A flood happens when water overflows or soaks land that is normally dry. There are few places on Earth where people don’t need to be concerned about flooding. Generally, floods take hours or even days to develop, giving residents time to prepare or evacuate. Sometimes, floods develop quickly and with little warning.

A flood can develop in a many ways. The most common is when rivers or streams overflow their banks. These floods are called riverine floods. Heavy rain, a broken dam or levee, rapid icemelt in the mountains, or even a beaver dam in a vulnerable spot can overwhelm a river and send it spreading over nearby land. The land surrounding a river is called a flood plain.

Coastal flooding, also called estuarine flooding, happens when a large storm or tsunami causes the sea to rush inland.

Floods are the second-most widespread natural disaster on Earth, after wildfires. All 50 of the United States are vulnerable to flooding.

Effects of Floods

When floodwaters recede, affected areas are often blanketed in silt and mud. This sediment can be full of nutrients, benefiting farmers and agribusinesses in the area. Famously fertile flood plains like the Mississippi River valley in the American Midwest, the Nile River valley in Egypt, and the Fertile Crescent in the Middle East have supported agriculture for thousands of years. Yearly flooding has left millions of tons of nutrient-rich soil behind.

However, floods have enormous destructive power. When a river overflows its banks or the sea moves inland, many structures are unable to withstand the force of the water. Bridges, houses, trees, and cars can be picked up and carried off. Floods erode soil, taking it from under a building's foundation, causing the building to crack and tumble. Severe flooding in Bangladesh in July 2007 led to more than a million homes being damaged or destroyed.

Floods can cause even more damage when their waters recede. The water and landscape can be contaminated with hazardous materials, such as sharp debris, pesticides, fuel, and untreated sewage. Potentially dangerous mold can quickly overwhelm water-soaked structures.

As flood water spreads, it carries disease. Flood victims can be left for weeks without clean water for drinking or hygiene. This can lead to outbreaks of deadly diseases like typhoid, malaria, hepatitis A, and cholera. This happened in 2000, as hundreds of people in Mozambique fled to refugee camps after the Limpopo River flooded their homes. They soon fell ill and died from cholera, which is spread by unsanitary conditions, and malaria, spread by mosquitoes that thrived on the swollen river banks.

In the United States, floods are responsible for an average of nearly 100 deaths every year, and cause about $7.5 billion in damage.

China's Yellow River valley has seen some of the world's worst floods in the past 100 years. The 1931 Yellow River flood is one of the most devastating natural disasters ever recorded—almost a million people drowned, and even more were left homeless.

Natural Causes of Floods

Floods occur naturally. They are part of the water cycle, and the environment is adapted to flooding. Wetlands along river banks, lakes, and estuaries absorb flood waters. Wetland vegetation, such as trees, grasses, and sedges, slow the speed of flood waters and more evenly distribute their energy. According to the U.S. Environmental Protection Agency (EPA), the wetlands along the Mississippi River once stored at least 60 days of flood water. (Today, Mississippi wetlands store only 12 days of flood water. Most wetlands have been filled or drained.)

Floods can also devastate an environment. The most vulnerable regions are those that experience frequent floods and those that have not flooded for many years. In the first case, the environment does not have time to recover between floods. In the second case, the environment may not be able to adapt to flood conditions.

In August 2010, Pakistan experienced some of the worst floods of the century. The annual monsoon, on which Pakistani farmers and consumers rely, was unusually strong. Tons of water drenched the nation. The Indus River burst its banks. Because the river flows almost directly through the narrow country, almost all of Pakistan was affected by flooding.

Millions of Pakistanis lost their homes, and almost 2,000 died in the floods. The province of Punjab, the country’s agricultural center, was particularly devastated. Rice, wheat, and corn crops were destroyed. The impact of the floods continued long after the monsoon dwindled and the Indus subsided. Pakistanis experienced food shortages, power outages, and loss of infrastructure. Outbreaks of cholera and malaria developed near resettlement camps. Experts estimated that the rebuilding effort would cost up to $15 billion.

Sometimes, floods are triggered by other natural disasters, such as earthquakes and tsunamis. In January 2011, a major earthquake struck off the coast of Miyagi Prefecture, Japan. The quake triggered a massive tsunami, its crest reaching as high as 40 meters (131 feet). The tsunami crashed more than 10 kilometers (6 miles) inland, flooding homes, businesses, schools, parks, hospitals, and the Fukushima Dai-ichi Nuclear Power Plant. A dam holding a reservoir burst, triggering another flood that destroyed homes.

