River Diversions Are Supposed to Save the Coast, Experts Say. What If They’re Wrong?

Denying climate change spells doom for Louisiana's coast

The marshes of Southeast Louisiana are washing away with salt water intrusion in St. Bernard Parish. (Photo by David Grunfeld)

On Aug. 20, Louisiana coastal advocates woke up to this headline:

“Mississippi River diversions led to land loss, not growth, study says: Implications are ‘obvious.’”

Were they shocked? Let’s just say it would be like Saints fans reading this: “Drew Brees traded to Falcons.”

After all, river diversions have long been the Drew Brees of our coastal hopes: the savior in our darkest hour. And for good reason. Diversions are the only affordable, long-term method of pouring enough mud into parts of Louisiana’s sinking coastal zone to keep those areas above the rising Gulf of Mexico.

So that headline quickly raised some important questions for anyone living south of Baton Rouge.

Let’s take them one at a time, with the most pressing first.

Q: Does the study say we should abandon diversions?

Definitely not. The lead author of the study, LSU oceanography and coastal sciences professor Eugene Turner, told me it means “that they should include the results of these studies in their modeling” to determine if they need to make changes in how the diversions are designed and operated.

Q: So what did this study find?

Actually, this study is an expanded look at an acknowledged problem of putting river water into the coastal marshes: The heavy nitrate load in the river from fertilizer runoff in farm states can cause marsh loss during storm surges and other flooding events.

This is another impact of our river levees. The sediment in the river is largely mineral in content. That mixture provides a very stiff and hard soil base that grips the roots of wetlands plants so tightly they can withstand most storm surges. But since levees blocked that annual layer of mineral-based mud, the top layers of soils in the marshes are now made from generations of highly organic decayed plant matter. This very loose mixture is less resistant to storm surge.

Worse, because nitrates decompose organic matter, river water can make them even more vulnerable to flooding events.

Two 2013 research papers brought this problem to the attention of the Coastal Protection and Restoration Authority. One study showed nitrates weakened saltwater marshes in Massachusetts. But the other research looked at projects putting river water in Louisiana coastal wetlands and found there were not net gains of wetlands — but losses in some areas, a finding it concluded should cause diversion designers concern.

Read the rest of the article at The Times-Picayune.

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