Floodwater is water that overflows from rivers, lakes, or oceans and spreads across land that's normally dry. When heavy rains fall for days, or when winter snow melts too quickly, rivers can't hold all the water and it spills over their banks. The water then flows through streets, into basements, across fields, and into buildings where it doesn't belong.

Floodwater is particularly dangerous because it's not like the clean water from your tap. It picks up all kinds of hazardous things as it flows: sewage, chemicals, oil, sharp debris, and even wild animals. It can knock you off your feet with surprising force, and just six inches of fast-moving floodwater can sweep away a person or carry a car away. That's why emergency workers warn people never to walk or drive through floodwater, even when it looks shallow and calm.

After a flood, floodwater leaves behind mud, damage, and destroyed belongings. Cleanup takes weeks or months because everything the water touched needs to be dried, cleaned, or thrown away. Some communities build levees (protective walls) or create special areas where floodwater can spread safely without damaging homes. Throughout history, civilizations have both benefited from floods (like the annual flooding of the Nile River in ancient Egypt, which fertilized farmland) and struggled to protect themselves from flooding's destructive power.