Environment
 

Get the science right: Diverting river sediment is key to saving the coast

The satellite image shows plumes of lost sediment that can only be recaptured by using the river to flood starved wetlands.

NASA/NWF

The satellite image shows plumes of lost sediment that can only be recaptured by using the Mississippi River to flood starved wetlands.

The Mississippi is certainly not our grandfather’s, or even our great-great grandmother’s river. We don’t generally have to face with dread the danger of crevasses and flooding each spring and ship captains don’t have to wait, losing money daily, at the mouth each fall hoping for the winter rise to float them over the bar. The U.S. Army Corps of Engineers has seen to that, and we’ve reaped the benefits. But we are also paying the price.

Our very survival depends on finding a third way between the wild untamed river and the straight-jacketed ship channel it has become. We can find that middle ground with the suite of restoration projects outlined in the 2012 Coastal Master Plan, including controlled diversions which harness nature. I welcome the open scientific and public dialogue now underway to make sure we do it right.

Along with Capt. George Ricks and other concerned citizens, I attended and spoke at the first meeting of the independent scientific panel on diversions assembled at the request of the Louisiana Coastal Protection and Restoration Authority (CPRA). At their invitation, Brig. Gen. Duke DeLuca, commander of the Mississippi Valley Division of the Corps of Engineers, thoughtfully laid out an overview from the corps’ perspective. He emphasized some uncertainties the panel needs to investigate as it advises the state on how best to build the river diversions proposed in the Master Plan. I did not hear DeLuca presuming to provide them with answers — only questions.

The question for the panel is not “should we build sediment diversions?” Diversions build more land for less money, and keep building land. The corps, other federal agencies, the state and scientists have long since acknowledged the need for sediment diversions to restore coastal Louisiana. And, as the general made clear to the panel, the corps supports using every weapon in the restoration arsenal.

Congress, on the recommendation of the corps, authorized diversions in 2007. Unfortunately, for a host of reasons, one being tightened federal purse strings, the corps has not been able to build those diversions.

In the meantime, because of dollars flowing from the Deepwater Horizon disaster, the state finds itself in a position to move forward.  The $40 million that the state is using for the Myrtle Grove—Mid-Barataria Diversion is from the BP criminal settlement, and cannot legally be used for marsh dredging projects. That money is disbursed through the Gulf Environmental Benefit Fund and is legally available only for barrier islands and river diversions.

Marsh creation using dredged sediment is another important tool in the toolbox, and the state proposes to spend almost $20 billion on it over the next 50 years—the biggest expenditure in the Master Plan. But dredging has severe limitations.

The river’s suspended sediment load, the mud that built so much of the delta, can’t be captured by dredges. It is that mud that we are wasting at the mouth — it’s the plume of sediment we see in satellite photos. There is four times as much mud in suspension in the river as sand on the bottom.

While diversions can utilize that suspended sediment, dredging is limited to the sand on the bottom.  It makes no sense at all, when we are losing 16 square miles a year, to leave 80 percent of the sediment on the table. But that is what diversion opponents are proposing — arguing it is an either-or question, when in fact both are needed desperately.

Diversions will also bring precious sediment and nutrients to help sustain the man-made marshes that we have already built and continue to build — marshes doomed to go the way of all marshes that are not sustained by the river. Without the ongoing benefit of new sediment, marshes built by dredge, like new cars, begin to depreciate as soon as you get them. And, yes, those marshes will do some of the work of pulling nutrients from the river water, feeding plant growth, and reducing the amount that gets into bays, or into the Gulf’s Dead Zone. In fact, the state devotes an entire appendix to it in the Master Plan.

By the time the Mississippi River began building our delta 7,000 years ago, the Ice Age glaciers were long gone. It was not runoff from the glaciers that built our delta. The delta grew steadily over the last 7,000 years, not in one episode. In that time the river deposited about 3 trillion tons of sediment. The wedge of sediment it deposited in Louisiana is 300 feet thick down in the Bird’s Foot Delta and it is over 150 miles wide and 175 miles long. It built land by switching course periodically, and by dividing its own channel periodically, while at the same time older, abandoned delta lobes eroded and subsided away.

