• The Mississippi: A Visual Biography by Quinta Scott

    "Great book and great blog - thanks for the first book I have seen that addresses the contemporary river, headwaters to gulf." Dan McGuiness, Audubon, St. Paul, Minnesota

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Sediment and the Mouth of the Mississippi

NASA Image of the Bird Foot Delta, the red is sediment streaming out of the delta, the black is oil from the BP oil gusher

To repeat what I have written before:

“Artificial levees, extending clear to the Gulf of Mexico, may have made human habitation of the delta south of Cape Girardeau possible, but prevented the Mississippi from refreshing its marshes with its sediment when it did flood. Revetments built of concrete mats may have stabilized the navigation channel, but reduced erosion of the banks, a source of sediment in the marshes. James Eads’ jetties at the mouth of the river may have allowed the river to cut a thirty-foot navigation channel through the sandbar blocking the South Pass of the modern delta, but delivered sediment carried by the river to very deep water at the continental shelf, where it washed away, never to be used for marsh building. Closure of the old distributaries of the river may have prevented flooding in the bayou country of Louisiana, but cut the flow of Mississippi sediment to the coastal marshes. Channel dams may have made navigation on the Upper Mississippi profitable, but they retained its sediment north of Alton, Illinois. Dams on the Missouri, from which the Mississippi drew sixty percent of its sediment, did the same. In short at the beginning of the twenty-first century, the coastal marshes south of New Orleans received eighty percent less sediment than they had at the beginning of the twentieth and eroded away.”

And to repeat what I wrote almost two years ago when researchers at Louisiana State University released a paper that that there is not enough sediment in the Mississippi as it flows through the Louisiana Coastal marshes to restore them by diverting sediment to them.

So it is ironic that the LA Times and other papers report that the navigation channel at the mouth of the Mississippi is silting in, has lost a foot of depth in the last year, and is half its width in some places. It seems the Corps of Engineers has had to cut its budget for dredging the navigation channel from $80 to $100 million a year to $63 million a year and will not start dredging the navigation channel until this month. Exporters are concerned that the cuts will hamper President Obama’s goal of increasing American exports.

Sediment and the mouth of the Mississippi has long been a problem. LaSalle ventured down the Mississippi to about Venice, just north of the Head of Passes in 1682. Five years later he went searching for the mouth of the river from the Gulf of Mexico. He never found it because the mouth was lost in all the sediment and willow trees that lined the passes. He ended up in Texas, where he was murdered.

The Bird's Foot Delta at the Mouth of the Mississippi, 1875

That sediment plugged the mouth of the Mississippi vexed the people of New Orleans in the eighteenth and nineteenth centuries and hampered the city’s development as a major port.

There were many proposals, including an 1832 proposed to build a canal through the east bank into Breton Sound and forget trying to turn the mouth into a viable navigation channel.

Capt. Amos Stoddard, who represented the United States when it took possession of Louisiana from Spain at the time of the Louisiana Purchase in 1804, explored the Atchafalaya for President Thomas Jefferson. In 1812 he published his observations in Sketches of Louisiana, where he noted:

“Nothing is more certain than that the Delta has gradually risen out of the sea, or rather that it has been formed by alluvious substances, precipitated by the waters from the upper regions. It is calculated that, from 1720, to 1800, a period of eighty years, the land had advanced fifteen miles into the sea; and there are those who assert, that it has advanced three miles within the memory of middle aged men.”–Sketches of Louisiana, 158

Entrance to Eads' Jetties through South Pass

James Buchanan Eads opened South Pass to navigation with jetties that scoured a 30-foot channel. When Eads started the process, South Pass was beginning to silt up and willows were taking root in the sediment. Eads’ workers cut the willows and turned them into mattresses which armored the bank. The river’s current scoured a 30 navigation channel.

Workers constructing willow mattresses

So now we have a problem of too much sediment and too little sediment at the mouth of the Mississippi. How we have engineered the whole of the Mississippi, as reflected in the opening quote has created both problems. So has how we have engineered the Atchafalaya.

When I first started research for The Mississippi: A Visual Biography, I learned from John McPhee, in his  wonderful article “Atchafalaya,” that “when a fast moving river meets a slow moving or still body of water, it deposits it load of sediment in the still body of water.” And, “when a river becomes too long and its slope to the sea too flat, it searches out a steeper, faster route to the sea.” The Atchafalaya is the Mississippi “wannabe.” Capt. Amos Stoddard understood that at the beginning of the nineteenth century.

“The Mississippi is known to seek new channels; and there is good reason to believe, that it has from time to time varied its course from one extreme of its valley to the other. The channel of the Chafalia, a few miles only from the head of it, is completely obstructed by logs and other materials. Were it not for these obstructions, the probability is, that the Mississippi would soon find a much nearer way to the gulf than at the present; particularly as it manifests a constant inclination to vary its course.”–Sketches of Louisiana, 167

Bayou Lafourche at Donaldsonville

In the twenty-first century, the Mississippi is too long and its slope to the sea to flat and the Atchafalaya is the steeper, faster route to the sea. But we decided in the twentieth century that we would not allow the Mississippi to divert to the Atchafalaya and leave the river south of Old River to silt in to a narrow channel like Bayou Lafourche, which carried the Mississippi to the Gulf of Mexico 1200 years ago. Doing so would destroy the ports of New Orleans and Baton Rouge. In the twentieth century we built the Old River Control Structure at the head of the Atchafalaya to prevent that from happening.

Old River Control Structure

While there may not be enough sediment in the Mississippi as it flows through Louisiana’s coastal marshes to restore them through diversion, there is enough to deposit in the flat navigation channel that carries shipping to the Gulf of Mexico, thus vexing navigation and the Port of New Orleans.

Bayou Baptiste Collette at Venice, Louisiana

By the way, Louisiana and the Corps of Engineers are considering cutting out the Bird’s Foot Delta,  allowing it to silt in and turn to barrier islands, and take up the 1832 proposal to cut a canal in the east bank of the river or enlarge Bayou Baptiste Collette,  and direct navigation to Breton Sound.

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