Detailed Findings

There are three categories of floods: flash flooding, river flooding, and coastal flooding.

The most lethal of these are flash floods, such as the one that occurred in 1972 in South Dakota that claimed 238 lives. The factors that contribute to flash floods are rainfall intensity and duration. Topography also plays a role — if the soil is too sodden with moisture, it may not be able to absorb rainfall. Flash flooding also can lead to mudslides, particularly in western states. Steep ravines and canyons can funnel high volumes of water, exacerbating flood intensity, as was the case in Colorado in 1976 in Big Thompson Canyon.

Other reasons for flash flooding are the rupture of a dam, slow-moving storms, or a surge of water released from an ice jam.

Some of the most severe flooding in our nation’s history, such as the disastrous event in 1927, occurred in areas around the Mississippi River. Hurricanes that have ravaged Florida and the Gulf Coast have brought with them storm surges that caused record damage in Louisiana in 2005 with Hurricane Katrina and in Texas with Hurricane Harvey.

Climate change has raised concerns of increased flooding and the impact that flooding from rising sea levels will have on those living on American coasts. Climatologists are concerned that melting snowfields and polar ice caps will cause more runoff and severe weather events, such as storm surges and coastal and river floods. Scientific advances, including better observational data, improved climate models, and more sophisticated detection methods have made it possible to attribute extreme weather events to rising global temperatures.

In some of these catastrophic flood events, human error, neglect, or miscalculation contributed to the scale of the disaster. In the case of the St. Francis Dam in Los Angeles in 1928, structural defects in the dam led to its collapse just two years after it was completed. At least 400 people were killed from the ensuing flood, and the calamity is the second greatest disaster in California history.

The good news is that humankind has become better at understanding the dynamics behind floods, and the use of advanced weather forecasting and satellite tracking can provide early warning. Evacuations ahead of recent hurricanes in Louisiana and Texas helped save lives.

Methodology

24/7 Wall St. compiled a list of the worst floods in U.S. history by the number of fatalities. Floods occur for various reasons — dams bursting, intense rainfall, massive water runoff from snowmelt, storm surge from hurricanes, even impact from tornadoes. In order to capture the scope of these tragic historical chapters in our nation’s history, we tried to include any and all weather-related events that could have contributed to floods. 24/7 Wall St. used government sources such as the National Weather Service Weather Forecast Office, The National Oceanic and Atmospheric Administration, and the United States Geological Survey as well as media reports about the flood events to create a list of the worst floods in U.S. history. The cost of damage was rendered in inflation-adjusted dollars.