New research indicates Mississippi River diversions could harm marshland

For decades, those leading the life-and-death struggle to keep southeast Louisiana from being swallowed by the Gulf of Mexico have had a battle cry: “Put the river back into the marsh.” The thinking is that the river should be allowed to build new land, just as it had done for millennia before flooding was controlled.

But what if pollutants in the river’s fresh water will kill the marsh before those sediments can do good?

Dissenters who posed this question have been treated as outliers, if not obstructionists.

Now a pair of recently released reports gives that question new relevance.

The reports provide new ammunition to diversion opponents who argue for other methods of coastal restoration, as well as scientists pushing for more research before river water is released on the wetlands.

But Garret Graves, head of the state’s Coastal Protection and Restoration Authority, is having none of it. He dismissed the latest findings as too little, too late to apply to the state’s Master Plan.

River diversions larger than Davis Pond, above, are the centerpiece of the state's Master Plan for the coast, but new research questions whether the river's water will do harm before its sediments can do good.

Coastal Protection and Restoration Agency

River diversions larger than Davis Pond, above, are the centerpiece of the state's Master Plan for the coast, but new research questions whether the river's water will do harm before its sediments can do good.

With coastal Louisiana facing a death sentence of sea level rise and subsidence, Graves invoked Donald Rumsfeld, arguing the state has to fight with the science it has, not the science it would like to have.*

“Look, if we were on a sustainable posture right now, sure, maybe we could spend a few more years and spend tens or hundreds of millions of dollars trying to perfect the science, but considering where we are in Louisiana, you have to take some risks,” Graves said.

“We are going to screw up sometimes, but the consequences of doing nothing are extraordinary,” he said. “It is simply not an option for us to sit back and wait” on more studies.

Further, Graves said, the land being built at the mouth of the Atchafalaya River with water from the Mississippi shows that diversions work. “That’s a diversion that is working perfectly,” he said, “and we need to replicate it.”

New studies revive old questions

Listen to The Lens' Bob Marshall discuss his reporting with WWNO's Eve Troeh.

Listen to The Lens' Bob Marshall discuss his reporting on this subject with WWNO's Eve Troeh.

River diversions have long been the great paradox of coastal rebuilding efforts – at once its most important, yet contentious element. Primary opposition has come from fishing interests that could see their target species pushed out of interior bays to the edges of the Gulf. Instead, they support “slurry pipelines,” which use suction dredges to mine sediment from rivers and offshore locations, mix it with water and pump into the sinking basins.

That technique has been used to build as much as 400 acres of land in as little as three months. But while the pipelines are a major component of the Master Plan, coastal authority research concluded that large diversions had two critical advantages: They built land at a much cheaper cost per acre and, once set up, they could continue building land as long as the river flows.

But for some diversion opponents in St. Bernard Parish, the debate over diversions isn’t just about moving their livelihoods southward. For years they’ve complained that the Caernarvon Freshwater Diversion — one of only two in the region — was actually destroying the marsh.

Opened in 1991, Caernarvon was conceived to support oyster fishers in Black Bay who were seeing their harvests decline due to rising salinity levels. It has a maximum capacity of 8,000 cubic feet per second, but its average discharge has been less than 2,000 cubic feet per second, a volume dwarfed by planned diversions that top 50,000 cubic feet per second.

But even at that small flow, fishers, trappers and others making a living off the wetlands claimed the lush blooms of freshwater plants masked a steady erosion of the soil below.

“The land is just wasting away, like it has cancer,” said Lionel Serigne, operator of a Delacroix Island boat launch and live shrimp business for more than 50 years.

“And now they got this plan to put more and bigger diversions? They ain’t helping us, they’re killing us.”

The top photo was taken after Hurricane Gustav on the west side of Bayou Terre aux Boeufs, which is in the outflow of the Caernarvon Diversion. The bottom photo was taken on the east side of the bayou, which is largely blocked from Caernarvon's fresh water. USGS researcher Chris Swarzenski said the images demonstrate the weakness of the soil caused by the nutrients in the river water.

U.S. Geological Survey

The top photo was taken after Hurricane Gustav on the west side of Bayou Terre aux Boeufs, which is in the outflow of the Caernarvon Diversion. The bottom photo was taken on the east side of the bayou, which is largely blocked from Caernarvon's fresh water. USGS researcher Chris Swarzenski said the images demonstrate the weakness of the soil caused by the nutrients in the river water.

Those impressions were given scientific weight in 2008 when noted LSU coastal scientist Gene Turner published research indicating the river water pouring from Caernarvon stunted root growth of critical marsh plants and accelerated decomposition of the highly organic marsh soil. His work echoed previous studies done in other parts of the world and was followed by more research in coastal Louisiana that reached the same conclusions.

