It's hard to get a sense of the Mississippi River's scale without having seen it. It drains water from 31 of the 48 contiguous states, along with parts of Canada, and its outflow can average up to 20,000 cubic meters of water every second. When something that big floods, it can be a staggeringly destructive event, one that can impact multiple states as the surge of water makes its way downstream to the ocean.

Accordingly, humans have been attempting to limit the impact of flooding by building structures that contain the river and direct its overflows. But as floods have continued to plague the river basin, including a massive 2011 flood, people have started to question whether the structures we've built have only made matters worse. A new study reaches back 500 years to gather data on past floods and answers that question with a yes.

Creating a history

For a river like the Mississippi, floods are erratic events, which means that picking out trends requires many years of data. A decade-long lull in flooding, for example, won't tell you if you're safer or lucky. And here, the relatively recent settling of the North American interior works against us; the first hardware to measure the flow of the river wasn't installed until nearly 1900. So how do you analyze a history that doesn't exist?

For a US-UK research collaboration, the answer was building your own history. The researchers relied on what are called "oxbow lakes," curved sections of potential river bed that are cut off from the main flow of the Mississippi unless there's a flood. During the floods, the river dumps heavier sediment than the normal accumulation in the lake. The research team took sediment cores and used the size and composition of the sediment grains to create a history of the river's floods.

That was supplemented by tree ring analysis. It turns out that trees don't exhibit their normal patterns of growth when they spend time submerged, and this shows up in the annual rings laid down as the tree grows. Combined, the two provided a record that stretches back to nearly 1500, with more sites providing data by the 1700s. The overlap with the time when instrument records became available shows that this record accurately captures the history of Mississippi floods, and some individual floods in the historical record can be picked out.

Climate and trends

The major trend in the data is obvious: things have gotten worse. "The highest rates of overbank flooding and the largest discharge events of the past 500 years have occurred within the past century," the authors conclude. Note that says "rate." The magnitude of recent floods is similar to that of those centuries ago; the problem is that these large floods are happening much more frequently. The increase started to become apparent about 150 years ago, which the authors note is about the same time that intensive agriculture in the areas surrounding the Mississippi and its tributaries picked up.

But that's not the only trend apparent in the data. There are also cyclical signals that correlate with major climate cycles. The strongest correlation is with El Niño events, but there's also a link to the Atlantic Multidecadal Oscillation, which means that floods are influenced by cycles occurring on both of North America's coasts. The authors suggest that El Niños result in more rainfall in general, leaving parts of the Mississippi drainage basin saturated and more prone to flooding. The Atlantic Multidecadal Oscillation, they suggest, influences the circulation of water-rich air north from the Gulf of Mexico.

Both of those influences have still held even as the frequency of flooding has increased.

There are two obvious things that could generate the trend, and both of them involve us. The first is climate change, which could change the patterns of precipitation in North America, directly influencing the amount of water in the Mississippi. The second involves our flood control efforts and the changes we make to allow large cargo vessels to use the river, which could inadvertently be increasing the frequency of floods.

To analyze this, the authors assumed that climate factors would act via El Niño and the Atlantic Multidecadal Oscillation. So they analyzed whether these had changed within the last century. This analysis suggested that changes in these two climate cycles could cause a five-percent increase in the flood frequency. The actual increase, however, was four times that. As a result, they conclude that the majority of the effect is due to "human modifications to the river and its basin."

And this, they argue, should have policy implications: "Our main finding—that river engineering has elevated flood hazard on the lower Mississippi to levels that are unprecedented within the past five centuries—adds to a growing list of externalized costs associated with conventional flood mitigation and navigation projects." But it's not clear that their analysis is quite so persuasive. Precipitation in the northeast area of the Mississippi drainage basin has been increasing both in total and in intensity in our warming climate, and that trend hasn't been linked to either of the two climate cycles.

There's still some work left to be done before we can definitively point the finger at our changes to the river itself. But the huge scale and associated costs of floods on the Mississippi mean that this is a possibility that needs to be taken seriously.

Nature, 2018. DOI: 10.1038/nature26145 (About DOIs).