Floods occur when the volume of water contained by the shores of a body of water or the banks of a waterway exceeds the body's capacity. The water moves above and outward from the typical area it occupies. The major causes of floods are heavy rains, melting snow or a blockage of water flow in a river.
Many factors come together to cause a flood. In every case, a flood is caused by an excess of liquid water beyond the environment's normal ability to carry it. The two places that water drains on land are into bodies of water and into the ground itself. This latter capacity of the ground to absorb water is very important, especially during rains. Typically, land that receives a large amount of rain also has a large ability to absorb it, preventing flooding up to its capacity. Once rain exceeds this capacity, however, it cause rivers and lakes to swell, and if they swell beyond their natural or artificial barriers, they spill out onto the land.
The capacity of land to absorb water is limited. Some places, particularly near mountains, receive water too quickly to be absorbed by the land. Such floods occur because of melting snow or storms in nearby mountains.