When the Mississippi River spilled over its banks late last month in Mark Twain's boyhood home of Hannibal, Mo., it was the sort of flood the Army Corps of Engineers expects to occur once every 10 or 25 years.
If Hannibal could only be so lucky.
The town saw similar floods in 1995, 1996, 1998 and 2001. Worse still was the 200-year flood in 2008. But none of those compared to the devastation in 1993, when river gauges at Hannibal measured floodwaters at levels expected only once in five centuries.
Similar stories have emerged along the length of the Big Muddy. In recent week, heavy rains created a monstrous swell that barreled from the river's upper reaches toward the Delta, shattering flood records as it went.
"Flooding is getting more frequent and more severe," said Robert Criss, a professor of Earth and planetary sciences at Washington University in St. Louis.
Criss is among a group of experts who believe worsening Mississippi River floods are due to a combination of changing climate and federal water projects over a century that have turned the river into a superhighway for shipping.
Construction of locks, levees and weirs, critics say, has created a deep passageway for shipping that has unnaturally constrained the river, forcing floodwaters higher in times of heavy rain. Meanwhile, they say, warming temperatures are forcing more water into the atmosphere, leading to heavier rainfall.
Criss blames the Army Corps, which he says has "dangerously underestimated" flooding potential along the Mississippi, promoting risky flood-plain development and pushing the National Flood Insurance Program to the brink.
"You know, if some casino were telling you the odds on their slots were this far off, somebody would be after them," Criss said. "It isn't like it doesn't matter. It is fraudulent to be stating flood statistics that are so erroneous."
To be sure, statistics are a poor predictor of whether a flood will occur in any given year. The arrival of a "100-year" flood is no guarantee that a similar disaster won't appear for another 99 years, just as hitting a number on a roulette wheel is no guarantee that a the winner won't hit another winner on the very next spin.
The corps disputes criticism of its flood calculations, arguing that Criss and others fail to recognize that a "100-year" flood has a 1 percent chance of showing up any given year, and could show up several times in a decade.
"I can tell you the ways he's couching them, I don't think that's a fair way to depict it," said Mike Petersen, spokesman for the corps' St. Louis District.
The corps' projections are based on historical data, he said. Climate change and urban development patterns affect the river and force the Army Corps to constantly update models and projects to reflect changes.
"We're always updating the yardstick," Petersen said. "It's a changing climate and a changing world, and we recognize that. It's not something that's lost on the Corps of Engineers."
Looking for a culprit
Criss says it's the Army Corps that misunderstands the statistics.
In a 2008 study, Criss ran a statistical test on the flood projections at various points along the Mississippi and found a 99.9 percent chance that they were incorrect.
Flood frequency projections across the entire system, he said, are off by a factor of 10, meaning that 100-year flood events should be reclassified as 10-year events and that risk, insurance premiums and official flood zones should be recalculated accordingly and independently.
The Army Corps "are the last people in the world at this point who ought to be doing it," Criss said. "Somebody independent needs to be doing it now. Talk about asking the fox to re-guard the henhouse. They have no credibility."
Many water experts blame climate change for worsening floods.
Peter Gleick, president and co-founder of the Pacific Institute, a nonprofit think tank based in Oakland, Calif., said that while it's impossible to attribute any weather event to climate change, "I'm completely sure that we're changing the climate."
"I'm also confident that because we're changing the climate, there is human influence on every weather event," Gleick said. "That influence may be incredibly tiny for every single weather event, but I'd like to argue that it is no longer zero."
Gleick noted that parts of the Mississippi River Basin have recently received major inundations of rain at levels 300, 400 and 600 percent above normal. The total picture points to human influence outside the normal variability of the weather.
"In effect, we're loading the dice and we're painting higher numbers on them," said Gleick.
Criss contends that climate change is a minor player in the worsening floods along the Mississippi. He blames the levees, weirs and dams -- projects that Congress ordered the Army Corps to build.
"Fundamentally, we've changed the landscape of the Mississippi River Basin," said Andrew Fahlund, senior vice president of conservation for the advocacy group American Rivers. "We've basically developed all the way up to the edge and really, the water has no place to go but to run off and create these massive floods."
'Not a realistic goal'
Construction of fewer levees and a return to the use of flood plains, Fahlund and Criss argue, would allow that river to spread out when heavy rain causes it to swell.
But industry groups contend that's a pie-in-the-sky solution, since towns and cities have already been built behind the thousands of miles of levees along the Mississippi.
"The notion of returning nature to its pristine state along the rivers of this country is just not a realistic goal," said Amy Larson, president of the National Waterways Conference, a group that represents a broad array of shippers, dredgers and local levee boards along the Mississippi.
Fahlund concedes that flood plains that could be realistically reclaimed are "limited" but said that should not deter the government from attempting to purchase certain low-lying farmlands, to contract farmers to allow their land to be flooded during wet seasons, or to relocate outright some small, rural communities built in flood-prone areas.
"We need a combined approach," said Fahlund. "Levees need to be our last line of defense, not our only line of defense."