Mississippi Flood - 60,000 square kms of the lower Mississippi River valley is submerged, displacing hundreds of thousands of people and killing 250
The great Mississippi flood of 1927 was the most destructive inundation in the history of the US. River levels had been rising since summer 1926, with some flooding of upstream areas. Then in April 1927 heavy rain fell, and by May cracks or “crevasses” were appearing in levees towards the Mississippi delta itself.
The levees broke. There were often arguments over whether to allow a particular levee to break naturally or dynamite it. Blasting a gap prevented the entire levee being washed away, and reduced the risk of flooding upstream.
New Orleans was saved by destroying levees, at the expense of other parts of Louisiana. Such decisions were often in the hands of property owners rather than those who would be most affected. Farmland, in an area stretching to 27,000 sq miles, was left under water on this occasion. About a million people were flooded out of their homes – 1%of the entire US population.
The official death toll was only a few hundred, but this was probably a severe underestimate. The deaths of many African Americans went unrecorded.
The 1927 flood changed the way rivers were managed. The US army corps of engineers, who built the levees, had previously insisted that embankments alone were sufficient; afterwards planned floodways or spillways were included as safety valves to relieve pressure on the levees.
The flood also had a significant social impact. Hundreds of thousands of homeless African Americans trekked north, leaving the land in search of industrial work and a better life.
Smithsonian - The Great Mississippi Flood of 1927 Laid Bare the Divide Between the North and the South
National Geographic - Man vs. Nature: The Great Mississippi Flood of 1927