Rain is pouring hard and fast—more than eight inches in just an hour, turning river water brown with mud. Earthworms wiggle up to the ground as the soil becomes too wet for them. A flood might be coming.
Just about any place on Earth can experience flooding. When so much rain falls that the ground can’t absorb it or waterways can’t hold it, the overflowing water becomes a destructive force. In the United States, flooding causes more death and damage than tornadoes, hurricanes, or lightning.
How floods develop
During a rainstorm, precipitation—or the water that comes from rain or snow—goes to different places. Some of it flows into streams, lakes, or city water systems. Other precipitation evaporates and returns to the atmosphere.
But much of the rainwater is absorbed by soil. It flows through the top layer of the ground, to plant roots below the surface. This helps provides plants with the water and nutrients they need to grow. The excess water moves deeper into the ground through layers of dirt and rocks where until it becomes part of natural underground wells called groundwater.
A habitat can naturally absorb a healthy amount of rainfall it needs to thrive. But too much rain can cause lake and river levels to rise and overflow their banks, or the soil to become too wet to absorb more water. And though you might think extremely dry habitats would welcome too much rain, it turns out that the parched dirt can’t absorb the rain fast enough to capture it all.
Severe coastal storms, quickly melting ice and snow, and collapsed barriers (like concrete dams) can also cause floods. Even damaged beaver dams can lead to an overflow of water that the surrounding earth can’t absorb.
Different types of floods
Floods can form slowly over several days or overwhelm an area with little warning. How quickly a flood forms often depends on the habitat.
Slow-forming river flooding happens in regions called floodplains. These are large, flat areas of land along waterways with very shallow banks. When big storms hit, the water overflows the banks and spreads out across the plains.
This type of flooding isn’t always bad. It brings nutrients to the surrounding soil, making it fertile for growing crops. That’s why many people live in or near floodplains.
But too much water can destroy crops and damage homes. Rushing water can disrupt ecosystems by moving aquatic plants and animals to other habitats. And if flooding is so severe that it flows into local water treatment facilities, experts must monitor the incoming water to make sure the water is safe.
Another type of flooding is called a flash flood, when a flood happens within six hours of a heavy rainfall—as much as eight inches an hour. With no place to go, the moving water will tear through highways, valleys, and canyons, washing away vehicles, roads, bridges, and houses.
Quickly melting ice and snow can also cause flash flooding, especially when mountain snowmelt overflows the waterways below. On coastlines, hurricane rains cause water levels to rise, and the high winds push that water onto land. Called a storm surge, this also causes flash flooding.
Flooding in the future
Scientists believe that warming temperatures caused by climate change are increasing the risk of floods all over the world, especially in coastal and low-lying areas.
Warmer water changes the patterns of ocean currents, which changes global weather patterns. This means that some places will receive more rainfall than the ground can absorb. Other places will get less rain so the land will be drier—and unable to handle rainfall when it does occur.
Scientists think that climate change could also cause stronger hurricanes, with more rain and higher winds causing bigger storm surges. A warmer climate could also mean more snowmelt overwhelming the soil; melting polar ice could cause sea levels to rise and increase flooding. (Find out how you can help slow climate change.)
How to survive a flood
Experts might issue a flash flood watch if weather conditions are right. It doesn’t mean flooding will happen but that meteorologists want people to be prepared. When experts are sure a flash flood is on the way, they issue a warning so people can evacuate immediately. Here’s how to keep you and your family safe.
Before a flood
• Know your neighborhood. Research how close you are to streams, drainage channels, canyons, and any other low-lying areas that might flood.
• Keep emergency contact phone numbers in one place so you can get in touch with family members.
• Put together an evacuation plan and share it with everyone in your household. Know what paths and routes you can use to quickly get to a high, dry place.
• Conduct flood drills to practice evacuating quickly.
• Maintain an emergency kit with a three-day supply of food and water. Experts recommend canned or dried foods that don't need to be cooked, and at least one gallon of water per day for each person and pet.
During a flood
• Never wait for orders to leave; if you think a flood might be coming, evacuate immediately.
• If flooding hasn’t started, move important items to upper floors of your house.
• If flooding has already started where you are, move to the highest place you can find, like a roof of a house or car. Bring as much food, water, and sheltering blankets as possible.
• Never walk through flood water, especially if it’s moving. Hazardous chemicals, sewage, and even wild animals are often in floodwater, and just six inches of moving water can knock you off your feet.
• If you must walk through water to escape, walk where the water is still. Never touch electrical equipment if you are standing in water, or even if you’re wet.
• If water is quickly rising while you’re in your car, leave your vehicle and move to higher ground, like a hill or bridge.
After a flood
• If you’ve evacuated your house, return only after you’re given the all-clear from officials.
• If you stayed put, check your house for damage to electrical systems and appliances. Get rid of any food that might be contaminated from floodwater (or because it’s been unrefrigerated for awhile).
• Check your home for wild animals, especially snakes. They can get washed into your home with the floodwater.
• Ask an adult before you use water to brush your teeth, make food, or even wash your hands. Flooding can contaminate water and make you sick.
Learn more about floods at National Geographic.
Flood safety tips from the Nat Geo Kids book Extreme Weather by Thomas Kostigen