Why Mississippi Floods Were Expected

     A combination of bad weather, ocean conditions and
     land development conspired to produce high waters.

Richard A. Lovett
Published online 13 May 2011 | Nature |
doi:10.1038/news.2011.289
http://www.nature.com/news/2011/110513/full/news.2011.289.html

Last year, it was Pakistan and Russia. This spring, all
talk of disasters attributable to freak weather
conditions turns eyes to the United States.

First, it was snowfalls that never seemed to end. After
that came tornadoes. Now, a massive slug of water is
working its way down the Mississippi River, forcing the
US Army Corps of Engineers to deliberately flood
farmland to spare riverside towns such as Cairo in
Illinois, and threatening near-record water levels all
the way to New Orleans. Nature looks at the underlying
causes of these extreme events, and how the surge might
have been predicted.

Why did it happen?

The simple answer is because it rained. A lot. Parts of
the US Midwest reported rainfalls up to four times the
norm in April. And that came on top of a winter that
saw some regions receiving unusually high snowfalls.

But that's only part of the answer. For decades, people
have been building shopping malls and parking lots that
cause water to flow quickly into rivers, rather than
soak into the ground. They've built levees that
constrict the flow of rivers, forcing water to travel
downstream faster. In places, this has been referred to
as a 'levee war', whereby one town's levees funnels
water downstream to become the next town's crisis.

"People don't realize how dramatically humans have
altered many of these river systems," says Len Shabman,
an economist at Resources for the Future, a think tank
in Washington DC.

But the much-publicized diversion of water into
Missouri farmlands to spare Cairo was actually a
success, Shabman adds. "That was always part of the
plan," he says. The federal government long ago
purchased easements - the right to flood the land -
from the farmers who own it, precisely for this
purpose. "The farmers may not have remembered they had
an easement," Shabman says. "But they were there."

Has anything like this happened before in the United
States?

Yes. The greatest flood of the twentieth century
occurred in 1927, but there were also large floods in
1937, 1973, 1993 and 2008, although only the 1927 flood
compared to this year's.

"This is the blessing and curse of farmers in the
American Midwest," says Bill Patzert, a climatologist
at NASA's Jet Propulsion Laboratory in Pasadena,
California. "They're blessed with rich farmland and the
rivers that irrigate it. The downside is that sometimes
they overflow."

Could this have been predicted?

Of course. Large snowfalls and heavy spring rains are a
classic formula for flooding. All of the water has to
go somewhere.

"By January or February, everybody should have known we
were going to have May floods," Patzert says. "To be
shocked and awed by these kinds of events is
disingenuous. It means you haven't read your history."

But that's only after the snow and rains hit.
Forecasting the weather patterns that produced them is
still a science of the future.

It may not be so very far away, however. Even before
the storms hit, a research group led by Upmanu Lall at
Columbia University in New York had been trying to
correlate a century's worth of floods in the Midwest to
continent-wide weather patterns.

What they found, Lall says, is a surprisingly
consistent pattern whereby a pair of high-pressure
systems - one over western Texas and another off the US
Atlantic coast - conspire to force moisture inland from
the Gulf of Mexico "like a funnel".

It is possible, he adds, that these persistent high-
pressure zones may be produced by two well-known
oceanographic patterns: La Nina and El Nino in the
Pacific Ocean (which mark alterations in warmer and
cooler conditions between that ocean's eastern and
western equatorial waters) and the North Atlantic
Oscillation (which results from weather patterns
between Iceland and the Azores).

If so, he says, it may someday be possible to predict
weather patterns likely to produce flooding in the
Midwest, perhaps 30-90 days in advance.

So why were people taken by surprise?

Partly because conditions have changed since 1927. The
population has soared and urban development has
encroached onto many areas that were once farmland.
There are simply a lot more people, and a lot more
infrastructure, in harm's way.

Nicholas Pinter, a geologist at Southern Illinois
University in Carbondale, who works on flood hydrology,
has a word for this: "hydro-amnesia". It causes people
to build in places that were flooded a generation ago
and will be flooded again a generation hence.

"In 1927, everyone had a boat," Patzert adds. "They
knew it was coming. One thing I noticed about this
particular flood was that all these people living in
harm's way didn't have boats in their backyards."

Did global warming play a part?

Maybe, but not a big one. In Northern Europe, Pinter
says, it's clear that global warming is producing
bigger floods. But in the Midwestern United States, the
impact is less clear.

Not that this lets us off the hook. A much bigger
factor is the degree to which we have altered the
rivers. "The river dynamics in no way resembles what it
did 200 years ago," Pinter says.

In The Netherlands, Shabman adds, there is an official
policy of leaving room for rivers. "In the US, we've
done the opposite," he says. "Then we're horrified when
the inevitable occurs."

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