in the winter, which would threaten power production.

Because waterfalls are barriers to navigation, canals are sometimes built to get around them. Niagara Falls prevents passage between Lake Erie and Lake Ontario on the Niagara River. In the 19th century, the Welland Canal was built to make passage between the two Great Lakes possible.

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A waterfall is a steep descent of a river or other body of water over a rocky ledge.

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When a stream or river flows over a precipice and plunges downward, it forms a waterfall. A typical waterfall is the kind in which a stream or river leaps over the edge of the precipice and falls freely through the air until it hits the lower level of the streambed. The term for a giant waterfall is cataract. With its great power and beauty, the cataract ranks among the most awesome of the natural wonders on Earth.

A cascade is usually a smaller kind of waterfall. It occurs when the streambed does not drop vertically, but instead slopes gradually to a lower level, and the stream makes short leaps from one rock on the slope to the next rock. If a riverbed slopes even less steeply, rapids may occur.

Waterfalls usually result from a difference in the resistance of rocks in the streambed. The stream erodes the softer portions, leaving a precipice of hard rock over which the river plunges. Sometimes other forces, such as glaciers or earthquakes, create a precipice in the path of a stream. Because of these differences in their formation, waterfalls vary greatly in appearance, from high and narrow to low and wide.

The world's highest known waterfall is Angel Falls in Venezuela. It is a slender stream that plunges 3,212 feet (979 meters) down the face of a rugged cliff. Surrounded by an almost impenetrable jungle, this silver ribbon of falling water was for many years seen only from airplanes.

Khone Falls, a series of cataracts on the Mekong River bordering Laos and Cambodia, has the greatest volume of the world's waterfalls. About 2,500,000 gallons (9,500,000 liters) of water per second flow over their rims, nearly double the volume of Niagara Falls. The double series of cataracts is caused by a resistant bed of basalt over which the river tumbles 45 feet (14 meters) to a pool 269 feet (82 meters) above sea level.

Niagara Falls, the famous cataract that spans the United States-Canada border, owes its reputation to great width, great volume, and beautiful clear waters. Its two branches present a spectacle of falling waters nearly 1 mile wide (1.6 kilometers). Four of the Great Lakes feed the avalanche that thunders over the precipice in never-ending floods of green, blue, and ivory-colored water.

name

location

volume in cubic feet (cubic meters) per second*

height in feet (meters)

*Mean annual flow.

**Total height represents more than one leap.

Niagara represents the commonest geological type of waterfall, the cap rock fall. It occurs where a hard formation of rock lies over softer layers. At Niagara, the escarpment, some 160 feet high, has a top layer of resistant limestone that is about 60 feet thick. Even the immense flow from the Great Lakes does not erode this top layer as it passes over the escarpment to empty into the lower-lying Ontario basin. A sharp-edged cliff remains-creating the magnificent cataract.

Under the hard top layer, however, soft layers of shale are eroded by backlash from the plunging waters. This cuts away support of the top layer, and from time to time a large block of it falls off. In this way the waterfall moves upstream, leaving behind a deep gorge. The Niagara gorge, for example, shows the falls have moved 7 miles upstream from the original location.

The geological formation of the Victoria Falls of the Zambezi River makes it one of the world's most unusual cataracts. Situated between Zambia and Zimbabwe, it spreads over a mile in width.

Unlike most rivers approaching a cataract, the Zambezi has no rapids to warn the river traveler of the approaching precipice. A shallow, broad river, the Zambezi continues to move slowly up to the brink of the falls. Then it spills abruptly over the 355-foot precipice with a thunderous roar.

The river seems to be dropping into a deep slot in the Earth, for a high gorge wall stands directly opposite the falls, crowding so close that the lower parts of the falls are hidden. The gorge wall, which has only one small exit channel, acts as a dam and halts the forward-rushing waters, dashing them back and up. This creates Victoria's uniquely heavy spray. Great columns of mist rise in the air, shot with rainbows. In floodtime these columns can be seen 70 miles away.

After water bursts from the gorge exit, it boils in whirlpools and rapids through a strangely zigzagging canyon. This rocky, Z-shaped channel and the precipice of the falls developed from a fault, or deep crack in the Earth's surface.

Flowing over a plateau of hard basalt, the Zambezi did not erode a deep bed. But when the river began to flow over the fault, its current gradually swept away loose rock fragments that had filled the zigzagging crack. The rift was carved deeper and deeper. Today the gorge walls rise 400 feet above the level of the river.

Other notable African waterfalls are Tugela Falls, Howick Falls, and Augrabies Falls, all in South Africa. Tugela Falls, on the Tugela River, is the world's second-highest waterfall, dropping 3,110 feet (948 meters) in five stages.

A third kind of waterfall is represented by the Upper and Lower falls in Yellowstone National Park. A popular tourist attraction, the falls are located in red-orange canyons that set off the white foam of the Yellowstone River's tumbling waters. Lower Falls, the more spectacular of the two, is 308 feet high.

