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.