Rain that accompanies hurricanes and cyclones can quickly flood coastal areas. The rise in sea level that occurs during these storms is called a storm surge. A storm surge is a type of coastal flood. They can be devastating. The storm surge that accompanied the 1970 Bhola cyclone flooded the low-lying islands of the Ganges Delta in India and Bangladesh. More than 500,000 people were killed, and twice that number were left homeless.

The strong winds associated with hurricanes and cyclones can also whip up and move huge amounts of water, forcing a storm surge far inland. In 2005, Hurricane Katrina brought huge amounts of wind and rain to the Gulf Coast of the United States. The city of New Orleans, Louisiana, was particularly hard-hit. The storm surge from Hurricane Katrina caused some of the city’s levees to break. Levees protect New Orleans from the Mississippi River. The river rushed in and flooded entire neighborhoods. Hundreds of people drowned, and the storm did more than $100 billion in damage.

Man-Made Causes of Floods

Floods can also have man-made sources. Many man-made floods are intentional and controlled.

Rice farmers, for instance, rely on flooded fields. Rice is a semi-aquatic crop—it grows in water. After rice seedlings are planted, farmers flood their fields, called rice paddies, in about 15 to 25 centimeters (6 to 10 inches) of water. Rice paddies must be carefully engineered to allow controlled flooding. Strong dikes or levees, as well as regulated channels for irrigation, are required.

Sometimes, engineers flood an area to restore an ecosystem. In 2008, the Grand Canyon was deliberately flooded. Water was released from dams on the Colorado River, which runs through the Grand Canyon. In 20 minutes, enough water was released from a dam at Lake Powell, Utah, to fill up the Empire State Building. Hydrologists, engineers, and environmentalists hoped that flooding the canyon would help redistribute sediment—which had been blocked up by dams—and create sandbars. Sandbars provide a wildlife habitat, often serving as a shallow bridge for animals such as beavers and bighorn sheep to cross from one side of the river to the other.

Dams control the natural flood plains of lakes and rivers. Hydrologists may intentionally flood areas to prevent damage to the dam or increase the water supply for agriculture, industry, or consumer use.

Engineers may also intentionally flood areas to prevent the possibility of worse flooding. When heavy rains caused the Souris River to flood in 2011, for example, the water level nearly reached the top of the Alameda Reservoir in Oxbow, Saskatchewan, Canada. Faced with the prospect of catastrophic flooding if the entire dam broke, engineers chose to release huge amounts of water. The reservoir remained intact, but the release contributed to massive floods in both Saskatchewan and the U.S. city of Minot, North Dakota.

Not all man-made floods are intentional, however. The natural banks of rivers and streams shrink as people develop land nearby. River banks are valuable real estate for housing, businesses, and industry. From Shanghai, China, to San Antonio, Texas, rivers are the sites of busy urban areas. In rural areas, factories use river currents to distribute runoff. To accommodate such development, river banks are paved with hard, non-porous materials. Soils and plants are replaced with concrete and asphalt, which can’t absorb water. An unusual amount of rain can cause these rivers to quickly overrun their concrete banks.

Australia is conducting an investigation of Brisbane’s development decisions after the Brisbane River overran its banks and flooded the country’s capital in 2011. Streets, downtown business districts, and bridges were destroyed. Water reached the third row of seats in the city’s rugby stadium. The flood waters were high enough (2 meters/6 feet) that bull sharks were spotted swimming up major streets.

Concrete banks also increase the amount of runoff flowing to nearby bodies of water. This increases the risk of coastal flooding. Venice, Italy, for instance, is frequently flooded as tides from the Adriatic Sea seep into the heavily developed islands on which the city rests.

Hydrologists, engineers, and city planners constantly work to reduce flood damage. Shrubs and plants create buffers to prevent runoff from seeping into flood plains, urban areas, or other bodies of water. The thick vegetation between a river and a flood plain is called a riparian zone.

Despite their efforts, people can also radically fail to control floods. The most famous flood in American history, the Johnstown Flood, was a man-made disaster. The tragedy killed 2,209 people and made headlines around the country.

