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As you can see from the image above, a lot more than water comes down a river. The Mississippi and its distributary, the Atchafalaya River, carry an average of 230 million tons of soil into the Gulf of Mexico every year. On flood years, the total runs a lot higher.

That’s going to cause downstream problems this year… all the way downstream to the ocean. But before we go there, let’s take a quick look at how the system works.

The graphic above shows sediment concentration and sediment discharge for major US rivers. The giant half-circle in the Gulf represents the Mississippi/Atchafalaya discharge. Why are the two rivers bundled together? Because they’re really the same river: the parentheses bracketing a 100-mile-wide delta.

Before 1927’s epic flood, the Mississippi was free to wander the delta, whiplashing between courses whenever a huge flood year like this one pushed it over its banks—a process known as delta switching. In the above image you can see some of its historical courses and when it ran them.

For much of the 20th century the Mississippi has been trying to leave its current course and flow back through its old Atchafalaya course. In his always-excellent WunderBlog, Jeff Masters explains:

There is a better way to the Gulf—150 miles shorter, and more than twice as steep. This path lies down the Atchafalaya River… Each year, the path down the Atchafalaya grows more inviting. As the massive amounts of sediments the Mississippi carries—scoured from fully 41% of the U.S. land area—reach the Gulf of Mexico, the river’s path grows longer. This forces it to dump large amounts of sediment hundreds of miles upstream, in order to build its bed higher and maintain the flow rates needed to flush such huge amounts of sediment to the sea. Thus the difference in elevation between the bed of the Mississippi and the Atchafalaya—currently 17-19 feet at typical flow rates of the rivers—grows ever steeper, and the path to the Gulf down the Atchafalaya more inviting. Floods like this year’s great flood further increase the slope, as flood waters scour out the bed of the Atchafalaya.





Of course it makes no difference to the Gulf of Mexico whether the floodwaters come down the Mississippi or the Atchafalaya. What matters to the ocean is what’s in the river water. The Mississippi drains the entirety of America’s breadbasket… therefore its fertilizerbasket, pesticidebasket, fungicidebasket, and manurebasket.

River water, with its organic nutrient loads, naturally acts as a fertilizer on ocean chemistry. Add in the runoff from farms and feedlots—with nitrogen and phosphorous loads from chemical fertilizers and manure—and you get a Miracle-Gro on steroids. Which, paradoxically, fuels an oceanic dead zone.

I wrote extensively about this problem in my Mother Jones cover, The Fate of the Ocean, a few years back.