What Are Floods

A flood happens when water covers land that is normally dry. Floods are the most common natural hazard in the world—a dangerous natural event that can harm people and property. Understanding floods helps communities prepare and stay safe when too much water arrives.

Heavy rain is one major cause of floods. When a lot of rain falls, rivers and streams can overflow their banks. The water spills onto nearby land, covering roads, fields, and neighborhoods. Melting snow can also cause floods. When winter snow melts quickly in spring, it sends too much water downstream all at once.

Flash floods are especially dangerous because they happen suddenly. A flash flood occurs when a large amount of rain falls in a short time, like during a thunderstorm. The water rushes through streets, neighborhoods, and low-lying areas with little warning. Flash floods can happen in minutes, giving people very little time to move to safety.

Floods cause serious damage to communities. They can destroy homes and wash away roads and bridges. Floodwater often carries dirt, chemicals, and waste that contaminate—or pollute—drinking water supplies. Floods make farmland unusable by covering crops with water and mud. Even a few inches of moving water can knock a person down, making floods dangerous to anyone caught in them.

Interesting Fact: The fastest-moving floodwaters can travel at speeds up to 25 miles per hour—faster than most people can run!