The creation of these falls began when the Yellowstone River eroded its bed down to lava dikes. These are hard rocks that extend upward toward the Earth's surface. At first the river wore down at an equal rate the softer rocks, both upstream and downstream, of a dike. As the dike began to protrude across the river like a dam, it slowed the current upstream and erosion was checked there. Downstream the water continued to spill over the dike with great force, eroding the riverbed more and more. Thus, the falls grew higher.

Because the dike extends deep into the Earth and is not underlaid with soft layers, this kind of waterfall is not undercut as is the caprock fall. It recedes upstream very slightly if at all.

Still other waterfalls develop through glacial deepening of a main stream valley. The seven waterfalls in a Yosemite National Park valley in California are examples. Among them is Ribbon Fall, one of the highest in North America. Located on the west slope of the Sierra Nevada Range, where it is fed by melting ice and snow, this fall has a drop of 1,612 feet (491 meters). The Yosemite falls flow from streams that are hanging tributaries. The valley of the river into which the streams empty was once deepened by a glacier. When the glacier retreated, the untouched tributary valleys were left high above the main valley. Now the tributaries leap down to the main river.

Among South America's many spectacular cataracts is the Kaieteur Falls of Guyana, a 823-foot (251-meter) cap rock fall of great beauty. Another is the Iguazú (Iguaçu) cataract, which is more than 2 miles (3 kilometers) wide, on the Argentina-Brazil border. Islands on the precipice divide the descending water into countless cascades.

In Europe there are no falls of great volume, but extremely high falls plunge from precipices of the Alps and Pyrenees. In Norway many lovely falls spill in lacy columns over the steep walls of fjords.

Churchill (formerly Grand) Falls, a little-known cataract of Labrador, forms a magnificent spectacle in a rugged Canadian wilderness. Australia has a number of high falls, among them the Wollomombi and Stony Creek (Wallaman) falls.

The strong currents near falls are often harnessed to generate electricity ( see waterpower). Today much of the water of Niagara Falls is diverted for the generation of hydroelectric power. On occasion so much water is diverted for this purpose that the waterfall itself weakens greatly.

Waterfalls are sometimes a disadvantage since they form a barrier to navigation. In the past, towns grew up by waterfalls because they marked the beginning of portages, where goods on boats were transferred to packhorses or other land transportation.

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When a stream or river flows over a precipice and plunges downward, it forms a waterfall. A typical waterfall is the kind in which a stream or river leaps over the edge of...

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You've probably at least seen a picture of a beautiful, majestic waterfall before, but how much do you know about these natural wonders?

Did you know there are many different types of waterfalls? Or that daredevils have gone over waterfalls inside of barrels throughout history? Keep reading to learn more!

What is a waterfall?

A waterfall is part of a river or other body of water that takes a steep drop over a rocky ledge or cliff. The water lands in a plunge pool, which is a deep pool of water found beneath waterfalls.

How are waterfalls formed?

Most waterfalls are formed from a process called erosion, when natural forces like water or wind wear away the earth over time.

As a stream flows, it carries sediment along with it. Sediment is usually made up of sand, dirt, pebbles, and pieces of larger rocks. The sediment rubs along the stream bed, which is made of both soft rock and hard rock, and wears it away over time.

The soft rock, like limestone or sandstone, erodes much more quickly, leaving only hard rock like granite. The granite that remains when the soft rock is worn away creates cliffs and ledges. Now you have a waterfall!

Waterfalls don't only form from erosion. They can also form because of earthquakes, landslides, volcanoes, or glaciers. Sometimes waterfalls form across cracks in the Earth's surface called faults.

How are waterfalls classified?

Scientists classify waterfalls in a few different ways. Some classify waterfalls based on volume. Others use width to classify waterfalls, and some use height.

One of the most popular ways to classify waterfalls is by type, which is based on the way the water flows or descends. Here are several types of waterfalls:

-A ledge waterfall is the classic waterfall, the curtain of water you picture in your mind when you hear the word "waterfall." It only partially touches the bedrock as it falls.

- Fan waterfalls create a fan shape because the water spreads out horizontally as it descends.

-A is a waterfall that descends over rocks that look like steps. These waterfalls aren't as steep or dangerous as most others, and they are usually safe to play in.

-On the other hand, a cataract is a big, powerful, and dangerous waterfall that is definitely not a good location for playtime.

- Punchbowl waterfalls are named for the extremely wide plunge pools at the bottom.

- Multi-step waterfalls are connected waterfalls that each have their own plunge pool.

-A is a very narrow waterfall that forces water through at extra high pressures.

-A block waterfall flows from a very wide stream.

-A is a waterfall that flows over a low angle slope.

- Plunge waterfalls are named because they plunge quickly over the ledge, not even touching the hard rock.

Did you know there were so many different types of waterfalls?

Waterfall Facts

-The roar from Victoria Falls in Zimbabwe is so loud that it can be heard from 25 miles (40 kilometers) away!

-Angel Falls, located in Venezuela, is the world's longest waterfall at 3,212 feet (979 meters). It is so long that the water usually turns to mist before it reaches the bottom.

