Flooding is the result of an area's water system, such as a river or tributary, receiving more water than it can hold within its banks. The overflow can be viewed either negatively or positively, based on the circumstance. Flooding can cost millions of dollars in property damage in addition to the costs of temporary and sometimes permanent displacement of human populations. In some cases, flooding of populated areas contributes to the spread of toxic materials, such as gasoline and pesticides, into the water system.
On the other hand, floods can provide valuable irrigation to crops and arid lands. The seasonal overflow of the Nile River in Egypt is one example of beneficial flooding; the Nile's waters enrich the soil with nutrients and quench the thirst of crops growing along its banks.
Heavy precipitation is the principal cause of flooding. Other factors include spring runoff after winters of heavy snowfall and inadequate drainage. In coastal areas, unstable weather conditions, such as hurricanes, are a seasonal concern. As well, tsunami waters pose a flooding threat after underwater earthquakes in the adjacent water bodies create tidal waves that advance to the shore.
Human populations have been drawn to these locations for their scenic attractions; the costs of this migration have increased through time as more people choose to live in coastal areas. Even in noncoastal areas, such as the central plains of North America, the choice of people to build their homes in natural floodplains has contributed to the extent of flooding.
Deforestation has been a major contributor to recent flooding disasters. The loss of trees and other vegetation has encouraged soil erosion and the level of adjacent water bodies has increased through runoff. Global warming, caused by the increase of carbon dioxide and other greenhouse gases in the atmosphere, has caused the melting of the global icecaps and added to the threat of flooding across the planet.
Urbanization of large areas of the world is another factor; unable to penetrate the concrete of modern cities, runoff precipitation collects in populated areas, causing problems for residents.
On a local level, governments have created strategies of coping with some forms of flooding. For populations living in flood plains, the construction of dikes and flood ways has helped relieve the problems residents often face annually. Reforestation of open areas has decreased the level of soil erosion and added to the natural defenses against flooding.
However, one of the principal ways of dealing with global flooding would be a decrease in greenhouse gas emissions. So far, many nations have been cautious in their approach to this issue, concerned that such measures would negatively affect their industrial economies.