In many arid regions of the world the soil has very poor water retention characteristics, or the amount of rainfall exceeds the ground's ability to absorb water. When a rainfall does occur, it can sometimes result in a sudden flood of water filling dry streambeds known as a "flash flood."
Many rivers that flow through relatively flat land border on broad flood plains. When heavy rainfall or melting snow causes the river's depth to increase and the river to overflow its banks, a vast expanse of shallow water can rapidly cover these flood plains . The floods deposit silt on the flood plains, improving their fertility, which has throughout history attracted agriculture and other human development. In order to preserve these farms and cities, some rivers prone to flooding have had extensive and elaborate systems of dikes constructed along their shores and surrounding nearby cities. Unfortunately, by restraining flood waters, these dikes can result in much greater flooding upstream and in locations where they break. The control of annual flooding, by dikes and by dams, also prevents the deposition of silt on the rich farmlands and can result in their eventual depletion. The annual cycle of flood and farming was of great significance to many early farming cultures, most famously to the ancient Egyptians of the Nile river and to the Mesopotamians of the Tigris and Euphrates rivers.
Monsoon rainfalls can cause disastrous flooding in some equatorial countries, such as Bangladesh, due to their extended periods of rainfall. Hurricanes can cause two types of flooding. One is from the leading edge of the hurricane when it hits land causing the surf to rise as high as 40 feet above sea level (in some cases). The other is from large amounts of precipitation sometimes associated with hurricanes. Under some freakish events associated with heat waves[?], flash floods from quickly melting mountain snow has caused loss of property and life.
In geological or prehistoric times, several great floods are known or suspected to have occurred with varying amounts of supporting evidence. These include:
Ancient mythology makes several references to a great flood, of which the most well known is probably the Noachian deluge of Genesis. Another very similar version is given in the Babylonian account of the Epic of Gilgamesh, which apparently derives from a similar Sumerian account. The Sumerians also referred to a great flood in other texts, such as the Sumerian king list, and another remarkably similar to the Biblical version, suggesting that the Genesis account has drawn influence from the the older Sumerian depiction.
A large percentage of the world's cultures have stories of a "great flood". The strong likelihood is: a global climatic change in recent geological time brought about some large deluge. Evidence is mounting from ice-cores in Greenland that the switch from a glacial to an inter-glacial period can occur over just a few months, rather than over the centuries earlier research suggested.