Each day brings news of more levees overtopped or breached. Some are smaller agricultural levees built to 30- or 40-year flood standards, while others are higher, designed to protect river towns.
Already, the flood seems likely to equal or exceed the 1993 floods that wreaked havoc in these states, taking 48 lives and causing more than $20 billion in damage. On Thursday, President Bush was expected to tour flooded Iowa counties.
The news of so many levees overtopping or breaching can come as a shock to residents who felt safe behind their walls.
But experts say it's hardly surprising, especially given the low standards to which most levees are built.
To qualify for the National Flood Insurance Program, structures simply need to be behind a levee built to a so-called 100-year standard, meaning there is a 1 percent chance in any given year that a flood will rise above the levee. In the Netherlands, on the other hand, levees for ocean flooding are built to a 10,000-year standard, and inland levees are designed at least to a 250-year standard and usually in excess of 1,250 years.
"Around the world, the 100-year standard is a joke," says John Barry, author of "Rising Tide," his book about the Mississippi River flood of 1927, and a member of a flood control authority that oversees six levee districts in metropolitan New Orleans. "We invest on the cheap."