Significant Floods in the United States During the 20th Century - USGS Measures a Century of Floods [open pdf - 1MB]
"During the 20th century, floods were the number-one natural disaster in the United States in terms of the number of lives lost and property damage. For more than 110 years the U.S. Geological Survey (USGS) has measured floods for the Nation's benefit while supplying additional streamflow data with its extensive stream-gaging network. Thirty two of the most significant floods (in terms of number of lives lost and (or) property damage) in the United States during the 20th century are listed according to the various types of floods. Internet sites for acquiring near-real-time streamflow data and other pertinent flood information are provided."
U.S. Geological Survey: http://www.usgs.gov/