That is the cycle we interrupted and why we are no longer gaining on the Gulf. If we ever hope to get back to equilibrium, we must restart the cycle. This is what diversions will do, but in a controlled way, making it possible for us to maximize the river’s land-building potential in the most strategic locations.

We’ll actually have it somewhat easier, in a sense, than nature did the last time around. We don’t have to fill a hole hundreds of feet deep and more than 9,000 square miles in extent. If we put diversions in the right places, we won’t even have to fill a hole as deep as the one the Wax Lake Outlet is trying to fill. And instead of trying to fill deep, high-energy open bays as we are doing at Wax Lake and West Bay, we could be filling very shallow areas of broken marsh, where the remnant marsh itself would help capture the finest grains of sediment.

The Wax Lake Outlet may have 10 percent of the river’s flow, but it is building 250 acres of land a year with far less than 10 percent of the river’s sediment. That is because each spring the Atchafalaya River can spread through the entire Atchafalaya Basin, including a swamp 16 miles wide and 60 miles long. That is a far cry from the Mississippi, which has little room between levees. If you ever visit the deltas building at the mouths of the Atchafalaya, you will no doubt be awestruck.  There you will find some of the most beautiful and richest wildlife habitat in North America.

And when the river falls, as it does it each year, the specks and reds move into Atchafalaya Bay — and into the passes and bayous and marsh ponds — to reap the bounty.  That also happens every year near Venice, and it will happen near Myrtle Grove when the diversion finally opens. River water and good fishing are not only compatible, they go hand in hand.

David Muth is director of the National Wildlife Federation’s Mississippi River Delta Restoration Campaign. The title was stated incorrectly in the article as first published. 

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  • Chris McLindon

    David,
    This is a very thoughtful piece with much good information. I appreciate your effort to continue the open scientific and public dialogue. Hopefully we can keep the vitriol to a minimum on this exchange. Your opening reference to a grandfather or great-great grandmother is particularly relevant to this conversation. Nearly every quote of cumulative land loss in south Louisiana is “since 1932”. This is simply because that is the year we had the first area-wide coverage of aerial photography with which to measure areas. The common conception has been that these photographs capture the way our wetlands are supposed to look, and that we need to take action to restore them to that state. This conception is exactly analogous to one that would suggest that a photograph of your grandmother from 1932 captured the way that she was supposed to look, and that you would be justified in taking her to a plastic surgeon with that photograph to begin a process of “restoration”.

    All things in nature have a life cycle. The coastal marshes have been built up by a series of historical deltas of the Mississippi; each one exhibited a natural cycle of creation, growth, and degradation. Each one in turn succumbed to the geophysical forces of subsidence, and we now find those deposits buried to depths of 100 feet or more, or as remnant offshore shoals that are 20 to 30 feet below sea level. What we are now witnessing is the abandonment cycle of the Mississippi Delta. By the process of natural avulsion, the focus of deltaic deposition has shifted to the Atchafalaya Basin. This is where new land is being created. In the not too distant future the marshes of the birdfoot delta will be below sea level following the natural progress of its predecessors. The argument that we should attempt to interfere with is cycle is highly suspect.

    The Expert Panel of Diversions just released its initial report this morning. The overwhelming theme of the report is the uncertainty of the entire proposition of diversions. They clearly state that “Analogs for controlled sediment diversions do not exist”. In other words there are no examples of sediment diversions that have successfully built new land at a rate that would have any significance in offsetting the rate of subsidence and sea level rise in order to provide meaningful flood protection. We have been attempting to control the processes of the Mississippi River for over 150 years. Every effort of enforce a measure of control has resulted in a set of unintended consequences that are an order of magnitude larger (and in the opposite direction) that whatever we were trying to accomplish. These efforts include channel cutoffs to shorten the course of the river, construction of levees to reduce the magnitude of flooding, the forced diversion of the river into its abandoned channel, and now the construction of diversions to attempt to imitate the land building function of the delta. The sooner we accept that fact that every effort we have made to control the natural processes of the river has been an abject failure, the sooner we will begin to see that it is our own culture, and its desire to bend nature to our will that needs to be controlled.