-Some waterfalls freeze for at least part of the year. Mountaineers (people who climb mountains) actually climb these frozen waterfalls to practice and test their skills. The Fang in Vail, Colorado, is a frozen waterfall that descends more than 100 feet (30 meters). That would be hard to climb!

-Waterfalls can be used to create hydroelectric power to provide electricity to homes and businesses.

-No two waterfalls are exactly the same, just like snowflakes and fingerprints!

-People have gone over Niagara Falls in barrels throughout history. There is even a Daredevil Exhibit documenting these crazy stunts with a collection of the actual barrels that were used.

-On October 24, 1901, a woman named Annie Taylor was the first to successfully go over Niagara Falls in a barrel.

Waterfalls look pretty, but they are also fascinating forces of nature, sources of hydroelectric power, and sites of wild daredevilry!

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You've probably at least seen a picture of a beautiful, majestic waterfall before, but how much do you know about these natural wonders? Did you know there are many different types of waterfalls? Or that daredevils have gone over waterfalls inside of barrels throughout history? Keep reading to learn more!

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A volcano is an opening in Earth's crust. When a volcano erupts, hot gases and melted rock from deep within Earth find their way up to the surface. This material may flow...

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Volcano facts | National Geographic Kids

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Check out these fascinating volcano facts with National Geographic Kids! What is a volcano? Why does it erupt? And where can you find the world's biggest?

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Take a bottle of soda. Shake it really hard. Gas bubbles will form inside, causing a buildup of pressure. Twist the cap off. Pressure will release from the bottle. Soda will shoot everywhere.

A volcanic eruption works much the same way. Pressure builds up inside a volcano as gas bubbles form in magma. Magma is hot liquid rock beneath the surface of the Earth. Gas bubbles in the volcano need to escape. So the volcano erupts. Boom!

A volcano is a mountain or hill with an opening. When a volcano erupts, magma is pushed up through the opening with great force. When magma reaches the Earth's surface, it is called lava. Lava can be as hot as 2,200°F (1,204°C).

Some volcanic eruptions are explosive. Lava shoots into the sky. Gas and hot pieces of rock and ash are also released. Other eruptions are effusive × effusive LEOPATRIZI/GETTY IMAGES overflowing (adjective) My effusive friend is enthusiastic about everything she does. . Lava oozes slowly from the volcano.

The intensity of an eruption depends on a few factors. One is the magma's temperature. Another is the magma's thickness. The amount of gas trapped within the magma is also an important factor.

GETTY IMAGES

The Ring of Fire

About 75% of the Earth's volcanoes are located in a region called the Ring of Fire. This 25,000-mile "ring' is located around the Pacific Ocean. It runs from the southern tip of South America and up the west coast of North America. It continues across the Bering Strait and then south through Japan to New Zealand.

The ring sits along the outline of several tectonic plates. Tectonic plates are the large puzzle pieces that make up the Earth's crust. The crust is Earth's outer layer. Most volcanoes form at the edges of tectonic plates. These huge plates are constantly shifting. Sometimes, the plates pull apart or slide into each other. When this happens, magma rises to fill in the space. Strong pressure and intense heat force the magma upward. It squeezes upward like toothpaste through a tube.

Heating Up

Volcanoes can be dangerous-especially to the millions of people around the world who live near one. When a volcano erupts, lava, hot ash, steam, and rocks are released. In addition, volcanic eruptions can cause other natural disasters. These include landslides, mudslides, earthquakes, and tsunamis.

The most destructive volcanic event in U.S. history occurred in 1980. That is when Mount Saint Helens, in Washington State, erupted. The eruption destroyed entire forests. It killed 57 people. Mount Saint Helens is located within the Ring of Fire. It is a composite volcano, or stratovolcano. This is the most common type of volcano. It is tall and cone-shaped. A composite volcano is made up of layers of rock and ash. Its eruptions are often violent.

Another historically important volcanic event occurred in 79 A.D. in Italy. That is when Mount Vesuvius erupted. It buried the city of Pompeii in lava and ash. The city and its people were lost. Centuries later, scientists discovered their remains.

Volcanoes can be destructive. But they can also create new land. The Hawaiian Islands were created by the eruptions of shield volcanoes. These large, low-lying volcanoes are made up almost entirely of layers of hardened lava.

Another type of volcano is the cinder cone. These shoot lava into the air like a sputtering fountain. A cinder cone eruption often comes from a single volcanic vent in the Earth's crust. Lava cools quickly in the air. It hardens into glassy rocks around the volcano's opening. One example of a cinder cone volcano is Sunset Crater, in Arizona.

ARCTIC-IMAGES/GETTY IMAGES

A Powerful Force

Volcanoes have played an important role in shaping our Earth. Ash from volcanoes adds helpful minerals to soil. This helps plants grow strong. Healthy plants produce oxygen for humans and animals to breathe. Volcanoes also remove heat from the Earth's interior. This helps keep our planet cool.

In recent years, a new benefit of volcanoes has surfaced. Scientists are turning to volcanoes to help power cities. Researchers are exploring geothermal energy. It involves tapping into the heat beneath the Earth's surface to generate electricity. In Iceland, 25% of the nation's energy is geothermal.