Johnstown, Pennsylvania, was on a flood plain at the meeting of the Stony Creek and Little Conemaugh rivers. As more people moved to the city, the banks of the rivers were paved and narrowed, causing yearly flooding. Residents were prepared for this. They watched the river and moved their belongings upstairs or onto rooftops as the city flooded.

However, residents were not prepared for the additional flood from an entire lake. Located in nearby mountains, Lake Conemaugh was a reservoir created by the South Fork Dam. The lake was an exclusive retreat for members of the South Fork Fishing and Hunting Club, which owned the dam. Lake Conemaugh contained 20 million tons of water.

On May 31, 1889, the dam broke and the water rushed down the river at 64 kilometers per hour (40 miles per hour). Johnstown’s leading industry was steel production, and the flood waters quickly became choked with industrial debris—steel cables, chemical solvents, glass, rail cars. The flood destroyed a wire factory, filling the water with tons of barbed wire. About 80 people died when floating wreckage caught fire.

Rebuilding Johnstown took years—the bodies of some victims were not found until 20 years later. Although the South Fork Fishing and Hunting Club failed to maintain the dam, members of the club successfully argued that the disaster was an “act of God.”

Flood Classification

Disaster experts classify floods according to their likelihood of occurring in a given time period. The most common classifications are a 10-year flood, a 50-year flood, and a 100-year flood. A 100-year flood, for example, is an extremely large, destructive event that would be expected to happen only once every century.

But this is only an estimate. What “100-year flood” actually means is that there is a 1 percent chance that such a flood could happen in any given year. In recent decades, 100-year floods have occurred more frequently. This may be due to global warming, the current period of climate change.

The Red River, which flows along the border of North Dakota and Minnesota, chronically floods. Anything over 8.5 meters (28 feet) is considered “flood stage” in the area. In 1997, the river crested at almost 12 meters (40 feet), a record level. In 2009, the record was beaten as the river flooded again, reaching a height of almost 12.5 meters (40.8 feet). The river flooded for 61 days.

Flash floods can develop within hours of heavy rainfall. Flash floods can be extremely dangerous, instantly turning a babbling brook into a thundering wall of water that sweeps away everything in its path. Most deaths from flooding occur as a result of flash floods. Flash floods do not have a system for classifying their magnitude.

Deserts are vulnerable to flash floods. Wadis and arroyos are dry river beds that only flow during heavy rains. Wadis can be dangerous during flash floods because they rarely have riparian zones to slow the flood’s energy. The city of Jeddah, Saudi Arabia, developed on the site of several wadis, and floods are frequent after heavy rains. More than 100 people died in flash floods in Jeddah in 2009. The floods developed so quickly that many victims drowned in their cars as streets became submerged.

Predicting Floods

Today, hydrologists study past flood patterns to help predict where and when floods will happen in the future. The predictions are only estimates, however. Weather, land, and climate can all change.

An area’s soil and groundwater provide clues about flooding. Pedologists, or soil scientists, work with hydrologists to determine how much water a region’s earth can absorb. Agricultural soil, for instance, can absorb much more water than sand or bare rock. Groundwater is water already in the earth—in soil, underground reservoirs called aquifers, and even porous rocks. The type of soil and the amount of groundwater tells hydrologists how much more water the earth can absorb.

Determining the amount of runoff in an area can also provide clues about the possibility of flooding. Runoff happens when there is more water than soil can absorb. Excess water overflows and runs on top of the land. Runoff can come from natural processes, such as icemelt. It can also come from human activity, such as excess irrigation, sewage, and industrial waste. Controlling runoff can help control floods.

Hydrologists work with meteorologists to evaluate snowfall and snowpack. Melting snow contributes to runoff and increases groundwater levels. When snow melts quickly, the ground may not have time to absorb the water. Snowfall is one of the biggest contributors to flooding, and cannot always be predicted. Rapid snowmelt in the Andes Mountains, for example, creates mudslides and floods that disable railways and bridges. In 2010, snowmelt flooding trapped 4,000 tourists in towns near the remote historic site of Machu Picchu, Peru, for two days.

