When water covers land that is normally dry, the result is a flood. Causes include prolonged heavy rain, rapid snowmelt, storm surge, or the failure of a dam or levee. Rivers may overflow their banks gradually, or water may rise quickly in smaller streams.

Floods damage homes, crops, and infrastructure. They also deposit fertile sediment in some settings while eroding soil in others. The depth, duration, and velocity of the water determine the severity of impacts. Even shallow moving water can sweep people and vehicles off their feet.

Mapping floodplains and building protective structures reduce risk, yet complete prevention is rarely possible. Real-time river gauges and precipitation forecasts provide the lead time needed for evacuation and property protection. Climate trends that alter rainfall intensity continue to influence flood frequency in many basins.

• Overflow of water onto dry land
• Caused by rain, snowmelt, or surge
• Damages property and disrupts life
• Managed through forecasts and infrastructure