Volcanoes may be unpredictable. But as we learn more about how they work, they will become even more important to life on Earth.

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Take a bottle of soda. Shake it really hard. Gas bubbles will form inside, causing a buildup of pressure. Twist the cap off. Pressure will release from the bottle. Soda will shoot everywhere. A volcanic eruption works much the same...

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Like other landforms, river valleys are always changing. Rivers continually wear away the land through which they flow, gradually making the valleys deeper. At the same time, other forces loosen material on the valley walls. These forces include rain, frost, and wind. The eroded material from the valley walls falls into the river and is carried away.

Valleys are usually narrow when they are first formed. Over time, however, most valleys become wider as the walls are worn away. One valley might grow wider than another because the river flows through a softer type of rock that erodes more easily. The force of the river's flow also affects how a valley changes over the years.

Canyons are very narrow, deep river valleys cut through hard rock. They have steep sides that run almost straight up and down. Canyons can be several thousand feet deep. Smaller valleys of this kind are called gorges. Some canyons are remarkable landforms. The Grand Canyon, in the U.S. state of Arizona, is 18 miles (0.2 to 29 kilometers) wide and more than a mile (1.6 kilometers) deep in some places. It was carved by the Colorado River.

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A valley is a long depression, or ditch, in Earth's surface. It usually lies between ranges of hills or mountains. Most valleys are formed by rivers that erode, or wear down,...

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Mountains and hills are made when the layers of rock and soil (called "strata") get folded up. There are always valleys in hills and mountains, in between the highest parts which are the "peaks". If the mountains are very high, then people who want to travel to the other side of the mountains must go by the valleys.

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Instantly access All About the Grand Canyon plus over 40,000 of the best books & videos for kids.

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Valley facts. A valley is a type of landform. A valley is a lower part in the land that sits between two higher parts which might be hills or mountains. Valleys often start as a downward fold between two upward folds in the surface of the Earth, and sometimes as a rift valley. A valley is made deeper by a stream of water or a river as it flows from the high land to the lower land, and into a lake or sea. Some valleys are made by glaciers which are slow-moving rivers of ice. Water or ice make a valley deeper or wider by erosion. Wind can also make valleys larger by erosion.

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In geology and physical geography, a plateau ( or plural plateaus or plateaux), also called a high plain or a , is an area of a highland consisting of flat terrain, that is raised sharply above the surrounding area on at least one side. Often one or more sides have deep hills. Plateaus can be formed by a number of processes, including upwelling of volcanic magma, extrusion of lava, and erosion by water and glaciers. Plateaus are classified according to their surrounding environment as intermontane, piedmont, or continental. A few plateaus may have a small flat top while others have wide ones.

Plateaus can be formed by a number of processes, including upwelling of volcanic magma, extrusion of lava, and erosion by water and glaciers.

Volcanic plateaus are produced by volcanic activity. The Columbia Plateau in the north-western United States is an example. They may be formed by upwelling of volcanic magma or extrusion of lava.

The underlining mechanism in forming plateaus from upwelling starts when magma rises from the mantle, causing the ground to swell upward. In this way, large, flat areas of rock are uplifted to form a plateau. For plateaus formed by extrusion, the rock is built up from lava spreading outward from cracks and weak areas in the crust.

Plateaus can also be formed by the erosional processes of glaciers on mountain ranges, leaving them sitting between the mountain ranges. Water can also erode mountains and other landforms down into plateaus. Dissected plateaus are highly eroded plateaus cut by rivers and broken by deep narrow valleys. Computer modeling studies suggest that high plateaus may also be partially a result from the feedback between tectonic deformation and dry climatic conditions created at the lee side of growing orogens.

Classification

Plateaus are classified according to their surrounding environment.

Intermontane plateaus are the highest in the world, bordered by mountains. The Tibetan Plateau is one such plateau.

Lava or volcanic plateaus are the plateau that occur in areas of widespread volcanic eruptions. The magma that comes out through narrow cracks or fissures in the crust spread over large area and solidifies. These layers of lava sheets form lava or volcanic plateaus. The Antrim plateau in Northern Ireland, The Deccan Plateau in India and the Columbia Plateau in the United States are examples of lava plateaus.

Piedmont plateaus are bordered on one side by mountains and on the other by a plain or a sea. The Piedmont Plateau of the Eastern United States between the Appalachian Mountains and the Atlantic Coastal Plain is an example.

Continental plateaus are bordered on all sides by plains or oceans, forming away from the mountains. An example of a continental plateau is the Antarctic Plateau in East Antarctica.

Large plateaus

The largest and highest plateau in the world is the Tibetan Plateau, sometimes metaphorically described as the "Roof of the World", which is still being formed by the collisions of the Indo-Australian and Eurasian tectonic plates. The Tibetan plateau covers approximately 2,500,000 km ² (970,000 sq mi), at about 5,000 m (16,000 ft) above sea level. The plateau is sufficiently high to reverse the Hadley cell convection cycles and to drive the monsoons of India towards the south.

