Mississippi River basin is getting wetter as climate change brings era of extremes

In the early morning hours of July 26, many St. Louis-area residents awoke to floodwater filling their homes, or to the din of car alarms from vehicles overtaken by murky brown water. Too much rain was falling far too fast.

The weather system dumped more than 9 inches on St. Louis — about a quarter of the city's annual average — compressed largely within a few hours. That same week, torrential rain storms settled on Eastern Kentucky, where up to 16 inches fell and water rushed into people's homes so swiftly that many didn't get out in time.

Forty people were killed in Eastern Kentucky. Two people died in St. Louis.

Longtime residents in both regions, no strangers to severe storms and flooding, said they'd seen nothing like it before — and they're right.

The rainfall totals obliterated previous records in each area by a margin that was difficult for some experts to fathom — topping St. Louis' single-day record by more than 2 inches, for instance. It was yet another example that rain isn't falling the way it used to, with both the magnitude and intensity of extreme rain events increasing throughout recent decades, across a large part of the country.

In Minnesota, mega-rain events have become more common since 2000, according to the Department of Natural Resources. The intense and widespread rains — like one that dumped 8.65 inches near Mankato in 2020 — are increasing water flow in rivers like the Mississippi. But they are also emblematic of a changing weather system that sees extremes in all directions, as drought has gripped parts of the state last year and this year.

The late July storms that devastated St. Louis and Eastern Kentucky helped showcase the risks in a climate that is growing hotter and wetter — and more prone to dumping extreme rains and flash flooding on communities whose creeks, streams and drainage systems are not equipped to handle it.

The shifting trends and escalating flood risk raise urgent questions about society's readiness to cope.

"We are not doing enough. This last flood sure proves it," said Bob Criss, a Washington University emeritus professor who studies regional flooding. "This problem is not going to get better. We've got to make it better."

'More fuel to work with'

After July's floods, the Mississippi River Basin Ag & Water Desk — a journalism partnership that includes more than 14 newsrooms — asked climate data nonprofit Climate Central to analyze 50 years of rainfall patterns.

Findings showed that the eastern half of the U.S. is getting far wetter on average, with some areas — including parts of the Mississippi River Basin — now receiving up to 8 more inches of rain each year than 50 years ago, according to data from the National Weather Service and the National Oceanic and Atmospheric Administration.

"Climate change models show further increases are likely in coming years," said Climate Central data scientist Jen Brady, who helped with the analysis.

And when it rains, it pours, rainfall intensity data shows. In other words, not only is more rain falling, but it's also falling harder in many places, according to further analysis from Climate Central.

The trend is largely tied to heat. As greenhouse gases from fossil fuels heat the Earth, that warming extends to the oceans and the Gulf of Mexico — a primary source of the atmospheric moisture for the eastern U.S. Warming oceans produce more water vapor, and a warming atmosphere can hold more moisture, which can then deliver more precipitation in short windows of time.

"We're getting warmer and we're getting wetter," said Pat Guinan, Missouri's state climatologist and a professor at the University of Missouri. "We are in an unprecedented wet period."

Recent decades have given rise to a stark divide across the continental U.S., with the western half of the country becoming increasingly arid and prone to drought, while the eastern half faces exceptional moisture, often delivered in bursts.

The Midwest is one region absorbing the brunt of all that water. Since 1958, the Midwest has seen a 42% increase in the amount of precipitation that falls during the most extreme events, said Ken Kunkel, a professor at North Carolina State University who studies extreme rainfall and is a lead scientist behind the National Climate Assessment.

"The future will be characterized by more extreme events, simply because our source of water vapor will be hotter," said Kunkel. "The system will have more fuel to work with."

That helps drive risks of high water even far from increasingly flood-prone major rivers, with widespread flash flooding as a separate and in some ways greater danger.

"At this point, no community in the country is safeguarded from flooding," said Laura Lightbody, director of the Pew Charitable Trusts' flood-prepared communities project. "We're seeing it flood in more places than ever expected before."

Don't just 'blame the weather'

Flash flood risk is shaped by more than weather alone. Pavement-heavy urban areas and places with constricted rivers or hilly topography compound the threat.

Around St. Louis, even an inch and a half of rain in an hour can trigger localized flash flooding, with small urban tributaries reacting to rainfall far faster and more dramatically than the region's major rivers. In St. Louis County, for example, creeks can rise as much as 10 feet in an hour.

Meanwhile, in one Appalachian town, the North Fork Kentucky River shattered its previous height record by more than 6 feet in July's flooding, rushing in fast enough to destroy the U.S. Geological Survey sensor designed to monitor the river.

That explosiveness means flash floods can present a far more sudden threat to people and property than the gradual rise of rivers carrying water collected from afar.

"You can get caught by rapidly rising water and it's hard to anticipate," Kunkel said.

Flash floods have become even more dangerous in many densely developed and heavily populated urban settings, say experts including Criss, who has shifted his research focus to flash floods in recent years.

Such flood risks vary widely but can be especially pronounced in small watersheds that are unusually sensitive to bursts of incoming water. One of the epicenters of damage near St. Louis, for example, occurred along the upper stretches of the River des Peres, a highly urbanized waterway that experts like Criss have identified as the most flash flood-prone system in Missouri.

The river essentially acts as an urban drainage ditch, converted largely into a straightened concrete sleeve that can be easily overwhelmed during storms, with water delivered more quickly and plentifully by the surrounding pavement and other impervious surfaces.

It's a combination that doesn't mix well with more extreme precipitation, raising the risk of flash flooding.

"Intense rainfall is just making it all the worse," said Criss. "[But] I don't want to just blame the weather all the time. That makes it like we're not responsible. ... There is plenty we can do in our local communities to improve the way we build and treat our creeks."

Possible precautions include buyouts of high-risk properties, limiting and corralling runoff, building water storage projects, and better floodproofing homes and basements.

Some similar risk factors are at play in rural Appalachia, another epicenter of this summer's flood damage. There, rugged terrain means homes are built almost exclusively along rivers and at the foot of mountains. Water travels quickly along the steep slopes to the residents below.

The region's history of strip mining and mountaintop removal also contributes to the danger, since the damaged land is often unable to absorb water as it naturally would. Even remediated mine lands often have compacted soil and grass, which isn't nearly as effective for flood mitigation as the forest that once occupied the land.

"It's like pouring water on a tabletop," explained Mary Cromer, of the Appalachian Citizens' Law Center, who recently told members of Congress about the flooding that put the law center and most of her town of Whitesburg, Ky., under water.

More rainfall presents urgent challenges and imperatives: How can a city retool the built environment to withstand stronger flooding? How do people get out of harm's way?

Most structures were built to withstand old conditions and expectations, not the climate trends happening today, nor future projections.

"Historical data is no longer a good predictor of the future," Kunkel said. As a result, "we're not building things to the level of resilience that we'd expect."

In freshly flooded places around the U.S., the slow process of recover is underway.

In Kentucky, residents were told to expect years of rebuilding. But some say climate change makes restoration a Sisyphean task, doomed to be repeated over and over unless precautions are undertaken.

Said Cromer: "We know flooding like this will happen again."

Star Tribune staff writer Chloe Johnson contributed to this report.