How a flood remade a flood

When the rains began to fall in northern Missouri during the first weekend of October, forecasters at a special center outside Kansas City grew worried.

It's their job to predict water levels on the Missouri River. The rains covered such a wide area that flooding looked inevitable. On Oct. 4, the Missouri Basin River Forecast Center issued its first alert: The Missouri would crest in Jefferson City, 4 feet above flood stage. More rain fell that night so by the following morning, the center had upped its forecast to 12.9 feet above flood stage.

But when the crest came a few days later, it exceeded flood stage by only 7 feet. The city saw closed roads and washed-out bridges but not the terrible inundation that had been predicted. What happened? The great flood of 1993 and flooding two years later scoured and reshaped the river and the great basin that drains into it. In places, levees haven't been rebuilt, allowing excess water to flow into flood plains. Creeks have changed course on their way to draining into the Missouri.

Since flood prediction depends on guessing not only how much water is flowing in but also when and where it will reach a certain point, the forecast center has had to throw out some old assumptions. "The routings have changed, and the ratings have changed. But we don't know how much," says Larry Black, the center's hydrologist in charge. "We're really shooting in the dark on some of these."

Eventually, the hydrologists will have enough experience with the new basin that their accuracy should improve. But if the river's flow is allowed to become more natural - as some environmentalists suggest it should - then it will also become more unpredictable. "It will make [forecasting] more difficult," Mr. Black says. "But we'll flat have to address it."