Other major plateaus in Asia are: Najd in the Arabian Peninsula elevation 762 to 1,525 m (2,500 to 5,003 ft), Armenian Highlands (≈400,000 km ² (150,000 sq mi), elevation 900-2,100 metres (3,000-6,900 ft)), Iranian plateau (≈3,700,000 km ² (1,400,000 sq mi), elevation 300-1,500 metres (980-4,920 ft)), Anatolian Plateau, Mongolian Plateau (≈2,600,000 km ² (1,000,000 sq mi), elevation 1,000-1,500 metres (3,300-4,900 ft)), and the Deccan Plateau (≈1,900,000 km ² (730,000 sq mi), elevation 300-600 metres (980-1,970 ft)).

Another very large plateau is the icy Antarctic Plateau, which is sometimes referred to as the Polar Plateau or King Haakon VII Plateau, home to the geographic South Pole and the Amundsen-Scott South Pole Station, which covers most of East Antarctica where there are no known mountains but rather 3,000 m (9,800 ft) high of superficial ice and which spreads very slowly toward the surrounding coastline through enormous glaciers. The polar ice cap is so massive that the echolocation measurements of ice thickness have shown that large areas are below sea level. But, as the ice melts, the land beneath will rebound through isostasy and ultimately rise above sea level.

A large plateau in North America is the Colorado Plateau, which covers about 337,000 km ² (130,000 sq mi) in Colorado, Utah, Arizona and New Mexico.

In northern Arizona and southern Utah the Colorado Plateau is bisected by the Colorado River and the Grand Canyon. How this came to be is that over 10 million years ago, a river was already there, though not necessarily on exactly the same course. Then, subterranean geological forces caused the land in that part of North America to gradually rise by about a centimeter per year for millions of years. An unusual balance occurred: the river that would become the Colorado River was able to erode into the crust of the Earth at a nearly equal rate to the uplift of the plateau. Now, millions of years later, the North Rim of the Grand Canyon is at an elevation of about 2,450 m (8,040 ft) above sea level, and the South Rim of the Grand Canyon is about 2,150 m (7,050 ft) above sea level. At its deepest, the Colorado River is about 1,830 m (6,000 ft) below the level of the North Rim.

Another high altitude plateau in North America is the Mexican Plateau. With an area of 601,882 km ² (232,388 sq mi) and average height of 1,825 m, it is the home of more than 70 million people.

A tepui, or tepuy, is a table-top mountain or mesa found in the Guiana Highlands of South America, especially in Venezuela and western Guyana. The word tepui means "house of the gods" in the native tongue of the Pemon, the indigenous people who inhabit the Gran Sabana.

Tepuis can be considered minute plateaus and tend to be found as isolated entities rather than in connected ranges, which makes them the host of a unique array of endemic plant and animal species. Some of the most outstanding tepuis are Neblina, Autana, Auyan and Mount Roraima. They are typically composed of sheer blocks of Precambrian quartz arenite sandstone that rise abruptly from the jungle, giving rise to spectacular natural scenery. Auyantepui is the source of Angel Falls, the world's tallest waterfall.

The Colombian capital city of Bogota sits on an Andean plateau known as the Altiplano Cundiboyacense roughly the size of Switzerland. Averaging a height of 2,600 m (8,500 ft) above sea level, this northern Andean plateau is situated in the country's eastern range and is divided into three main flat regions: the Bogotá savanna, the valleys of Ubaté and Chiquinquirá, and the valleys of Duitama and Sogamoso.

The parallel Sierra of Andes delimit one of the world highest plateaux: the Altiplano, (Spanish for "high plain"), Andean Plateau or Bolivian Plateau. It lies in west-central South America, where the Andes are at their widest, is the most extensive area of high plateau on Earth outside of Tibet. The bulk of the Altiplano lies within Bolivian and Peruvian territory while its southern parts lie in Chile. The Altiplano plateau hosts several cities like Puno, Oruro, El Alto and La Paz the administrative seat of Bolivia. Northeastern Altiplano is more humid than the Southwestern, the latter of which hosts several salares, or salt flats, due to its aridity. At the Bolivia-Peru border lies Lake Titicaca, the largest lake in South America.

The highest African plateau is the Ethiopian Highlands which cover the central part of Ethiopia. It forms the largest continuous area of its altitude in the continent, with little of its surface falling below 1500 m (4,921 ft), while the summits reach heights of up to 4550 m (14,928 ft). It is sometimes called the Roof of Africa due to its height and large area.

Another example is the Highveld which is the portion of the South African inland plateau which has an altitude above approximately 1500 m, but below 2100 m, thus excluding the Lesotho mountain regions. It is home to some of largest South-African urban agglomerations.

In Egypt are the Giza Plateau and Galala Mountain, which was once called Gallayat Plateaus, raising 3,300 above sea level.

The Western Plateau, part of the Australian Shield, is an ancient craton covering much of the continent's southwest, an area of some 700,000 square kilometres. It has an average elevation of between 305 and 460 m.