Modern technology helps researchers predict floods. Doppler radar, for example, shows scientists where a storm is most severe. Doppler uses motion to detect weather patterns and create computerized images of rainfall. Automated gauges placed in rivers measure the height and speed of river currents, and the amount of rain received. Geographic information system (GIS) maps made with this information help scientists warn people if a river will overrun its banks and flood areas nearby.

Preventing Floods

For thousands of years, people have tried to prevent and control floods. Yu the Great, for example, is a legendary figure in Chinese history. Around 2100 BCE, Yu developed a way to control the devastating floods of the Yellow River. Yu studied data from previous Yellow River floods, noting where the flow was the strongest and flood plains were most vulnerable. Instead of damming the river, Yu dredged it—he and a team of engineers made river channels deeper to accommodate more water. Yu also oversaw the construction of numerous irrigation canals, which diverted the flow of the river’s mainstem during times of flooding.

It’s not always possible to prevent floods, but it is often possible to minimize flood damage. Structures around rivers, lakes, and the sea can contain flood waters. Levees, runoff canals, and reservoirs can stop water from overflowing.

Levees are usually made of earth. They are built by piling soil, sand, or rocks near a river’s banks. Levees may also be made of blocks of wood, plastic, or metal. They may even be reinforced by concrete. Levees in New Orleans, for example, use compacted earth, wooden beams, iron rebar, steel pilings, and concrete to hold back the mighty Mississippi River.

Runoff canals are man-made channels. These structures are connected to rivers and direct excess water away from buildings and residences. One of the first canals in North America was constructed in about 200 BCE to control the seasonal flood waters of Lake Okeechobee, Florida. Today, southern Florida is criss-crossed by runoff canals that redirect the flow of the Everglades, the “River of Grass” that runs from Lake Okeechobee to the Atlantic Ocean and Gulf of Mexico. These canals redirect flood water away from urban areas in southern Florida and toward irrigation canals primarily used for fields of sugar cane.

Natural and man-made reservoirs help prevent flooding. Natural reservoirs are basins where freshwater collects. Man-made reservoirs collect water behind a dam. They can hold more water in times of heavy rainfall. In April 2011, the government of Ethiopia announced plans for a large dam on the Blue Nile River. The Grand Ethiopian Renaissance Dam, which would be the largest dam in Africa, would create a reservoir capable of holding 67 billion cubic meters (2.4 trillion cubic feet) of water. The dam would prevent flooding downstream and provide the nation with hydroelectric energy.

Conserving wetlands also reduces the impact of floods. Wetlands provide a natural barrier, acting as a giant sponge for storm surges and flood plains. The swamps and bayous of southern Louisiana and Mississippi, for instance, protect inland areas from both coastal and riverine flooding. Wetlands absorb the storm surge from hurricanes that hit the area from the Gulf of Mexico. Wetland riparian zones that line the Mississippi River protect fertile flood plains as the river overflows its banks.

Many governments mandate that residents of flood-prone areas purchase flood insurance and build flood-resistant structures. Massive efforts to mitigate and redirect floods have resulted in some of the most ambitious engineering efforts ever seen. The Thames Barrier is one of the largest flood-control projects in the world. The Thames Barrier protects the urban area of London, England, from floods from storm surges that rush up the River Thames from the Atlantic Ocean. A series of 10 steel gates span the river near London’s Woolrich district. Each gate can hold back 9,000 tons of water, and disappears into the river when the water is calm.

Perhaps the most extensive and sophisticated flood-prevention program is the Zuiderzee Works in the Netherlands. The Netherlands is a low-lying nation that is plagued by coastal flooding from the North Sea. Beginning in the 1200s, the Dutch began to erect a series of massive dikes and levees on its coast. In the 1900s, Dutch engineers worked to isolate and dam an entire inlet of the North Sea, the Zuiderzee. The largest part of the Zuiderzee Works is the Afsluitdijk, a 32-kilometer (20-mile) dike that cuts off the Zuiderzee from the North Sea. In addition to protecting the Netherlands from flooding, the Zuiderzee Works has drained parts of the Zuiderzee for development.

flood
Another word for flood is deluge (pronounced day-LOOJ or DEH-looj).

Apres Moi, le Deluge
"After me, the flood" (in French, "apres moi, le deluge") is a phrase attributed to the French King Louis XV or his mistress, Madame de Pompadour.

