Scientists, environmentalists, and anyone who lives within a hundred miles of the winding Mississippi River will tell you—have told you, repeatedly, for 150 years—that efforts to tame the river have only made it more feral. But scientists would like more than intuition, more than a history of 18th-century river level gauges and discharge stations, more than written and folkloric memory. They would like proof.

Luckily, rivers inscribe their history onto the landscape. Which is why Samuel Muñoz, a geoscientist from Northeastern University, found himself balancing on a pontoon boat with a hole in the middle, trying to jam 30 feet of aluminum irrigation pipe into the muddy bottom of a 500-year-old oxbow lake. Muñoz and his team thought that if they could just pull up good cores of that mud, the layers would be a chronology of forgotten floods—a fossil record of the river’s inconstancy made not through petrification but implication.

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.

Then Muñoz’s team checked their work against another record: tree rings. Inundate an oak tree for a couple weeks and that year’s growth ring will show damage at the cellular level. So they took core samples from trees, living and dead, in the Mississippi flood plain—the oldest going back to the late 1600s. The ring damage matched. Not exactly, maybe, but close enough. They knew they were seeing floods for which no one had numbers. Muñoz’s team had created a record of Mississippi River floods two centuries older than any other. They published that work in the journal Nature Wednesday.

Here’s where the fun part starts. Muñoz’s team then compared those floods with meteorological data—hunting for some link between flooding and climate. They especially looked at temperature changes on the oceans—El Niño events in the Pacific and Atlantic Multidecadal Oscillation. “There’s this really obvious increase in both how often the river has been flooding over the last century and how big those floods were,” Muñoz says. “The default explanation is that there’s something going on with the climate that would explain that.” There was: More El Niño meant more floods.