Floods are natural processes. Throughout history they have shaped the landscape, provided habitat for wildlife, and created rich soils. Cumulatively, floods have also been our nation's greatest disaster, disrupting lives, and often causing significant economic losses.
Floods happen when the water draining from a watershed, whether from rainfall or melting snow, exceeds the capacity of the river or stream channel to hold it. Water overflows onto the nearby low-lying lands called “floodplains”. In hilly and mountainous areas flooding is likely to be rapid, deep and dangerous. In relatively flat flood plains, land may stay covered with shallow, slow moving water for days or even weeks. Some coastal flooding may be caused by the storm surge associated with tropical storms and hurricanes.
Human activity often leads to flood damage. When people use flood prone areas along rivers, streams and the immediate coast, they do two risky things. First, their homes, businesses and activities get in the way of the natural overflow of the waterway. Sooner or later, they will be damaged or destroyed. Second, their buildings, pavement, landscaping, roads and other facilities take up space in the normal floodplain that is needed to carry extra water during a flood. This forces the flood water to move farther away from the natural waterway, flooding more land.