The phrase is a casual way of expressing irresponsibility, something like "When I leave a project, I don't care if a catastrophe happens. It no longer concerns me."

Toxic Flood
There are many examples of toxic materials, from pig manure to coal slurry, flooding communities. One of the most unusual was the 2010 rupture of a chemical storage tank at an aluminum factory in Ajka, Hungary. The bright-red sludge was responsible for at least four deaths, as well as the relocation of hundreds of Hungarians. The toxic sludge, which included lead and arsenic, was eventually diluted by the Danube River.

London Beer Flood
In 1814, vats containing 1.47 million liters (388,333 gallons) of beer spilled in the St. Giles area of London, England. Several homes and businesses were destroyed, and seven people drowned.

Boston Molasses Flood
In 1919, an 8.7 million-liter (2.3 million-gallon) tank of molasses exploded in the North End area of Boston, Massachusetts. The wave of molasses crested as high as 3 meters (10 feet) and moved as quickly as 56 kilometers per hour (35 miles per hour). A train was lifted off its tracks, and 21 people died. Six months later, Boston Harbor remained brown with molasses.

Costliest U.S. Floods
As of July 2011, according to the Federal Emergency Management Association (FEMA):
Hurricane Katrina (2005)
$16.2 billion
Hurricane Ike (2008)
$2.6 billion
Hurricane Ivan (2004)
$1.2 billion
Tropical Storm Allison (1989)
$1.1 billion
Louisiana Flood (1995)
$585 million

Flood as a War Tactic
In 1937, the Chinese government destroyed the dike at Huayuankou, on the Yellow River, to stop the Japanese invasion. The invasion continued by a different route, but the environmental devastation of the flooding was immense. At least 800,000 people drowned, and more than a million were made homeless. More than a thousand square kilometers of farmland was underwater. Flooding changed the course of the Yellow River to such an extent that its mouth moved dozens of kilometers to the south. Ten years later, the dike at Huayuankou was rebuilt and the Yellow River resumed its previous course.

"The Hero of Haarlem"
A popular story concerns a young boy from the town of Haarlem, Netherlands, who notices a leak in the town's dike. The Spaarne River is flowing through a tiny hole in the barrier, threatening to flood the town. The young boy plugs the leak with his finger, and stays there all night. Adults find him the next morning and permanently repair the leak. Although first written about by an American (Mary Mapes Dodge, in her book Hans Brinker, or the Silver Skates), the story is from the Netherlands.

The story has been changed and retold many times. In most versions, the dike is holding back the North Sea, not a river. In some versions of the story, the young boy freezes to death during his all-night stay at the dike.

Flood Myths
Stories about great, Earth-drowning floods are common throughout world cultures. Many stories are remarkably similar: A deity warns a virtuous man about a catastrophic flood. The man builds a large boat, saving himself, his family, animals, and plants from the flood, which destroys the rest of the Earth. Eventually, the man releases two birds to see if they bring back vegetation (which can only grow in soil). A bird returns, and human civilization is saved.

The most famous version of this flood myth is probably the story of Noah, recorded in the Torah, the Bible, and the Quran. Another version is the Mesopotamian legend of Utnapishtim, recorded in the Legend of Gilgamesh, one of the earliest works of literature, predating the Torah by more than a thousand years. The Maasai legend of Tumbainot, the Altai myth of Nama, and the Hawaiian myth of Nuu are all remarkably similar.

100-year flood
Noun

flood that has a one-percent chance of occurring any year. 

absorb
Verb

to soak up.

accommodate
Verb

to provide or satisfy.

act of God
Noun

legal term for a catastrophic event that cannot be foreseen or controlled.

adapt
Verb

to adjust to new surroundings or a new situation.

Afsluitdijk
Noun

(32 kilometers/20 miles long, 7 meters/23 feet above sea level) dike in the Netherlands that partly dams the North Sea.

agribusiness
Noun

the strategy of applying profit-making practices to the operation of farms and ranches.

Noun

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

ambitious
Adjective

eager to achieve wealth, power, status, or a specific goal.

Noun

an underground layer of rock or earth which holds groundwater.

archaeological site
Noun

place where evidence of the past is being studied by scientists.

arroyo
Noun

deep channel or canyon, often dry except during flash floods. Also called a wadi.

asphalt
Noun

chemical compound made of dark, solid rocks and minerals often used in paving roads.

bank
Noun

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

barbed wire
Noun

twisted metal with sharpened points, often used for fences.

