Flood, flow of water from a river, lake, or ocean over normally dry land. Most floods are destructive to homes and other property, and can leave the land barren by carrying off topsoil. Some can be helpful: the annual floods of the Nile River in Egypt made the desert land of the Nile Valley fertile by depositing rich soil carried from far upstream. River floods, which are the most common type, are usually caused by heavy rains and sudden melting of ice and snow. Ocean floods are most often caused by hurricanes or other strong storms that raise the height of waves and push water far inland. Floods also occur when wave heights are raised by earthquakes or volcanoes. Abnormally high tides can also occasionally cause flooding. Lakeshore flooding is also usually caused by strong storms.