The North Island Volcanic Plateau is an area of high land occupying much of the centre of the North Island of New Zealand, with volcanoes, lava plateaus, and crater lakes, the most notable of which is the country's largest lake, Lake Taupo. The plateau stretches approximately 100 km east to west and 130 km north to south. The majority of the plateau is more than 600 m above sea level.

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In geology and physical geography, a plateau ( or plural plateaus or plateaux), also called a high plain or a , is an area of a highland consisting of flat terrain, that is raised sharply above the surrounding area on at least one side. Often one or more sides have deep hills.

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I've thought a lot about home during the quarantine. The place and the idea. The way it calls to us, and the way the pulse of daily life can sometimes drown it out.

My home is near the courthouse square in Oxford, Mississippi, a vibrant modern college community. That's where we wake up and make scrambled eggs for our toddler. But I'm from a nearby place called Clarksdale, birthplace of the Delta blues, the town that gave the world the likes of Muddy Waters, Sam Cooke, and Nate Dogg -a place where on a clear night I can hear plantation blues, the protest soul of Stax Records, and the heavy sound of G-Funk LA all mix in the open air.

On a recent Saturday afternoon, my wife, Sonia, and I took our two-year-old daughter, Wallace, on a drive into the Delta. The plan was to sit somewhere for a picnic. We'd stop in Clarksdale and pick up some takeout at an old-school Italian roadhouse called Ramon's, drive out in the country, spread out a blanket, and just be there for a while.

Clarksdale is 64 miles by car from Oxford, but it feels farther than that. It feels like something out of a novel or a play. In fact, it is. Tennessee Williams grew up in my neighborhood. Two doors to the left of the house where we moved when I was five was a big white mansion. The woman who lived in it was named Blanche.
(Related: This is the ultimate Mississippi road trip.)
Heading into town, I turned off of the highway and we bumped along a busted country road. We looked out the right window of the car and saw a nearly burned down house, still smoldering. A soot-stained chimney reached into the blue sky. Out the left window, a tall white grave marker sat in the middle of a pecan orchard. The son of a prominent local farmer had recently died of an opioid overdose. I worked for the farmer a long time ago, and he's a good man, funny and generous. I saw a bench situated next to the grave, and in my mind I could see the outline of the farmer and his wife, year after year, growing old on that bench, watching leaves come and go on the trees.

"It's a town full of ghosts," Sonia said as we arrived.

That's Clarksdale.

In late summer, the Delta is covered in vast swaths of white, and by the end of October, the cotton fields are mostly picked clean as the people leave one season behind and begin preparing for it all to start again.

Photograph by William Widmer, Redux

Clarksdale sits at the intersection of Highways 61 and 49, which form the teardrop shaped wedge of fertile farmland known as the Mississippi Delta.

Photograph by William Widmer, Redux

The places we haunt

We are a nation of ghosts these days. In a strange twist, the places we're haunting are our own lives. To me right now, the idea of traveling isn't so much about making new experiences as it is communing with old ones. I think that's why I wanted takeout from Ramon's.

I texted ahead to place our order.

In the late 1800s, Sicilians landed in New Orleans on lemon boats and worked in the sugarcane towns of Louisiana, and in the cotton towns of Mississippi. Open any phone book in the Mississippi Delta and all the vowel-ending names give away a time and a place that has mostly been forgotten. The Italians speak with southern accents now.

Whatever gets rolled under the wheel can be reborn; home is an idea and if the idea lives, then home lives, too.

Beverly Ely runs Ramon's, after her wonderful husband, Thomas, passed away. Her daughter Scottie, and her granddaughters Meri Hollis and Sara, are in there a lot of the time, too. When I think of what home tastes like, I imagine their fried shrimp, and their red gravy ladled atop tubular cavatuna noodles and veal cutlets. Their veal might be my dying wish. In this time of social distancing, which is just a fancy word for loneliness, I find myself craving it.
(Related: Here's how to eat local across the American south.)
We pulled up out front and waited. The restaurant is squat and long, with weathered siding and an old faded sign. The original owners wanted to name it Raymond's. The sign painter messed up and the mistake stuck.

Their dining room is closed. Beverly still goes to work and sits behind the low counter in the front room. Scottie says her mom is really struggling with that. Beverly has a reputation as a tough lady, but I think she loves how this little town passes by her counter. On one recent visit, I saw my friend Fred's parents sitting there with the local parish priest.

Scottie talks to her late father a lot when she's in there alone. The restaurant isn't her primary source of income but patrolling the dining room in this little joint is her way of paying tribute to her family, of making sure some essential idea about them endures. She likes that there is a community that exists around Ramon's. And she hopes it can survive, not just this moment, but the inevitable passage of time.

The virus is super-charging a fear she always has about what time does to cherished people and places. I think right now Americans are realizing something that people who grew up in towns like Clarksdale have always known: We live in a country that has been blowing away like poorly managed topsoil. This virus isn't destroying as much as revealing how fragile our way of life has been for generations.

Legacy of longing

Placed on the back seat next to Wallace, the Italian food smelled amazing.

