In the latter part of August 1926, the sky darkened over much of the central United States and a heavy rain began to fall. Rain pelted first Nebraska, South Dakota, Kansas and Oklahoma, then edged eastward into Iowa and Missouri, then Illinois, Indiana, Kentucky and Ohio. The great storm lasted for days. It was followed by another low-pressure system heavy with moisture moving up the Mississippi Valley and pouring precipitation over this same region. And then another.

On September 1, water poured over the banks of dozens of streams and flooded towns from Carroll, Iowa, to Peoria, Illinois, 350 miles apart. On September 4, floods deluged much of Nebraska, Kansas, Iowa, Illinois and Indiana, killing four people. The Mississippi River rose rapidly in the upper Midwest and washed out bridges and railroads. A few days later another storm brought flooding to towns from Terre Haute, Indiana, to Jacksonville, Illinois. Seven people died. More rains fell. On September 13, the Neosho River rose to record heights and roared through southeastern Kansas, killing five. In Iowa, 15 inches of rain fell in three days.

Through September and October, floods in Nebraska, South Dakota, Oklahoma and elsewhere were the greatest ever. At Vicksburg, Mississippi, the river gauge had never exceeded 31 feet in October. That month, it topped 40 feet.

The Mississippi River is not simply a stream that begins in Minnesota and runs south to the Gulf of Mexico. Its fingers stretch from near New York and North Carolina in the east to Idaho and New Mexico in the west. All fall, the rains continued over almost this entire basin. On December 13, in South Dakota the temperature fell 66 degrees in 18 hours, followed by an intense snowstorm. Helena, Montana, received 29.42 inches of snow. In one day, 5.8 inches of rain fell on Little Rock, Arkansas. The Cumberland River rose to the highest level ever recorded and flooded Nashville. The Tennessee River flooded Chattanooga, killing at least 16 and making thousands homeless over Christmas. On New Year’s Day, the Mississippi itself went above flood stage at Cairo, Illinois, and would stay above flood stage for 153 consecutive days. In January, Pittsburgh flooded, Cincinnati flooded, Louisville flooded. To the west, outside Oklahoma City, 14 Mexican migratory workers drowned.

All spring the rains continued, punctuated by five separate storms, each greater than any storm in the preceding ten years. The largest came on Good Friday. From 6 to 15 inches of rain poured down over an area in excess of 100,000 square miles, north into Missouri and Illinois, west into Texas, east almost to Alabama, south to the Gulf. New Orleans received the greatest rainfall ever recorded there; in 18 hours, 14.96 inches fell. Still, New Orleans did not flood, chiefly because the river had broken levees hundreds of miles upstream, so the flood crest never reached the city.

It would become, until Katrina, the nation's greatest disaster. But the story of the 1927 flood, like that of Hurricanes Katrina and Rita, is not simply a story of nature's force. As Gifford Pinchot, then governor of Pennsylvania and a pioneer environmentalist, said after surveying the 1927 destruction, "This isn't a natural disaster. It's a man-made disaster."

Since before the Civil War, the U.S. Army Corps of Engineers had regulated the Mississippi River, and since the 1880s had insisted upon a "levees-only policy"—that is, a belief that levees alone were all that was needed to control flooding. The corps believed that if the quantity of water in a river increases, the current will accelerate. This is generally true. The theory also assumed that a faster current would scour the riverbed more than a slower current, and thus deepen the river. This is also true. But the corps further concluded that such scouring would deepen the river enough to accommodate even a huge flood. This was not true. In fact, all the scientific data about the river up to that time—most of it collected by the corps itself—contradicted that assumption. Nonetheless, the corps opposed building spillways and floodways to let water out of the river, and it had closed off natural reservoirs to maximize the amount of water in the river.

Then, in 1927, the disaster that critics of the corps had long expected finally arrived.

When it was over, the Mississippi River and its tributaries had killed people from Virginia to Oklahoma, flooding the homes of approximately 1 percent of the U.S. population. At its widest point, north of Vicksburg, Mississippi, the river became an inland sea nearly 100 miles across. No one knows the death toll; officially, the government said 500 people died, but a disaster expert who visited the flooded area estimated that more than 1,000 perished in the state of Mississippi alone. The Red Cross fed roughly 650,000 for months, many for a year; 325,000 lived in tents for months, some of them sharing an eight-foot-wide crown of a levee—the only dry ground for miles, with flooded land on one side and the river on the other, their hogs, mules and horses in tow but not their dogs, which were shot for fear of rabies. The worst of the flooding occurred in April and May. Not until September did the floodwaters drain from the land.

The devastation left a legacy of change far beyond the flooded regions—changes that are still being felt today. The first involved the river itself. The 1927 flood ended the debate over the levees-only policy and forced engineers all over the world to look at rivers differently. Most recognized they could not dictate to a great river; they could only accommodate its awesome power.

Since 1927, the lower Mississippi has not burst the levees, although it came dangerously close in 1973 and 1997. But containing the river had an unintended consequence. Previously, the river had deposited so much sediment that it actually created all the land from Cape Girardeau, Missouri, to the Gulf of Mexico. With the levees preventing periodic flooding, the sediment no longer replenished south Louisiana. And the land began to sink, making it more vulnerable to hurricanes. The loss has been greatly worsened by pipelines and shipping channels that cut through the vast marsh and speed erosion.