Noun

swampy backwater of a river or lake.

benefit
Verb

to be helpful or useful.

Bhola cyclone
Noun

(1970) storm that caused widespread damage and death in Bangladesh and India.

brook
Noun

small flow of water, larger than a rill but smaller than a river.

buffer
Noun

a cushion or shield.

bull shark
Noun

shark able to survive in freshwater habitats.

cable
Noun

strong set of cords or wire ropes.

canal
Noun

artificial waterway.

catastrophic
Adjective

very bad.

century
Noun

100 years.

Noun

waterway between two relatively close land masses.

cholera
Noun

infectious, sometimes fatal disease that harms the intestines.

chronic
Adjective

recurring or happening frequently.

city
Noun

large settlement with a high population density.

city planner
Noun

person who plans the physical design and zoning of an urban center.

climate
Noun

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

Noun

gradual changes in all the interconnected weather elements on our planet.

coastal flooding
Noun

process where a storm or tsunami causes the sea to rush inland, as a storm surge. Also called estuarine flooding.

Colorado River
Noun

(2,335 kilometers/1,450 miles) river in the western U.S. and Mexico, draining into the Gulf of California.

concern
Verb

to worry or take an interest in.

concrete
Noun

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

consumer
Noun

person who uses a good or service.

contaminate
Verb

to poison or make hazardous.

crest
Noun

the top of a wave.

Noun

agricultural produce.

Noun

steady, predictable flow of fluid within a larger body of that fluid.

cyclone
Noun

weather system that rotates around a center of low pressure and includes thunderstorms and rain. Usually, hurricanes refer to cyclones that form over the Atlantic Ocean.

dam
Noun

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

debris
Noun

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

deliberately
Adverb

on purpose.

Noun

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

destructive
Adjective

harmful.

devastate
Verb

to destroy.

development
Noun

construction or preparation of land for housing, industry, or agriculture.

Noun

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

disable
Verb

to weaken or make useless.

disease
Noun

harmful condition of a body part or organ.

dispose
Verb

to throw away or get rid of.

distribute
Verb

to divide and spread out materials.

divert
Verb

to direct away from a familiar path.

domestic animal
Noun

animal that has been tamed for work or to be a pet.

domesticate
Verb

to tame or adapt for human use.

Doppler radar
Noun

weather tracking system that reads the direction and speed of moving objects, such as drops of precipitation.

downstream
Noun

in the direction of a flow, toward its end.

dredge
Verb

to remove sand, silt, or other material from the bottom of a body of water.

drench
Verb

to soak or cover with water.

drywall
adjective, noun

wide, flat board, usually made of plaster or wood pulp, that is often used to construct the interior walls of buildings.

dwindle
Verb

to shrink.

earth
Noun

soil or dirt.

earthquake
Noun

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

economy
Noun

system of production, distribution, and consumption of goods and services.

Noun

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

elaborate
Adjective

complex and detailed.

Noun

height above or below sea level.

engineer
Noun

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

enormous
Adjective

very large.

environment
Noun

conditions that surround and influence an organism or community.

environmentalist
Noun

person who studies or works to protect the Earth's ecosystems.

Environmental Protection Agency (EPA)
Noun

U.S. government organization whose mission is to "protect human health and the environment."

erect
Verb

to build or raise.

erode
Verb

to wear away.

Noun

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

estimate
Verb

to guess based on knowledge of the situation or object.

estuarine flooding
Noun

process where a storm or tsunami causes the sea to rush inland, as a storm surge. Also called coastal flooding.

evacuate
Verb

to leave or remove from a dangerous place.

evaluate
Verb

to decide something's worth.

Everglades
Noun

vast swampy region flowing south of Lake Okeechobee in Florida.

excess
Noun

extra or surplus.

exclusive
Adjective

limited to a few characteristics.

extensive
Adjective

very large.

farmer
Noun

person who cultivates land and raises crops.

fertile
Adjective

able to produce crops or sustain agriculture.

Noun

region extending from the eastern Mediterranean coast through Southwest Asia to the Persian Gulf.

flash flood
Noun

sudden, short, and heavy flow of water.

Noun

overflow of a body of water onto land.

