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Floods can be defined as an overflow or downpour of water accumulating in an area where water is normally absent. Floods usually occur within a short period of time due to the soil's inability to absorb the water fast enough. According to the United States Geological Survey, floods were the natural disaster that caused the highest number of deaths and the most property damage in the United States during the twentieth century. Of all the natural disasters, floods are the most common and occur in the most places, with the only exception being fire. Flooding results in heavy currents that have the capacity to loosen structures and collapse foundations, destroying even the toughest of buildings.

The most common type of flood is the regional flood. Regional floods typically occur during the winter and spring months when the snow melts too rapidly or an excessive amount of water falls too quickly during spring rains or thunderstorms. Additionally, regional floods can result from tropical storms or hurricanes occurring along the coast or even far inland due to drastic changes in weather patterns. Floods can occur with no warning, but often occur over a period of days. If cold temperatures keep the ground frozen or the ground is already immersed with water, the water will run off into rivers . However, all too quickly the water rises above the banks of the rivers and flows onto dry land. Other types of floods include flash floods, ice-jam floods, storm-surge floods, damfailure floods, and debris, landslide , and mudflow floods.

Regional floods across the U.S. that have occurred since 1990, include the Trinity, Arkansas, and Red Rivers in Texas, Arkansas, and Oklahoma in April of 1990, each caused by recurring thunderstorms. The number of reported deaths was 17 and the approximate cost of damage was one billion dollars. In January of 1993, the Gila, Salt, and Santa Cruz Rivers in Arizona flooded due to persistent winter precipitation , causing 400 million dollars in damages with the number of deaths unknown. From May through September of 1993, the Mississippi River Basin in the central U.S. flooded due to excessive rainfall, causing 48 deaths and 20 billion dollars in damages. In May 1995, flooding occurred in the south central U.S. from recurring thunderstorms causing 32 deaths and over five billion dollars in damages. Winter storms in California killed 27 people and caused three billion dollars in damages between January and May of 1995. Torrential rains and snowmelt caused flooding in the Pacific Northwest and western Montana in February of 1996 and again between December 1996, through January 1997. This caused nine deaths and one billion dollars in damages and 36 deaths and over two billion dollars in damages, respectively. The Ohio River and its tributaries flooded in March of 1997, causing more than 50 deaths and 500 million dollars in damages from a slow-moving frontal system. Snow melting caused the Red River of the North in North Dakota and Minnesota to flood between April and May of 1997, causing eight deaths and two billion dollars in damages. In September 1999, Hurricane Floyd destroyed eastern North Carolina, causing 42 deaths and six billion dollars in damages.

Flooding does not always prove destructive. Occasionally, floods can be beneficial, leaving soil laden with minerals and organic matter from the debris carried by the flood. Annual flooding of the Nile River enabled agriculture to be the foundation for Egyptian civilization. At the same time each year, the Nile River would flood, providing enough water to the soil to make lands fertile. With little rainfall, the Egyptians were dependent on the annual flooding to sustain their agriculture. Accordingly, Egypt's soil containing minerals and organic debris was a result of river sediment brought by the yearly floods.

See also Debris flow; Mud flow; Sedimentation

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