It was 78 years ago, in an era far different from the present, that hundreds of thousands of flooded Mississippi River valley residents spent months in temporary housing. They received meals from the Red Cross. They cursed the federal government's failure to help. In many cases, they left their hometowns and started anew in strange places hundreds of miles away. Yet New Orleans itself was spared heavy damage. Still, the scope of the devastation from that Great Mississippi Flood may most closely resemble in American history what Hurricane Katrina has wrought over the past two weeks, and its impact still to come.

The 1927 disaster displaced, at least temporarily, nearly 1 percent of the nation's people, many of them sharecroppers. One effect was a spike in the Great Migration of rural black Southerners to Pittsburgh and other Northern industrial cities, which involved resettlement over decades of at least 5 million individuals.

Other calamities have uprooted Americans on a massive scale, most notably the 350,000 Okies who ventured west in the 1930s when their farmland turned barren in the Depression Dustbowl. Disasters such as the 1906 San Francisco earthquake or the Chicago fire of 1871 destroyed tens of thousands of homes, although residents of those cities didn't necessarily leave town.