The coastal restoration establishment countered that Caernarvon was a bad analogy because it was designed to move water, not land-building sediment. Further, the state coastal authority’s research showed that the diversions would put so much sediment in the wetlands — along with the water — that they would build land and heal the ecosystem.

And they had another reaction to scientists urging caution on diversions: Southeast Louisiana was running out of time. As the coast continued to collapse, that community made no excuses for its sense of urgency.

New research: Fertilizer from the river weakens marshes

The new papers refocus scientific attention on that debate.

The New England study was led by Linda Deegan, an LSU alumna and now a senior scientist at the Marine Biological Laboratory at Woods Hole, Mass.* Her team meticulously added concentrated nitrogen and phosphorous to tides flowing into an unpolluted coastal salt marsh. The primary plant in that marsh – Spartina cordgrass – also dominates wetlands targeted for some river diversions south of New Orleans.

In the first few years of the project, the nutrients ignited an explosion of growth in leaves and stems, but by the fifth year the edges of the marsh began “literally falling apart,” said team member John Fleeger, a professor emeritus at Louisiana State University. The pollutants weakened the root structure and speeded decomposition of the organic soil. The combination of stunted, weakened roots and less stable soil led to increased erosion from regular tidal currents.

“When we first started this work,” Fleeger said, “it was thought salt marshes would be able to sequester excess nutrients and neutralize them with little impact on the marsh itself, but that hasn’t proven to be the case.”

Graves said Deegan’s research did not mimic conditions found on Louisiana’s coast. Deegan’s study used nutrient loads “double to 600 percent” of those found in the Mississippi, and the tidal cycle in the New England study was more frequent and much stronger than in Louisiana, Graves said.

Fleeger agreed that there are differences between the study area and Louisiana, but he argued that the study has a bearing on Louisiana’s Master Plan because the changes caused in the New England marsh have been recorded in other research around the world.

Do diversions build land? We don’t really know

The second report, a review of published research on the impact of the state’s freshwater diversions, was requested by Graves’ agency. It looked for any conclusions that could be drawn about those diversions, including their ability to build new land, encourage plant growth and stop erosion.

The panel criticized much of the science that has become standard reference material for the state’s coastal plan as sparse, insufficient and inconclusive. One key finding: “Little evidence was available that any Freshwater Diversion in the Louisiana deltaic plan has significantly reversed the rate of marsh degradation and land loss.”

While the panel saw some evidence of small-scale land-building adjacent to outflows, it noted that some papers also showed that the freshwater diversions actually compromised plant growth, soil building and wetland elevation.

And, most troubling to outside researchers who have reviewed the report, the authors concluded that the Coastal Protection and Restoration Authority may be forging ahead without proper science to guide its decisions.

The authors finished by urging the agency to conduct comprehensive research to test alternative hypotheses on how wetlands respond to nutrient-rich river water.

One editor of the report, Chris Swarzenski of the U.S. Geological Survey’s Louisiana Water Science Center in Baton Rouge, said the team was surprised to discover the state and federal agencies had never undertaken in-depth research to find out how the river water would affect coastal wetlands.

“That’s actually one of the most shocking findings,” he said. “It seems everyone has just taken for granted the slogan ‘Put the river back in the marsh’ as the key to addressing the problem without ever going out in a scientific way to find out what that would do.

“It’s almost inconceivable to me that would happen, but apparently it has.”

That review panel was drawn from researchers outside Louisiana to avoid any claim of bias in a subject that has been hotly debated inside Louisiana coastal research circles for some time, Graves confirmed.

But he dismissed the report as unusable, saying the authors did not follow the state’s mandate.

“It largely didn’t answer some of the key questions the state had posed to the scientists who were putting this together,” Graves said. “Questions like informing us or guiding us on how to design, operate and maintain sediment diversions called for specifically in the Master Plan. This largely looked at the role of freshwater diversions that exist today, and again we don’t have any of those in our Master Plan.”

In fact, while the Master Plan 2012 lists only “Sediment Diversions,” critics point out that at least four of the nine move river water into the marsh at the rate of 5,000 cubic feet per second or less, which they contend is not large enough to move large loads of sediment deep into adjoining basins.

The panel’s chairman, John Teal of Woods Hole, replied to Graves’ critique with both fire — and compassion.

He said Graves’ charges were so inaccurate he “could not respond to them in language fit for publication.” But he relented.

“Is he right in those charges? I can respond to his claims with one word: No!” Teal said. “We did exactly what the state asked us to do in every way.”

He provided what he said was a copy of the state’s requests, which supported his point.

“They asked us to look at what had been done on freshwater diversions, and that’s what we did,” he said.

But he also understood Graves’ sense of urgency. “Look, everyone knows the only chance you have down there is to get sediment into the wetlands,” said Teal, who has been involved in Louisiana coastal research for decades.

