The Mississippi, North America’s largest river and the fourth-largest river in the world, is a powerful natural force. So powerful, it appears, that the 2010 Gulf of Mexico oil spill changed course due to the river’s outflow, sparing the Louisiana coast from worse oil pollution than it might have otherwise experienced.

Douglas Jerolmack, an assistant professor in the Department of Earth and Environment Science in the School of Arts and Sciences, teamed with postdoctoral investigator Federico Falcini and Italian geoscientist Bruno Buongiorno Nardelli to show how the Mississippi river wielded such power. In the immediate aftermath of the explosion of the Deepwater Horizon drilling rig (which caused the oil leak), the scientists began tracking the movement of the surface oil slick using maps of the oil extent published online daily by the National Oceanic and Atmospheric Administration (NOAA).

“We also noticed that NOAA was publishing forecasts, based on computer models from several universities, of where they thought the oil slick was going to move in the next day or two,” Jerolmack said.

But the researchers quickly realized that the forecasts weren’t accurate—something else was directing the oil’s movement. Their thoughts turned to the Mississippi River.

Examining satellite data on sea-surface levels, the scientists observed a large “bulge” in the ocean close to the Mississippi Delta. This mound of water, about 10 centimeters higher in elevation than the surrounding ocean and 50-100 kilometers (31-62 miles) in diameter, appeared to be giving the surface oil slick a push.

“The Mississippi was flooding at the time,” explains Jerolmack. “When the discharge from the river was high, we saw this big mound and we saw the oil move away from the shore. And then when the flood went down, we saw the mound disappear and the oil come right back and made landfall.”

Using a mathematical model, the researchers confirmed that the mound created by the flooding river waters was sufficient to force the oil away from shore, as was observed in the first weeks of the leak. That prevented some of the worst of the sticky crude from oiling the shores, though the oil did hit Louisiana’s coasts after the floods died down.

Jerolmack says this phenomenon of peaks and valleys in the ocean surface directing the flow of oil slicks had never been investigated before. In the future, he says taking these variations into account may help create more accurate predictions of oil slick migration.