Floods are usually harmful to people and homes. Floods are sometimes helpful by making the land very fertile. Seacoast floods usually kill hundreds of thousands of people, and destroy millions of homes. Seacoast floods usually result from hurricanes and big storms. A giant sea wave caused by an earthquake is called a Tsunami.
Most rivers overflow once every two years.
Too much rain at one time sometimes floods rivers. Sudden melting of snow and ice does too. When this happens the riverbed has ten times as much water as the river can hold. Heavy rain or thunder storms usually cause flash floods. They mostly occur in the mountains.