Noun

flat area alongside a stream or river that is subject to flooding.

flood stage
Noun

level at which a river, creek, or other body of water may cause damage to lives, property, or businesses.

Noun

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

foundation
Noun

structure on which a building is constructed.

frequent
Adjective

often.

freshwater
Noun

water that is not salty.

fuel
Noun

material that provides power or energy.

gauge
Noun

measuring device, usually mechanical.

Noun

any system for capturing, storing, checking, and displaying data related to positions on the Earth's surface.

Noun

increase in the average temperature of the Earth's air and oceans.

government
Noun

system or order of a nation, state, or other political unit.

Noun

water found in an aquifer.

Noun

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

Noun

part of a body of water deep enough for ships to dock.

harvest
Noun

the gathering and collection of crops, including both plants and animals.

hazard
Noun

danger or risk.

hepatitis
Noun

disease of the liver.

herd
Noun

group of animals.

hurricane
Noun

tropical storm with wind speeds of at least 119 kilometers (74 miles) per hour. Hurricanes are the same thing as typhoons, but usually located in the Atlantic Ocean region.

Hurricane Katrina
Noun

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

Noun

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

hydrologist
Noun

person who studies the distribution, circulation, and properties of water.

hygiene
Noun

science and methods of keeping clean and healthy.

icemelt
Noun

flowing water created by melting ice.

industry
Noun

activity that produces goods and services.

infrastructure
Noun

structures and facilities necessary for the functioning of a society, such as roads.

inland
Adjective

area not near the ocean.

inlet
Noun

small indentation in a shoreline.

insurance
Noun

money paid in good health to guarantee financial or physical health if injury or damage occurs.

intact
Adjective

whole or complete.

international
Adjective

having to do with more than one country.

Noun

watering land, usually for agriculture, by artificial means.

irrigation canal
Noun

channel dug between a source of water and crops. Also called an irrigation ditch.

Noun

body of land surrounded by water.

isolate
Verb

to set one thing or organism apart from others.

Johnstown Flood
Noun

(1889, Johnstown, Pennsylvania) disaster caused by the failure of a dam, resulting in more than 2,200 deaths.

Noun

body of water surrounded by land.

Noun

the geographic features of a region.

Noun

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

magnitude
Noun

intensity of an earthquake, represented by numbers on a scale.

mainstem
Noun

largest river or channel in a watershed or drainage basin.

malaria
Noun

infectious disease caused by a parasite carried by mosquitoes.

massive
Adjective

very large or heavy.

meteorologist
Noun

person who studies patterns and changes in Earth's atmosphere.

migrate
Verb

to move from one place or activity to another.

minimize
Verb

to make smaller.

Mississippi River
Noun

(3,734 kilometers/2,320 miles) river in the central United States.

mold
Noun

type of fungi that forms on the surface of materials.

Noun

seasonal change in the direction of the prevailing winds of a region. Monsoon usually refers to the winds of the Indian Ocean and South Asia, which often bring heavy rains.

mosquito
Noun

insect capable of piercing the skin and sucking the blood of animals.

mudslide
Noun

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

natural disaster
Noun

an event occurring naturally that has large-scale effects on the environment and people, such as a volcano, earthquake, or hurricane.

Noun

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

non-porous
Adjective

not permeable by a substance such as air or water.

nonprofit organization
Noun

business that uses surplus funds to pursue its goals, not to make money.

Noun

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

outbreak
Noun

sudden occurrence or rapid increase.

overwhelm
Verb

to completely overpower.

particularly
Adverb

specifically.

pedologist
Noun

person who studies soil.

pesticide
Noun

natural or manufactured substance used to kill organisms that threaten agriculture or are undesirable. Pesticides can be fungicides (which kill harmful fungi), insecticides (which kill harmful insects), herbicides (which kill harmful plants), or rodenticides (which kill harmful rodents.)

piling
Noun

structure, usually made of metal or wood, hammered vertically into the ground to serve as a foundation or wall.

plague
Verb

to consistently bother, torment, or annoy.

power plant
Noun

industrial facility for the generation of electric energy.

predict
Verb

to know the outcome of a situation in advance.

primarily
Adverb

first or most important.

prone
Adjective

vulnerable or tending to act in a certain way.

