Science Question of the Week - natural disasters, floods - April 05, 2002

This spring marks the 75th anniversary of what great natural disaster in the US, and why was the damage so severe in some locations?

In the autumn of 1926, heavy rains plagued America's heartland. The precipitation didn't let up much during the winter, and by early spring, the soil was soaked from Minnesota to Missouri. The rains persisted into April, setting the stage for one of the most devastating, widespread floods in the history of the US.

Across the northern tier of states, snow steadily accumulated from December through February. When this snow melted in early spring, some of it was absorbed by the soil, but because the soil layers were already close to being saturated, most of the meltwater was flushed into streams and waterways leading to the Mississippi River.

Snowmelt flooding is an annual occurrence during the early spring for many watersheds of the upper Mississippi River basin. In some years, if warm spring weather quickly melts deep winter snows over the upper Mississippi River basin, snowmelt by itself is sufficient to significantly raise the level of the water in big rivers, like the Missouri, the Ohio and the Mississippi. In those years when snowmelt occurs coincident with heavy spring rains, disastrous flooding can be expected.

In late March of 1927, flood waters of the Mississippi and it's tributaries menaced the upper Mid West. By mid April, the flood crest had worked its way down to Mississippi and Arkansas. The "Big Muddy" rose to unprecedented heights after violent spring storms poured even more water into the already swollen river. The flood waters on the Mississippi were one mile (1.6 km) wide, approximately 100 feet (32 m) deep and were moving down-river at about 9 miles (14 km) per hour - an impressive clip for a river having such a shallow slope. As the crest approached, tributaries to the Mississippi actually flowed backwards since the volume of water passing by on the big river was much higher than the water level of the rivers feeding into it.

It's said that people who live in flood-prone areas along the Mississippi River fear only God and the Mississippi. They had a right to be fearful in the spring of 27'. On April 15, 1927 there was good, bad and ugly. The good was that this day was Good Friday, the bad was that the 15th was tax day (income taxes have to be paid on this date) and the ugly was that more than 10 inches of rain deluged areas on the lower Mississippi River near Greenville, Mississippi.

The Mississippi River flooded nearly the entire length of its course as levees busted apart in more than 100 places.The "Big Muddy" and its tributaries flooded more than 26,000 square miles (67,340 square km) in seven states, and in a few locations along its course, the Mississippi River became an 80 mile (128 km) wide inland sea. Nearly 700,000 people were evacuated or left homeless by flood waters. Although, 247 people were confirmed dead, the actual total of people who lost their lives in the flooding was over 1,000.

In contrast to the floods of 1927, the flooding of the Mississippi River in 1993 (the most recent great flood) was a summer event, crests occurred in June and July. Basically only the upper portion of the Mississippi River basin, north of the confluence with the Ohio River, experienced severe flooding. Forty seven people were killed during the 1993 flooding, and the area flooded was about 60% of the area inundated in 1927. The peak flows in 1993 were measured at about 1 million cubic feet per second, compared to almost 2.5 million cubic feet per second in 1927.

The "Father of Waters" (another name for the Mississippi River) drains part of 31 states and 2 Canadian provinces. The total length of the Mississippi and all of its tributaries approaches 300,000 miles (480,000 km), of which nearly 15,000 miles (24,000) are navigable! Ordinarily, for rivers that aren't bounded by levees, floodwater is permitted to spread out over its floodplain, which is the natural zone of land where rivers can dispose of their excess water. However, to protect villages, towns and cities from all of this water, a system of levees has been built up over the years.

The first levees were 3-foot dikes built by French pioneers in 1718 to protect the settlement of New Orleans. Many of the levees along the Mississippi have been built or rebuilt by the US Army Corps of Engineers. Some are more than 50 feet high (16 m). Levees help manage the flow of the river, but it's arrogant to believe that the Mississippi can be completely controlled. While levees and floodwalls protect people on the floodplain from most floods, they may not hold during the largest floods, with recurrence intervals of more than 100 years. During the 1993 floods, over 5,800 miles (9,280 km) of levees were damaged. About 17% of federal levees were damaged, but up to 77% of locally constructed levees failed - most failures occurred south of St. Louis.

Levees can fail from the tremendous water pressure exerted by floodwaters, or because of the weight of the water moving over top of them or because of an accident (a barge or ship running-aground). Whatever the reason, when they collapse or burst, the force of the river is then concentrated in a small geographic area. When levees broke near Greenville, Mississippi on April 22, 1927, twice the flow of Niagara Fall rushed inland, and 10 feet (3.2 m) of water swamped the city an its environs. In portions of the lower Mississippi basin, water remained above flood stage for 2 months!

In subsequent years, most noticeably January 1937 (also a rain and snow flood) and 1993, big floods have broken through levee systems, resulting in loss of lives and terrible damage. Although Greenville, Mississippi has been spared, thus far.

Interestingly, in 1927 the Red Cross was directed by the Secretary of Commerce, Herbert Hoover, to help clean up the flooding and to help provide for those who were suffering the floods consequences. The publicity generated by the massive relief effort was primarily responsible for propelling him into the White House in 1929 as our 31st president. Ironically, a few months later, a largely manmade disaster (the collapse of the stock markets) initiated the " Great Depression, " which pretty much insured that Hoover would be a one-term president.

For more about this see the PBS program American Experience "Fatal Flood."

Also see the September 1927 and the June 1937 issues of National Geographic Magazine.