The Great Mississippi Flood of 1927

April 15, 1927:

The Great Mississippi Flood of 1927, the most destructive river flood in U.S. history, begins.
The flood began when heavy rains pounded the central basin of the Mississippi in the summer of 1926. By September, the Mississippi's tributaries in Kansas and Iowa were swollen to capacity. On Christmas Day of 1926, the Cumberland River at Nashville topped levees at 56.2 feet (17 m), a level that remains a record to this day, even exceeding the devastating 2010 floods.
Flooding overtook the levees causing the Mounds Landing to break with more than double the water volume of Niagara Falls. The Mississippi River broke out of its levee system in 145 places and flooded 27,000 square miles (70,000 km²). This water flooded an area 80 km (50 mi) wide and more than 160 km (99 mi) long. The area was inundated up to a depth of 30 feet (10 m). The flood caused over $400 million in damages and killed 246 people in seven states.
The flood affected Arkansas, Illinois, Kentucky, Louisiana, Mississippi, Missouri, Tennessee, Texas, Oklahoma and Kansas. Arkansas was hardest hit, with 14% of its territory covered by floodwaters. By May 1927, the Mississippi River below Memphis, Tennessee, reached a width of 60 miles (97 km).
On April 15, 1927, 15 inches (380 mm) of rain fell in New Orleans in 18 hours. More than 4 feet (1.2 m) of water covered parts of the city, and influential bankers in town met about how to guarantee the safety of the city, with the scale of flooding upriver already known. A few weeks later, about 30 tons of dynamite were set off on the levee at Caernarvon, Louisiana and sent 250,000 ft³/s (7,000 m³/s) of water pouring through. This was intended to prevent New Orleans from experiencing serious damage, but flooded much of St. Bernard Parish and all of Plaquemines Parish's east bank. As it turned out, the destruction of the Caernarvon levee was unnecessary; several major levee breaks well upstream of New Orleans, including one the day after the demolitions, made it impossible for flood waters to seriously threaten the city.

An excellent book on the flood is Rising Tide: The Great Mississippi Flood of 1927 and How it Changed America.  It is fascinating.