I wanted to eat it on the little square of grass where Muddy Waters' house used to stand. It's the spot where he sang plantation songs that Alan Lomax and John Work recorded for the Library of Congress. The home is gone now, transplanted to a nearby museum, but when I was in high school, the wreck of it still stood abandoned alongside a cotton field.

The road there runs through the old Stovall Plantation, where Muddy Waters worked. It was owned then by Colonel William Howard Stovall, a World War I flying ace who died just six years before I was born. His family still owns it. Not much changes around here.
(Related: From bluegrass to blues, here's where to experience the music of the American south.)
Muddy's genius was that he took that nostalgia for homeplace music, combined it with his pain and anger at that place, and let it rip-loud, dangerous, and alive. He played this new invented sound in juke joints like Chicago's Smitty's Corner or Theresa's Lounge, to crowds of newly urban country folk like him, many of them in the city to leave behind the same plantation fields he had fled.

A few miles from Stovall's, I pulled up to Muddy's grassy patch on Burnt Cane Road. We spread out the blanket and Wallace sat down, gripped a fried shrimp, and took a bite. She pointed at the clamshell of spaghetti and I twirled up a forkful and she ate it with delight.

Sitting there with her, in a place where something used to be, I never felt more strongly connected to my hometown. I grew up serenaded by the lullabies of vanished worlds. By songs of regret and decay. Now I find I crave those feelings. I find myself communing with them, seeking them out, as if the bittersweet in them might inoculate me against future pains and regrets.

I've been worried the virus might leave a similar inheritance for my daughter. I worry that she, too, might grow up surrounded by stories of places lost, of how things used to be. I wondered if bringing her back here was good or bad, whether to sit tight or run from that spot.

I looked at the empty field behind us as we ate, the cotton picked clean and the stalks rolled under in preparation for another cycle. It was getting close to time to plant-if farmers can get crop loans, if the rain will hold off.

Wallace started making the noise she makes when she has to go to the bathroom.

"Home," she said with urgency.

We laughed and started to gather up our things.

"Home," she said.

Country blues

I put on some Muddy Waters for the drive back.

That glorious noise he invented rarely made it on to his recordings thanks to commercial-conscious record-label men who'd softened him. But seven years before he died, he finally laid it down on "Hard Again," recorded on Blue Sky Records in 1976.

In the beginning of the first tune, his voice is accompanied by a single slide guitar. Everything he had overcome in his life lives inside his cry, at once wounded and defiant. The band slides into gear, the younger musicians confronted with something they'd read about in books but never truly experienced, as if it weren't 1976 any longer but the end of the Depression, as if they were standing in cotton fields surrounded by gnarled trees along Stovall Road. Muddy is speaking from the soul of a tractor driver and guitar picker and moonshine runner whose first memories of Alan Lomax included shock that a white man would share a water cup with him.
(Related: Take a road trip through Alabama's civil rights history.)
I've been thinking recently about how these specific blues could be the soundtrack for a country trying to emerge from quarantine in one piece. A friend I trust told me that sentiment sounds like a kumbaya, and I know what he means. There is real pain and irreducible violence in the music. It records a very particular history.

But I think I also know the grace in Muddy's music is that he has given it to everyone, even the descendants of his oppressors.

That makes me hopeful. Whatever gets rolled under the wheel can be reborn; home is an idea and if the idea lives, then home lives, too, no matter what individual pieces of it slip through our hands. The human spirit is in flight not only when it triumphs, but when it struggles and strives. That's the message in the empty spaces between Muddy's band and his voice. You cannot miss it. He's an older man talking to his younger self, singing to him, and sometimes laying back and letting those guitar strings ring out like a church bell calling to worship, letting him know he will struggle, yes, but he will emerge and survive.

At the end of the track, the band hollers with delight. You can hear it on the record-Muddy hops around the studio, on fire, reborn.

We listened to it the whole way home.

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What the Mississippi Delta teaches me about home-and hope

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Finding struggle and resilience on a road trip through the birthplace of the blues.

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River Delta Facts For Kids | AstroSafe Search

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Discover River Delta in AstroSafe Search Educational section. Safe, educational content for kids 5-12. Explore fun facts!

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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.

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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.

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Islands are pieces of land surrounded by water. Hawaii is a famous group of islands. Some islands are formed by underwater volcanoes.

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GEOGRAPHY AND LANDFORMS

Hawaii sits over 2,000 miles west of California.

One of world's longest island chains, Hawaii is the only U.S. state completely made up of islands. But only 7 of its 132 islands are inhabited: Hawaii (also known as the Big Island), Maui, Molokai, Lanai, Oahu, Kauai, and Niihau.

The Hawaiian Islands are volcanic islands. They have formed as the Earth 's crust, made up of giant rocky slabs called tectonic plates, moves over a particularly hot spot in the molten layer beneath the crust. The heat melts the rock that makes up the crust, turning it into magma. Then once the magma breaks through to the surface of the Earth's crust it cools and forms new land.

