Flooding is caused in a variety of ways. Winter or spring rains, coupled with melting snows, can fill river basins too quickly. Torrential rains from decaying hurricanes or other tropical systems can also produce river flooding. Repeated heavy rain from thunderstorms over a period of weeks contributed to the Mississippi River Flood of 1993. Occasionally, floating debris or ice can accumulate at a natural or man-made obstruction and restrict the flow of water.
Water held back by the ice jam or debris dam can cause flooding upstream.
Subsequent flash flooding can occur downstream if the obstruction should suddenly release. Coastal flooding occurs when strong onshore winds push water from an ocean, bay or inlet onto land. This can take the form of surges associated with tropical storms and hurricanes, or can be associated with non-tropical storms such as nor'easters . Urban flooding may occur as land is converted from fields or woodland to more paved areas, losing its ability to absorb rainfall. Urbanization increases runoff 2 to 6 times over what would occur on natural terrain.