A flood is an overflow of water on land. Sometimes a river might receive extra water, either from heavy rains or other natural disasters. When this happens, the water overflows from its normal path in the river bed and onto the dry land. This is called a flood. Flash floods happen quickly. Extreme flooding can also be caused by a tsunami or a large storm that will cause the sea to surge inland. The most deadly flood was in 1931 in China and it had killed about 2,500,000 people.
During a flood, people should move themselves and their most precious belongings to higher ground quickly. The process of leaving homes in search of a safe place is called evacuation. Floods occur at irregular intervals and vary in size, duration and the affected area.
Water naturally flows from high areas to low lying areas. This means low-lying areas may flood quickly before it begins to get to the higher ground.
Whenever there are more rains than the drainage system can take, they can cause floods.
Rivers can overflow their banks to cause a flood. This can happen when there is more water upstream than usual, and as it flows downstream to the adjacent low-lying areas there is a burst and water gets into the land.
Sea water can be carried by massive winds and hurricanes onto the dry coastal lands and cause flooding. This is made worse if the winds carry rains with themselves. Sometimes water from the sea resulting from a tsunami can flow inland to cause damage.
Dams are man-made structures mounted to hold water flowing down from a highland to a lowland. The power in the water is used to turn propellers to generate electricity. Sometimes, too much water held up in the dam can cause it to break and cause overflow in the area. Excess water can also be intentionally released from the dam to prevent it from breaking and that can also cause floods.
In cold regions, heavy snow over the winter usually stays unmelted for sometime. There are also mountains that have a ice cap on them. Sometimes the ice suddenly melts when the temperature rises, resulting in massive movement of water into places that are usually dry. This is usually called a snowmelt flood.