From Wired (Adam Rogers):
Basically, the Mississippi meanders. Sometimes the river curves around so tightly that it just pinches off, cutting across the peninsula and leaving the bigger curve high, if not dry. That parenthesis of water alongside the main channel is an oxbow. In a flood, water churns up chunks of sediment and spreads into the oxbow. When the flood waters recede, the layer of coarse sediment sinks to the oxbow’s bottom, where it remains.
So Muñoz’s team humped their pontoon boat all the way from Woods Hole, Massachusetts to three oxbows whose birthdates they knew—one from about 1500, one from 1722, and one from 1776—and jammed pipe into the lakebed with a concrete mixer. “It vibrates so hard, your hands fall asleep,” Muñoz says. “And then you have 300 or 400 pounds of mud you’re trying to get back up.” But it worked.
The cores were a map of time, with today at the top and the oxbow’s birthday at the bottom. In between: A peak of the radioactive isotope cesium-137 marked 1963, when humans started testing nuclear bombs. Using technique called optically stimulated luminescence to date, roughly, when a layer was last exposed to sunlight, they spotted classic floods, like 2011, which caused $3.2 billion in damages, and 1937, which required the largest rescue deployment the US Coast Guard had ever undertaken.
The important part, though, was that the characteristics of the layers for floods they had numbers on could tell them about the magnitude of floods they didn’t. They got 1851, 1543, and on and on…
AFTER A PARTICULARLY devastating flood in 1927—637,000 people lost their homes, perhaps up to 1,000 killed, $14 billion in period-adjusted damage—human beings deployed the US Army Corps of Engineers to wage all-out war on nature to protect industry, farms, and trade. People tried to warn the government even as construction began on the Mississippi’s infrastructure—channelization, dredging, dams in the upper stretch, and along the middle and lower levees, concrete mats along the banks called revetments, and gates.
“All that increases the amount of water and the speed that water goes during a flood. What we’re saying is, we can’t explain the increase we’re seeing with climate alone,” Muñoz says. “But for the first time, we can go back further, to a state in which the river wasn’t dominated by human activities. We can really show that the way the river behaves today is not natural.”
Even that look at the prelapsarian Mississippi may not change much. Warnings that flood control would lead to uncontrolled floods date back to at least 1852, when a famous engineer named Charles Ellet warned in a report to Congress that the whole idea was going to lead to disaster. Yet the US Army Corps of Engineers’ Mississippi River and Tributaries Project remains in full, multi-billion-dollar effect. (Representatives for the Corps of Engineers did not return multiple requests for comment.)
Now, Muñoz’s inferential datasets don’t convince every river researcher. Bob Criss, a hydrogeologist at Washington University at St. Louis, says he doesn’t completely buy Muñoz’s team’s particle-size correlations and tree-ring cell biology. “It’s just a bunch of voodoo and sound bites,” Criss says. “I certainly don’t object to his conclusion. But I don’t think it’s robust.”
Criss definitely does buy the idea that engineering has made flooding worse, though. He says straight-ahead numbers like stage measurement (the height of the river) are enough to tell you that. Levees upriver send more water downriver. Revetments move that water faster. What might have been slow-spreading floodwaters when they were unconstrained turn into neighborhood-destroying mini-tsunamis when they burst all at once from behind failing levees…
And [Victor] Baker buys what Muñoz has come up with. “Levees protect against little floods. If you have a super-big flood that exceeds the capacity of the levee, the levees make that worse.” he says. There have been bigger floods than people remember—but the landscape recorded them. And if humans learn to play those recordings back, maybe we can find a new way to get ready for the waters yet to come.