The Earth's crust is always moving just a little bit, but the hot spot that produces magma isn't. So over time as the crust moved, but the hot spot remained-creating a series of volcanic islands. Hawaii's most active volcano is Kilauea, and you can see it at Hawai'i Volcanoes National Park on the Big Island. Kilauea has been erupting for over 30 years, and each year, its lava expands Hawaii by over 40 acres.

Hawaii is known for its beautiful beaches-some of them with unusual colors. Many beaches are filled with white sand, but other Hawaiian shores are covered with green, red, pink, and even black sand.

Whether you like hiking, biking, kayaking, sailing, swimming, or just sitting on the beach, Hawaii is the state for you. Near the water, you can relax as palm trees blow in the island breeze. Travel toward to the center of one of the big islands and you can hike through dense tropical rain forests and experience stunning waterfalls. Don't forget to dive in the waters and snorkel near Hawaii's coral reefs.

On Hawaii you can experience yet another environment: the volcano Mauna Loa's dry lava is so much like parts of the moon's surface that astronauts once walked on it to practice for lunar voyages. Mount Waialeale on Kauai is considered on of the rainiest spots on Earth, getting 384 inches of rain a year on average.

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Get facts and photos about the 50th state.

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Encyclopedic entry. An oasis is an area made fertile by a source of freshwater in an otherwise dry and arid region. Oases (more than one oasis) are irrigated by natural springs or other underground water sources.

Subjects

Earth Science, Geography, Human Geography, Physical Geography

An oasis is an area made fertile by a source of freshwater in an otherwise dry and arid region. Oases (more than one oasis) are irrigated by natural springs or other underground water sources. They vary in size from a cluster of date palms around a well or a spring to a city and its irrigated cropland. Dates, cotton, olives, figs, citrus fruits, wheat and corn (maize) are common oasis crops.

Underground water sources called aquifers supply most oases. In some cases, a natural spring brings the underground water to the surface. At other oases, manmade wells tap the aquifer. In some oasis settlements, these wells might be centuries old and might have been diligently maintained for generations to preserve access to their life-giving water.

Sands blown by desert winds threaten wells as well as agricultural areas in oases. Sand can destroy crops and pollute water. Communities have traditionally planted strong trees, such as palms, around the perimeter of oases to keep the desert sands from their delicate crops and water.

Some of the world's largest supplies of underground water exist beneath the Sahara Desert, supporting about 90 major oases there. The Sahara is the largest desert on Earth-about the size of the continental United States. Though there are many oases there, traveling between them can take days because the desert is so vast.

For this reason, oases in the Sahara and throughout the world have become important stops along trade routes. Merchants and traders who travel along these routes must stop at oases to replenish food and water supplies. This means that whoever controls an oasis also controls the trade along the route-making oases desirable to political, economic, and military leaders.

Al-Hasa, Saudi Arabia, has been an important farming area for the Arabian Peninsula for thousands of years. Today, it continues to be a leading agricultural region, producing dates, rice, corn, sheep, cattle, and eggs. The al-Hasa region also lies above one of the richest oil fields in the world, making the oasis an important center of international trade.

Rivers that flow through some deserts provide permanent sources of water for large, elongated oases. The fertile Nile River valley and delta in Egypt, supplied with water from the Nile River, is an example of this type of large oasis. At 22,000 square kilometers, it might be the largest oasis in the world.

Fast Fact

Meadow in the Mojave
The Las Vegas Valley, in the U.S. state of Nevada, is a popular tourist destination known for gaming and entertainment. Before the arrival of casinos, however, Las Vegas was the site of a natural oasis in the Mojave Desert. Springs brought water from the regions aquifer to the surface, ultimately flowing into the Colorado River. Las Vegas means the meadows in Spanish, and was named when the oasis was discovered by Mexican merchants in 1829.

The Las Vegas oasis has dried up. Development drained the springs. The areas empty aquifers are now used to store water from Lake Mead, an artificial lake created by the Hoover Dam on the Colorado River. Water is a major environmental and political issue in Las Vegas and throughout the U.S. Southwest.

Media Credits

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Illustrators

Mary Crooks, National Geographic Society

Editors

Jeannie Evers, Emdash Editing, Emdash Editing

Producer

National Geographic Society

other

Last Updated

October 19, 2023

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Encyclopedic entry. An oasis is an area made fertile by a source of freshwater in an otherwise dry and arid region. Oases (more than one oasis) are irrigated by natural springs or other underground water sources.

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This desert oasis is a time capsule of Egypt’s grand past

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Fossilized whales, temples to crocodile gods, and a thriving pottery scene draw day-trippers from Cairo to the bucolic Fayoum region.

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Siwa Oasis Facts For Kids | AstroSafe Search

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Discover Siwa Oasis in AstroSafe Search Educational section. Safe, educational content for kids 5-12. Explore fun facts!

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A river delta is a landform created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment. The size and shape of a delta is controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution. River deltas are important in human civilization, as they are major agricultural production centers and population centers. They can provide coastline defense and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position.

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A river delta is a landform created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment.

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Test your knowledge about landforms with this fun quiz.