“I agree with him that you don’t have time left, that you probably should move forward even though the science isn’t complete, mistakes and all. He’s right about that.

“But what [the report]l was saying is that while [the coastal agency] moves forward, they have to conduct research to monitor the results, so that they know what mistakes they make, and what is working.

“We found that wasn’t the case concerning freshwater diversions.”

Graves: Atchafalaya shows that diversions work

Graves insisted the state is aware of the harm that freshwater-only diversions like those at Caernarvon and Davis Pond can do the marsh plants, saying if those federal projects were built today, the state would insist that they be designed to move sediment.

Several times, he came back to one point: “Nature is showing us how to get this done” at the mouth of the Atchafalaya River and at the West Bay Diversion south of Venice.

The Atchafalaya, a distributary of the Mississippi that captures 30 percent of its flow north of Baton Rouge, began building a small delta at the end of the Wax Outlet Canal near its natural mouth after the flood of 1973. Since then it has added about 39 square miles of land thick with healthy, freshwater vegetation.

Bob Marshall reports on the new research with Lens partner Fox 8 News.

At West Bay, opened in 2004, land and healthy vegetation began to rise above the water after the 2011 flood.

“To look at the Atchafalaya Basin and to say there is not thriving vegetation there, with the exact same water that is in the Mississippi River, is a fundamentally inaccurate statement,” Graves said.

However, some scientists say the comparison is invalid because the soil on the newly built Wax Lake and West Bay deltas is different from those in the path of many of the planned diversions.

Swarzenski, who studies wetland soils, said the new deltas are comprised primarily of mineral soils, which provide a firmer foundation for plant roots and cannot be broken down by fertilizer compounds.

But the marshes along the leveed Mississippi, blocked from sediments for more than a century, now are highly organic.

“That’s sort of the whole issue,” said Swarzenski. “These organic soils are broken down by the [compounds in the fertilizer pollution] rather quickly. That’s the concern about having river water flood them.”

And that leads to a question that scientists concerned about the diversions say hasn’t been answered: How much sediment can be deposited on these sinking marshes to rebuild new land, without causing more rapid subsidence?

“These are questions some of us believe should have been answered before planning these diversions and should still be answered before we just plow ahead,” Swarzenski said.

“Yes, we know things are desperate and we only have one chance to get this done. All the more reason we make sure we’re doing it right – because if we get it wrong, we won’t have a second chance.”

*Correction: The original version of this story misidentified Donald Rumsfeld and Linda Deegan.

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About Bob Marshall

From 2013 to 2017, Bob Marshall covered environmental issues for The Lens, with a special focus on coastal restoration and wetlands. While at The Times-Picayune, his work chronicling the people, stories and issues of Louisiana’s wetlands was recognized with two Pulitzer Prizes and other awards. In 2012 Marshall was a member of the inaugural class inducted into the Loyola University School of Communications Den of Distinction.

  • Americascity

    You should dig and find reports about how wetlands assimilate nutrients (fertilizers) that come down river, too. There’s plenty of these. There will always be studies coming out that dissent.

    Weren’t we recently discussing how the freshwater was encouraging plants to have shallower roots due to the nutrients being more readily available in the river water, ie higher in the soil column? This presumption didn’t state anything about killing the plants, but rather we work on a better operation plan of the freshwater diversions.

  • Donald, not David Rumsfeld.

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  • Steve Myers

    Of course; that was my lapse. Now fixed; thanks for the heads-up.

  • Scott Eustis

    Argh. i think the point, in all of this confusion, is that we’ve got to build the gates of the sediment diversion, be more honest about what freshwater diversions are in the master plan, and then monitor the results, adapting the operation of the diversion gates over time in order to build land with the least shock to the system possible. We’ve got to study as we go, and this article positions the scientists (study forever) against the agency men (do without thinking).

    Pulsing the water only at the rising stages, when the river is carrying the heaviest sediment load, rather than leaving the gates open all spring and summer, could be part of a solution. But I don’t think the state’s modelling is quite up to snuff to predict this, yet, but Louisiana can’t afford to miss another opportunity like we had in 2011. Build the gates now, then keep them shut, even.

    There’s the question of whether Caernarvon will ever be a sediment diversion, since its gates are positioned over the Thalweg, and not over the Sandbar (like Myrtle Grove and the Bonnet Carre). Caernarvon was designed to not move sediment–it will probably be best to leave those gates shut, and spend the money elsewhere.

    I forget whether this NOAA report assumes or concludes that Caernarvon is a sediment diversion. but it is in the paper. I was surprised to see that Mardi Gras Pass and Bayou Lamoque are considered sediment diversions by this paper. i think the definition needs to be more strictly defined, based on the engineering behind the Myrtle Grove project, and what we see the Bonnet Carre doing.

