New Study: River Diversions Don't Build Land; State Disagrees

Aug 28, 2019

The Mississippi River plays a critical role in Louisiana’s plan to combat coastal land loss. The state wants to divert part of its flow into the dying marshes as a way of building back some of the land.

But, a recent study by LSU researcher Gene Turner says the benefits of using the river might not outweigh the drawbacks. WWNO’s Travis Lux spoke with Turner about the study, and the response from the state.

This transcript has been lightly edited for clarity:

Q: Back in the 1990s and early 2000s as some of the coastal marshes were getting saltier, the state built these structures to freshen them up. The idea is to divert freshwater from the Mississippi River into the marsh. They’re called freshwater diversions. Your study tries to answer this question: how did that affect land loss nearby? Did it worsen it? Make it better? Stay the same? What did you find?

Turner: We found that the ares the diversions have gone into, which were the original project areas -- and showed the land loss has actually accelerated a little bit in both of those areas.

Q: The state of Louisiana has strongly disagreed with your conclusions. Chip Kline, the board chairman of the Coastal Protection and Restoration Authority (CPRA), addressed one of the freshwater diversions you studied, Davis Pond, during a recent CPRA meeting.

He said, "Go to Davis Pond, and watch the commanding General of the Mississippi Valley Division of the US Army Corps of Engineers jump off of an airboat and jump up and down on land that wasn't there two years ago."

I’ve been to some of these places. I haven’t been to Davis Pond but I’ve been to some of these places where the river has spilled its banks and over time built new land -- and walked there. So I’m wondering how to square these things -- what you’re saying with this study and what the state is saying.

Turner: We used the state’s definition of land loss and gain for a much larger area. Some areas, of course, have changed over the years. Some areas have gained, and other areas have lost more. The question is, whether there was a net change in land loss or gain over a much larger area. That’s the difference in perspective that was mentioned in the news clip.

Q: So in other words, are you essentially saying that diversions might have built land in some places, but overall if you zoom out, the net change is a negative one?

Turner: That’s right. And also, the area that we looked at was not only in the immediate area of the outfall [of the diversion], but far downstream.

Q: What is the likely reason for that?

Turner: Well, we don’t know exactly, but there are three reasons. One is the scour from the physical presence of the water flow going through. The second is that far down from the diversion, it floods the plants longer than they would otherwise. Much like if you watered your lawn for five days in a row. That would cause a problem. The third is that the river isn’t the same as when these wetlands were built. It has many more nutrients in it. Nitrogen and phosphorous in particular.

Q: This also has implications for the state’s efforts to continue to rebuild coastal land. One of the things the state wants to do is build sediment diversions -- which are a little bit different from the things you studied, which are freshwater diversions. Freshwater diversions weren’t meant to build land, though maybe they built a little bit. Sediment diversions, on the other hand, are designed to build land. So isn’t comparing these two things -- freshwater diversions and sediment diversions -- isn’t that like comparing apples and oranges?

Turner: I wouldn't use that analogy. It’s the same river water going in there. It may be there were different intents to use that water -- whether it has more or less sediment in it. But it’s still the same river. So, the problem is -- as demonstrated in the studies we did, and others -- the nearfield deposition of sand than the far-downstream effects of the higher water and the nutrients being added.

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