The situation where the presence of a large amount of water in the wrong place causes property and/or human losses. Floods are one of the most dangerous and devastating natural disasters known to mankind. While early warning systems have improved since the time of Noah, increased competition for land resources have made more people vulnerable to flooding.

Not much water is required to do damage, as anybody who has noticed the effects poorly maintained plumbing can do may testify. A mere four inches of water will mess up your carpet. Six inches of water flowing swiftly can knock you off your feet. Cars can be carried away with two feet of flood water. Aside from putting lives at risk through drowning and landslides, floods have many indirect effects that are hazardous to human health, like contaminating water supplies.

Most floods are caused when weather patterns produce more rain than a region can handle, either through channeling the water out to sea through rivers, or by being absorbed by the soil. Water naturally flows downhill, feeding into streams and then rivers. As a river collects more water, it is liable to burst its banks, spilling water into surrounding low-lying flat areas, called flood plains. Debris can block river systems, causing more water to be diverted.

There are different kinds of floods:

River floods , where a river's banks perennially burst every year, are the most common type of flood. Many communities around the world expect their local river to seasonally ]flood]; indeed wet rice farming is dependent upon a crop being saturated with water in the wet season.
Coastal floods happen when huge amount of ocean water is deposited inland usually by off-shore winds like hurricanes and tropical storms, or occasionally by tsunamis.
Flash floods are frighteningly swift floods that can occur in narrow gullies or creek beds (arroyos) in barely minutes.
Urban floods occur in cities where the transformed terrain prevents water from being drained adequately (and instead goes into people's cellars).
Ice Jams are floods caused by ice obstructing the flow of water, diverting it into a flood plain.
Dam Bursts are the consequences of dams or locks being accidentally or deliberately damaged.

The worst flood in history occurred in September 1887 when the Yellow River in China burst its banks and killed over 900,000 people. Another flood in 1939 claimed around half a million Chinese lives, and recent floods have caused considerable damage.