The 1927 flood's political and social consequences were possibly even more significant than its environmental legacy. The flood made Herbert Hoover president of the United States. A logistical genius, Hoover had already earned the nickname "the Great Humanitarian" for overseeing the distribution of food in occupied Belgium before the United States entered World War I. After the war, back in the United States, he ran food programs for Europe. In 1927, Hoover was Secretary of Commerce, and President Calvin Coolidge put him in charge of the rescue, care and rehabilitation of nearly a million people. He seized the opportunity. The flood crest took weeks to snake down the Mississippi River, giving the press the chance to cover battle after battle to hold the levees. It made for a bigger story than Katrina. In all of this, Hoover performed masterfully—organizing rescue fleets and displaced persons camps as well as the delivery of food and supplies—and he made sure everyone knew it. "The world lives by phrases," he once said. Portrayed as a hero in papers all across the nation, he confided to a friend, "I shall be the nominee, probably. It is nearly inevitable."

Hoover's presidential campaign began the shift of African-Americans from the Republican Party to the Democratic. The press had created Hoover's candidacy, and a potential scandal was brewing about abuses of—and virtual slavery imposed on—blacks in some of the refugee camps that he oversaw. This would have undermined Progressive support for him, threatening his candidacy.

How could he head off the scandal? There is irony in the answer. Since the South was then solidly Democratic, few whites were active in GOP politics, leaving the party of Lincoln in the hands of African-Americans throughout the region. Although blacks could not vote in most elections in the South, they could do so at the Republican National Convention. Hoover, both to protect himself from the abuse charges and to secure core delegates, in essence reached a deal with the national African-American leadership. He named a "Colored Advisory Commission" to investigate the abuses, and in return for the commission whitewashing the scandals and supporting his candidacy, Hoover promised to break large plantations into small farms and turn sharecroppers into owners. Robert Moton, head of the commission and the Tuskegee Institute, said this would be "the greatest boon to the Negro since emancipation."

The blacks kept their word; Hoover broke his. This very personal betrayal snapped the emotional connection between the national African-American leadership and the GOP, and made it easier for Democrat Franklin Delano Roosevelt to attract black support for his policies four years later.

The 1927 flood also changed the face of many cities. The black migration out of the South had begun in World War I, but slowed to a trickle in the 1920s. In the flood's aftermath, hundreds of thousands of African-Americans moved from the flooded region to Detroit, Chicago, Los Angeles and elsewhere. In the 1930s, this migration dwindled, and did not pick up until after World War II and the mechanization of agriculture.

But the most important and most subtle change generated by the flood involved the way Americans viewed government. Before the flood, Americans generally did not believe government had a responsibility for individual citizens. Consider the yellow fever epidemic that had struck New Orleans in 1905: U.S. public health officials would not help New Orleans until the city put up $250,000—in advance—to cover federal expenses. Americans accepted this. Likewise, when a 1922 flood left 50,000 in Louisiana homeless, Governor John Parker, a close friend of Hoover's, refused not only to tap the federal government for help, he declined even to ask the Red Cross, declaring, "Louisiana has not asked for aid and will not."

Though the federal government in 1927 had a record surplus in its budget, not a dollar of federal money went in direct aid to any of the one million flood victims. (Hoover established private reconstruction corporations—they were failures.) The only money that the U.S. government spent was on supplies and salaries for military personnel who participated in the rescue.

But Americans believed that the federal government should have done more. John Parker, no longer governor but then in charge of helping the 200,000 homeless in Louisiana, reversed himself and desperately sought all the outside help he could get. Across the nation, citizens demanded that the federal government take action. The sentiment became concrete a year later, when Congress passed the 1928 Flood Control Act, a law that would cost more than anything the government had ever done except fight World War I; the law would also set a precedent by giving the federal government more authority to involve itself in what had been state and local government decisions.

Today, many people are wondering if Hurricanes Katrina and Rita will have a similarly large impact on American life. Clearly, they will in some areas. Government on all levels will reexamine its ability to respond. Designers of major projects will give environmental forces a higher priority. Population will shift at least regionally, permanently affecting such cities as Jackson and Houston, not to mention New Orleans, Baton Rouge and Lafayette, and possibly extending to Atlanta and points in between. Political pressure to address global warming will likely increase, since most experts believe that a warmer Gulf of Mexico means, at the very least, more intense hurricanes.

But will Katrina and Rita change the way Americans think about even larger questions? The storms, like the 1927 flood, ripped open the fabric hiding some of the most disquieting parts of American society. It made George W. Bush sound almost like a liberal Democrat when he spoke of the "legacy of inequality" and said "poverty has roots in a history of racial discrimination, which cut off generations from the opportunity of America. We have a duty to confront this poverty with bold action."

If it is too early to tell what the largest long-term effects of these hurricanes will be, clearly it has rekindled the debate, begun during the flood of 1927, over the federal government’s responsibility to citizens.