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

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NASA/WWF

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

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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