Rivers, lakes, or seas may flood the land. River floods are more common, though lake and seacoast floods can be more serious. However, sometimes floods may be helpful. For example, the yearly floods of the Nile River built up the plains of Egypt and made the Nile Valley one of the most fertile regions in the world. These floods brought fertile soil from lands far to the south and deposited the soil on the Egyptian plains.
Most rivers overflow their normal channels about once every two years.
When a river overflows land where people live, it causes a flood. When a river overflows land where people do not live, it is said to be in flood. Common causes of river floods include too much rain at one time and the sudden melting of snow and ice. Under such conditions, rivers may receive more than 10 times as much water as their beds can hold. Heavy rains, sometimes from thunderstorms, can produce flash floods if small rivers or streams rise suddenly and overflow.