    I wish Denise Reed had been quoted in this article. She convened the NOAA panel. In my opinion, having read the NOAA paper, the main takeaway is that diversions need to be monitored.

    Of course, Louisiana DEQ and CRPA have been cowards when it comes to regulating the excess nitrogen (not to mention the herbicides) in the river. Agriculture is generally exempt from EPA regulation under the Clean Water Act, but petrochemical plants and the Anhydrous Ammonia plants in Louisiana are not.

    EPA said it would clean the Mississippi, back in 1998. They have also failed to act.


  • Scott Eustis said “argh”, I say yuck- and ask the knee jerk questions:
    Who funds these studies from the Marine Biological Laboratory at Woods Hole, Mass.?
    or are they self/under funded and fishing for more funds?
    Alas… it’s not polite to criticize what I know little about- so sorry “LSU Grad”, but our ecosystems health insurance will use your data to deny coverage and send our future goes to the hospice.
    As always, big thanks to Bob Marshall and the Lens for exposing, so the blame is on us if we allow the Feds to continue to drag on funding fixes because a few petri dishes in a Massachusetts laboratory.
    My take- Are they right? Who care- it’s risks vs rewards when you make a vaccine.
    Yes- Newton’s 3rd law “for every action there is an opposite and equal reaction” is correct- and
    Yes- fresh water diversion has problems- but the laws of nature can’t be quantified like he laws of physics, and they evolved both the Hummingbird and the Airbus the ability of flight.
    Ok i’m getting Hume, Lock, and Kant stupid deep, but we need to quantify and sell our poor and small State….
    We may only have 4.5 million people, but over 6,000,000+ Speckled trout are caught in our waters annually, and we make 1/3rd of your energy, yet loose a football field of American Soil every 38 minutes…

    Save us now, or pay for it later and a clock is ticking before it’s too late…
    Best from Freret,
    Andy Brott

  • Scuttlebutte
  • Scuttlebutte
  • Still waiting for Bob Marshall to figure out that injecting partisan politics into our coastal erosion catastrophe does not help whatsoever. I think I’ll be waiting a lot longer. Thankfully, he went a little light on it in this article.

  • Wow. Really?
    Diversions cause WATER to flow which will “cause erosion” where it is flowing… THEN as the water ladden with sediment slows down it drops its sediment and then disperses. The problem with Caernarvon is that “it has a maximum capacity of 8,000 cubic feet per second, but its average discharge has been less than 2,000 cubic feet per second, a volume dwarfed by planned diversions that top 50,000 cubic feet per second.” AND IT HAS NEVER BEEN ALLOWED TO WORK AS DESIGNED! Because the oystermen in St. Bernard whine.
    All the data indicate that we are loosing MORE land to subsidence due to staving the wetlands of sediment than sea level rise due to global warming. Ignoring the fact that our subsidence problem only exists because we shut the river sediments off from the wetlands is to ignore the weight of the KNOWN at our peril. How does anyone think we got the wetlands in the 1st place? This is a dynamic system, not a set of Legos. We’ve waited and worried over political and scientific in-fighting long enough.

  • Scuttlebutte

    Caernarvon was designed for regulation of estuarine salinities–it was not designed for coastal restoration purposes, if that is what you are suggesting. Also, its design capacity is 8,000 cfs, but its maximum discharge at any point in time is largely determined by difference in stage between the river and the receiving Breton Sound estuary, and as a result it cannot be operated at 8,000 year-round (when the river is low enough, it will even run in reverse if operated). Caernarvon siphons water off of the surface of the water column of the river, which is very sediment-poor in general (read the references provided earlier for more on this subject).

    What data indicate we are losing more land to subsidence due to starving wetlands of riverine sediment? Is it a fact that subsidence only exists because of ‘starving wetlands of sediment’? Most of Lousiana’s wetlands maintain or nearly maintain elevations exceeding relative sea-level rise without the aid of river diversions, via organic matter accretion; marshes play a much more widespread role than diversions in maintenance of coastal elevations. The coastal plain may have been built by deposition of river sediment, but it is maintained by coastal wetlands. Also, note that the river should have abandoned its current route in favor of the Atchafalaya probably sometime around the 1990s, in which case the area below New Orleans would be dependent on transgressive delta processes (such as marsh vertical accretion, marine reworking of seidments, etc.) to maintain elevations.


    Does anyone besides science “experts” comment about what did and is actually happening with the existing natural crevasses south of Belle Chasse, La.? There is an actual existing “model” presently operating in this area. The Birds Foot Delta is stated by the LACPRA as an un-restorable area yet the Master Plan proposes to create diversions upriver that are supposed to do what the BFD cannot do. ??? I swell a fish!!!! Could it be $$$$ and power at work???

  • Dan Brown

    This makes no sense. If this were the case, wouldn’t the Atchafalaya Basin be much worse off? It’s the same water.