Noun

division of a country larger than a town or county.

radically
Adverb

completely or extremely.

railway
Noun

stretch of railroad between two points.

Noun

liquid precipitation.

rapid
Adjective

very fast.

real estate
Noun

property and the business of buying, selling, and developing land.

rebar
Noun

metal bar, usually steel or concrete, used to reinforce concrete structures.

recede
Verb

to retreat or withdraw.

redistribute
Verb

to give away an amount of something in a different way.

reduce
Verb

to lower or lessen.

refugee camp
Noun

temporary shelters built for immigrants who have fled their homes due to environmental or social conflict.

remote
Adjective

distant or far away.

Noun

natural or man-made lake.

retreat
Verb

to go back to a familiar or safe place.

rice paddy
Noun

rice field.

riparian zone
Noun

area surrounding a river, stream, or other body of flowing water.

Noun

large stream of flowing fresh water.

riverine flood
Noun

process where a river or stream overflows its banks.

rock
Noun

natural substance composed of solid mineral matter.

rugby
Noun

team sport similar to soccer, but where players are allowed to carry the ball, block with the hands and arms, and tackle.

Noun

overflow of fluid from a farm or industrial factory.

sand
Noun

small, loose grains of disintegrated rocks.

sandbar
Noun

underwater or low-lying mound of sand formed by tides, waves, or currents.

sedge
Noun

grass-like plant native to wetlands.

Noun

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

seedling
Noun

young tree or other plant.

seep
Verb

to slowly flow through a border.

semi-aquatic
Adjective

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

severe
Adjective

harsh.

sewage
Noun

liquid and solid waste material from homes and businesses.

sewer
Noun

passageway or holding tank for liquid waste.

shrub
Noun

type of plant, smaller than a tree but having woody branches.

Noun

small sediment particles.

snowfall
Noun

amount of snow at a specific place over a specific period of time.

Noun

layers of snow that naturally build up during snowfalls.

soil
Noun

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

solvent
Noun

substance that dissolves another substance.

sophisticated
Adjective

knowledgeable or complex.

steel
Noun

metal made of the elements iron and carbon.

storm
Noun

severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

Noun

abnormal rise in sea level accompanying a hurricane or other intense storm. Also called a storm tide.

Noun

body of flowing fluid.

submerge
Verb

to put underwater.

subside
Verb

to return to a lower level.

subway
Noun

underground railway; a popular form of public transportation in large urban areas.

sugar cane
Noun

tall grass that is harvested to extract sugar from its sap or juice.

Noun

land permanently saturated with water and sometimes covered with it.

technology
Noun

the science of using tools and complex machines to make human life easier or more profitable.

Thames Barrier
Noun

flood-control project that protects London, England, from flooding of the River Thames, made of 10 moveable steel gates.

tourist
Noun

person who travels for pleasure.

toxic
Adjective

poisonous.

trigger
Verb

to cause or begin a chain of events.

tsunami
Noun

ocean waves triggered by an earthquake, volcano, or other movement of the ocean floor.

typhoid
Noun

infectious, sometimes fatal disease that harms the intestines. Also called typhoid fever.

Noun

developed, densely populated area where most inhabitants have nonagricultural jobs.

vegetation
Noun

all the plant life of a specific place.

Noun

small human settlement usually found in a rural setting.

vulnerable
Adjective

capable of being hurt.

wadi
Noun

deep channel or canyon, often dry except during flash floods. Also called an arroyo.

wading bird
Noun

bird with long, thin legs adapted for walking and feeding in shallow water.

Noun

movement of water between atmosphere, land, and ocean.

waterway
Noun

body of water that serves as a route for transportation.

wave
Noun

vibrations (oscillations) around a fixed location, usually involving a transfer of energy from one point to another.

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

Noun

the breaking down or dissolving of the Earth's surface rocks and minerals.

weather pattern
Noun

repeating or predictable changes in the Earth's atmosphere, such as winds, precipitation, and temperatures.

Noun

area of land covered by shallow water or saturated by water.

Noun

uncontrolled fire that happens in a rural or sparsely populated area.

Yu the Great
Noun

(220-2100 BCE) Chinese leader, engineer, and hydrologist.

Zuiderzee Works
Noun

(Netherlands) series of dams and drainage systems designed to isolate and dam the Zuiderzee, an inlet of the North Sea.