  • Shawn Stephens
  • TraveLAr

    “The sooner we accept that fact that every effort we have made to control
    the natural processes of the river has been an abject failure, the
    sooner we will begin to see that it is our own culture,…”

    Hi. I could use more clarification of this. My understanding is that we have been very successful at channeling the river, exactly what we wanted for commerce. Your position seems to argue that we should just leave it alone, but with what levee status? Stop maintaining them? Return them to pre-1932 condition? I’m a bit lost.

  • Chris McLindon

    You are correct. We have been very lucky in the seven or so decades since the catastrophic floods of the early twentieth century, just as New Orleans had been lucky with its storm surge protection for several decades before Katrina. What Katrina tragically revealed is that every engineering system has its failure point. For the Mississippi River the Corps calls this the “project flood” – the one that will exceed the capacity and overwhelm the system. The 2011 flood reached 85% of project flood, it might have been a 100 year storm, but somewhere in the future there is a 200-year or a 500-year flood coming. When it does, the system is most likely to fail at the Old River Control Structure, and the river will complete its change of course down the Atchafalaya. I think it is safe to say that most people knowledgeable with the situation would agree that it is not a question of if the river changes course, only when. It might be 100 years from now, but it might be this spring. This is the way the river wants to go. This is a way a natural system should work, and this is the path we should follow to achieve coastal restoration.

    Our interaction with the river over the past 150 has been a study of unintended consequences, and as such it contains some great ironies: the primary place where new land is being built is in the place where we are actively trying to prevent it from happening, and the primary place where wetlands are being lost is in the place where we are trying to prevent that from happening. This is a direct measure of our ineptitude in trying to control the Mississippi. Since the middle part of the twentieth century the Mississippi has sought a preferred path to the Gulf down the Atchafalaya. This is part of the natural cycle of avulsion by which the historical deltas spread sediment across the coastal plain. One river channel is abandoned, and a new one takes over, and that is where the new land is built – at the delta of the new channel. The Old River structure was constructed to prevent this from happening in order force the flow of the river down its old channel, so the new delta only receives a fraction of the sediment that it would in a natural system.

    http://blogs.discovermagazine.com/crux/files/2013/01/land-loss-map.jpg

    The USGS Land Loss map shows areas where marsh has converted to open water in color codes. The reds and oranges are marshes that subsided below the surface between the 1950s and 1970s. The purples are marsh cover that has been lost in the past 10 years. It is all concentrated around the mouth of the Caenarvon Diversion. Several very reputable scientists including Linda Deegan and John Teal, who are on the Expert Panel on Diversions, and Eugene Turner at LSU have determined that nutrient loading of the salt marshes by diversion of river water through the Caenarvon has caused the root systems of the salt marsh grasses to weaken, and they are ripped up during hurricanes. The biggest cause of land loss in the past 10 years has been coastal restoration itself.
    We need to take a step back. Nature is showing us the way it wants to go. We need to work with it, not against it. Real restoration is only going to come from removing engineering structures from the marsh, not building new ones. The Corps estimated that the maintenance cost for the proposed Morganza to the Gulf levee would be $700,000,000 per year. Based on Bob Marshall’s article last week, we should probably plan on spending that much to maintain the flood protection infrastructure we have in place. We are going to need every dollar we can get to maintain the systems we have. None of the natural or artificial openings of the lower Mississippi River have built any significant new land in the past few years, and it is unlikely that any new ones will. We should stop wasting money on them.

    Eventually a new delta system will fully emerge in the Atchafalaya. When it does we will be on our way to real restoration of our wetlands, and a viable solution to the dead zone problem. We need to figure out how to get from here to there by the lease catastrophic path. Right now we allow 30% of the Mississippi flow down the Atchafalaya, it wants 100%. What if we started to increase the flow at regular increments, say 5% a decade so that 50 years from now 80% of the flow would be going down its intended path. This would allow time for all of the adjustments that would need to take place, and we might end up getting where we need to be without a big disaster.

  • Clay Kirby

    First off, here’s the actual text of The Water Institute’s initial report:

    PDF http://t.co/UD1Nxz0Mja

    The USGS map also shows green. The Atchafalaya’s natural delta past Morgan City has been growing. Furthermore, the Wax Lake Outlet has been growing at a pace of 250 acres a year.

  • gefff

    Thank you for reiterating the logic of connecting the delta with the river basin. Guess that is why they were connected in the first place? When will humans learn we are not smarter than nature?