Human changes to Mississippi River have increased risk of floods

A new study led by the Woods Hole Oceanographic Institution (WHOI) has revealed the flood history of the Mississippi River over the last 500 years. The researchers found a drastic increase in the size and frequency of extreme floods in the past century, primarily due to levee system construction.

The experts also linked flooding in the Mississippi with the natural shift of water flow and temperatures in the Pacific and Atlantic Oceans.

The newly-established record of flooding in the Mississippi River, which is the very first of its scope, highlights the fact that river engineering has intensified flooding to unprecedented levels in recent years.

“The floods that we’ve had over the last century are bigger than anything we’ve seen in the last 500 years,” said lead author Sam Muñoz.

The study revealed that, in the past 150 years, the severity of a 100-year flood has increased by 20 percent. A flood of this magnitude only has a 1 percent chance of occurring in any given year.

“There’s been a longstanding question about the extent to which all the changes we have made to the Mississippi River – one of the most engineered rivers in the world -have altered the probability of really large floods,” said Muñoz.

According to the research, at least 75 percent of the elevated flood risk can be directly attributed to human modifications of the river.

The experts also noted that when El Nino brings more storms and rainfall to North America, the ground surrounding the Mississippi River becomes unusually saturated. If extreme rainfall is also triggered by oscillation in the Atlantic Ocean, flooding is very likely.

“We’re able for the first time to really parse out how the natural variability of the climate system influences flooding, and then how people have modified that,” said Muñoz.

The team plans to extend the flood record even further back in time to understand what drives flooding in other major river systems across the globe.

The study is published in the journal Nature.

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By Chrissy Sexton, Earth.com Staff Writer