Encyclopedic Entry

This temporary levee is made of sand bags.

Photograph by Jennifer LaVista, U.S. Geological Survey

Long Levees

The levee system along the Mississippi River has some of the longest individual levees in the world. One of these levees stretches south along the river from Pine Bluff, Arkansas, for an entire 611 kilometers (380 miles).

Living near water is a wonderful thing—except when there’s a flood. So people build levees. A levee is a natural or artificial wall that blocks water from going where we don’t want it to go. Levees may be used to increase available land for habitation or divert a body of water so the fertile soil of a river or sea bed may be used for agriculture. They prevent rivers from flooding cities in a storm surge. But if a levee breaks, the consequences can be disastrous.

Levees are usually made of earth. The natural movement of a body of water pushes sediment to the side, creating a natural levee. The banks of a river are often slightly elevated from the river bed. The banks form levees made of sediment, silt, and other materials pushed aside by the flowing water. Levees are usually parallel to the way the river flows, so levees can help direct the flow of the river.

Levees can also be artificially created or reinforced. Artificial levees are usually built by piling soil, sand, or rocks on a cleared, level surface. In places where the flow of a river is strong, levees may also be made of blocks of wood, plastic, or metal. Where the area beside a river or other body of water is in particular danger, levees may even be reinforced by concrete.

People have been building and reinforcing levees since the beginning of civilization. As early as 2500 BCE, the Indus Valley Civilization, with urban centers in what is today Mohenjo Daro and Harappa, Pakistan, used levees to protect land near the Indus River. Farmers were able to grow crops like cotton and rice.

In addition to creating living space and cropland, levees can also provide a measure of protection from invaders. Levees can make a river like a moat, preventing people from easily invading territory on the other side. Destroying levees can also stop invading forces. In 1938, Chinese leaders intentionally broke levees on the Yellow River to prevent the Japanese military from advancing. More than 500,000 people, Japanese and Chinese, died in the resulting flood.

Artificial levees need to be protected. They have to stand up to erosion, or wearing away, by the nearby water. Sometimes, trees and plants like Bermuda grass are planted near levees to anchor the soil. Engineers need to maintain levees with structural work to reinforce the boundaries.

In emergencies, temporary levees can be made of sandbags. These soak up the water and usually prevent excess water from seeping past the sand.

Artificial levees prevent flooding. But they also create a new problem: levees squeeze the flow of the river. All the river’s power is flowing through a smaller space. Water levels are higher and water flows faster. This puts more pressure on levees downstream and makes the water more difficult to control. If levees break, it also makes containing the flood more difficult.

Since the 18th century, levees have protected Louisiana and other nearby states from flooding by the Mississippi River. When Hurricane Katrina struck New Orleans in 2005, the levees could not withstand the storm surge. The levees broke, and water flooded 80 percent of the city.

Levees on the Sea

Although most levees exist to control rivers, they can also exist on the coast. The country of the Netherlands has an elaborate system of dikes, levees, and dams to hold back the North Sea. Land for farms, industry, and residential use has been created from land that was once the ocean floor.

The Bay of Fundy, which borders the provinces of New Brunswick and Nova Scotia, Canada, has one of the highest tidal ranges in the world. The tidal range reaches more than 17 meters (55 feet) in some places. To make the most use of land that would otherwise be underwater during high tide, Canada has constructed levees along parts of the Bay of Fundy.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

anchor

Verb

to hold firmly in place.

artificial

Adjective

created by people and industry.

bank

Noun

a slope of land adjoining a body of water, or a large elevated area of the sea floor.

Bermuda grass

Noun

grass with deep roots.

civilization

Noun

complex way of life that developed as humans began to develop urban settlements.

Encyclopedic Entry: civilization

concrete

Noun

hard building material made from mixing cement with rock and water.

cotton

Noun

cloth made from fibers of the cotton plant.

crop

Noun

agricultural produce.

Encyclopedic Entry: crop

dam

Noun

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

dike

Noun

a barrier, usually a natural or artificial wall used to regulate water levels.

Encyclopedic Entry: dike

downstream

Noun

in the direction of a flow, toward its end.

earth

Noun

soil or dirt.

elaborate

Adjective

complex and detailed.

elevate

Verb

to raise higher than the surrounding area.

engineer

Noun

person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

erosion

Noun

act in which earth is worn away, often by water, wind, or ice.

Encyclopedic Entry: erosion

excess

Noun

extra or surplus.

flood

Noun

overflow of a body of water onto land.

Encyclopedic Entry: flood

high tide

Noun

water level that has risen as a result of the moon's gravitational pull on the Earth.

Hurricane Katrina

Noun

2005 storm that was one of the deadliest in U.S. history.

Indus Valley Civilization

Noun

(2500-1500 BCE) civilization that flourished in the Indus River Valley, in present-day Pakistan.

legally protected

Adjective

having laws to keep something safe.

levee

Noun

bank of a river, raised either naturally or constructed by people.

Encyclopedic Entry: levee

maintain

Verb

to continue.

metal

Noun

category of elements that are usually solid and shiny at room temperature.

military

Noun

armed forces.

moat

Noun

trench around a castle, filled with water, to prevent or delay attack or invasion.

neighborhood

Noun

an area within a larger city or town where people live and interact with one another.

Encyclopedic Entry: neighborhood

overwork

Verb

to demand too much of someone or something.

parallel

Adjective

equal distance apart, and never meeting.

plastic

Noun

chemical material that can be easily shaped when heated to a high temperature.

province

Noun

division of a country larger than a town or county.

Encyclopedic Entry: province

result

Verb

to end after a series of actions.

rice

Noun

grass cultivated for its seeds.

river bed

Noun

material at the bottom of a river.

rock

Noun

natural substance composed of solid mineral matter.

sand

Noun

small, loose grains of disintegrated rocks.

sandbag

Noun

bag filled with sand or earth and placed near a river to prevent flooding.

sediment

Noun

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

Encyclopedic Entry: sediment

seep

Verb

to slowly flow through a border.

silt

Noun

small sediment particles.

Encyclopedic Entry: silt

soil

Noun

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

structural

Adjective

having to do with the frame or support of a construction such as a bridge or building.

tidal range

Noun

the difference in height between an area's high tide and low tide.

urban center

Noun

densely populated area, usually a city and its surrounding suburbs.

ward

Noun

neighborhood or political district in some large cities.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writer

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrator

Tim Gunther
Mary Crooks, National Geographic Society

Editor

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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