"Floods are the most chronic and costly natural hazard in the United States, causing an average of 140 fatalities and $5 billion damage each year (Schildgen, 1999). Despite advances in flood science and implementation of Federal hazard-reduction policies, damage from flooding continues to escalate (Pielke and Downton, 2000). Damage from floods results from a combination of the great power of flowing water and the concentration of people and property along rivers. In the United States, about 3,800 towns and cities of more than 2,500 inhabitants are on floodplains (Miller and Miller, 2000)."
U.S. Geological Survey Circular 1245
U.S. Geological Survey: http://usgs.gov/