There are few places on Earth where flooding is not a concern. Any area where rain falls is vulnerable to floods, though rain is not the only cause.
How floods form
A flood occurs when water inundates land that's normally dry, which can happen in a multitude of ways.
Excessive rain, a ruptured dam or levee, rapid melting of snow or ice, or even an unfortunately placed beaver dam can overwhelm a river, spreading over the adjacent land, called a floodplain. Coastal flooding occurs when a large storm or tsunami causes the sea to surge inland.
Most floods take hours or even days to develop, giving residents time to prepare or evacuate. Others generate quickly and with little warning. So-called flash floods can be extremely dangerous, instantly turning a babbling brook or even a dry wash into rushing rapids that sweep everything in their path downstream.
Climate change is increasing the risk of floods worldwide, particularly in coastal and low-lying areas, because of its role in extreme weather events and rising seas. The increase in temperatures that accompanies global warming can contribute to hurricanes that move more slowly and drop more rain, funneling moisture into atmospheric rivers like the ones that led to heavy rains and flooding in California in early 2019.
Meanwhile, melting glaciers and other factors are contributing to a rise in sea levels that has created long-term, chronic flooding risks for places ranging from Venice, Italy to the Marshall Islands. More than 670 U.S. communities will face repeated flooding by the end of this century, according to a 2017 analysis; it's happening in more than 90 coastal communities already.
People walk on a catwalk in a flooded St. Mark's Square during a period of seasonal high water in Venice, Italy, on October 29, 2018.
Impacts of flooding
Floods cause more than $40 billion in damage worldwide annually, according to the Organization for Economic Cooperation and Development. In the U.S., losses average close to $8 billion a year. Death tolls have increased in recent decades to more than 100 people a year. In China's Yellow River Valley some of the world's worst floods have killed millions of people.
When floodwaters recede, affected areas are often blanketed in silt and mud. The water and landscape can be contaminated with hazardous materials such as sharp debris, pesticides, fuel, and untreated sewage. Potentially dangerous mold blooms can quickly overwhelm water-soaked structures.
Residents of flooded areas can be left without power and clean drinking water, leading to outbreaks of deadly waterborne diseases like typhoid, hepatitis A, and cholera. (Learn here about flood preparation and safety tips.)
Flooding, particularly in river floodplains, is as natural as rain and has been occurring for millions of years. Famously fertile floodplains such as the Mississippi Valley, the Nile River Valley in Egypt, and the Tigris-Euphrates in the Middle East have supported agriculture for millennia because annual flooding has left tons of nutrient-rich silt deposits behind. Humans have increased the risk of death and damage by increasingly building homes, businesses, and infrastructure in vulnerable floodplains.
To try to mitigate the risk, many governments mandate that residents of flood-prone areas purchase flood insurance and set construction requirements aimed at making buildings more flood resistant—with varying degrees of success.
Massive efforts to mitigate and redirect inevitable floods have resulted in some of the most ambitious engineering efforts ever seen, including New Orleans's extensive levee system and massive dikes and dams in the Netherlands. Such efforts continue today as climate change continues to put pressure on vulnerable areas. Some flood-prone cities in the U.S. are even going beyond federal estimates and setting higher local standards for protection.