The Mississippi River delta – what the corps knew and when they knew it!
Editor’s note: Tom Sands is a distinguished authority on the lower Mississippi River, and I have long respected his broad experience, vision and judgment with respect to “re-plumbing” the river system south of Cairo, so as to save the delta while enhancing navigation. General Sands, now a coastal engineering consultant, is retired from the US Army Corps of Engineers as former commander of the Mississippi Valley Division.
He thoughtfully forwarded this link to the December, 1897 issue of National Geographic Magazine, which features a fascinating piece on the Mississippi River delta that is absolutely relevant to current policy discussions. After reading this article by a civil engineer named E.L. Corthell (his professional titles are listed as C.E., D.Sc., etc.) I was inspired to comment on what I discovered to be a surprisingly sophisticated state of delta knowledge in 1897.
I especially call your attention to a provocative discussion that is even more apropos today than it was in 1897: flood protection vs. Mississippi delta survival.
Although I’ve never taken a geology course for credit I’ve long worked with coastal geologists and consider myself reasonably well informed about the earth sciences,* at least as they relate to south Louisiana.
It is now considered a truism that, prior to the widespread construction of artificial river levees, spring overbank river floods and crevasse splays had for millennia added new sediments that elevated the landscape of what was to become south Louisiana, in a process that roughly offset subsidence and maintained the deltaic surface above sea level.
Even the most loyal levee lovers now acknowledge that constructing dikes around NOLA ended sediment input, permanently consigning the drained and “protected” landscape to unmitigated sinking relative to sea level.
I had assumed that this sediment starving levee effect was a twentieth century concept .** I had also assumed that the levee effect was not acknowledged by the public until the nineteen eighties – and not by the corps until the nineteen nineties.
In other words it had always been my understanding that the corps’ traditional 19th century levees-only decision to dike and channelize the lower Mississippi River was carried out in blissful ignorance of the long term effects of this policy. I had assumed that the Mississippi River and Tributaries (MR&T) project was authorized in 1928, absent knowledge that the levee system would starve the delta of sediments, with long term devastating consequences.
Thus I was surprised to discover in Corthell’s article that the harmful levee effect was recognized by experts at least as long ago as 1897 – and by authorities with the corps, no less! One of these exceptionally informed folks with strong corps connections was James Buchanan Eads, the brilliant engineer who designed the first steel bridge to span the Mississippi River (at St. Louis, MO). Mr. Eads, who died in 1887, figured notably in John Barry’s fabulous book Rising Tide, which details the national effects of the 1927 flood.
The following three topics discussed in the article are of particular scientific interest and a fourth is relevant to current policy discussions. Quotes from the article are shown in italics:
1) Lateral and vertical extent of the delta:
Dr. Corthell reported (without a reference citation) that the Mississippi delta extended upriver about 100 miles from New Orleans. He wrote that the most precise datum available on delta thickness was measured while drilling for potable water in Lafayette Square in New Orleans. Apparently the drill tool broke and was abandoned at 1,042 feet, at which depth pieces of driftwood were still coming to the surface in the drill cuttings. In other words the floor of the Holocene had not been reached. I’m very curious why well water was being sought in NOLA, adjacent to North America’s largest source of flowing freshwater!
2) Subsidence rate and delta instability:
Corthell reported a subsidence rate based on observed sinking over nineteen years of a 17th century Spanish battery close to where Mr. Eads installed his jetties along South Pass in 1877. This datum suggested that subsidence at the mouth of the river was occurring at a constant rate of one-half of one-tenth of a foot per annum (i.e., five feet/century). This closely conforms with modern subsidence estimates in the Belize Delta of about five feet per century. It should be noted that this number is not net (eustatic) elevation change, i.e., it does not include sea level rise, which was considered to be nil, or any correction for riverine sediment accretion or organic production. A
Here’s another quote that sounds very modern, It is a fact well known to people living in the delta of the Mississippi that large tracts of land were long ago abandoned in consequence of overflow by Gulf waters due to the sinking of the lands.
Not only are these lands unstable in a vertical direction they are also found to be so in a lateral direction. It is an interesting engineering as well as physical fact that an accurately measured base line exactly 700 feet was found, after a lapse of five years to be 712 feet in length. Any ideas about this, geologists?
3) Sea level rise:
As to the question of the rising of the Gulf level, careful investigations and inquiries around the entire Gulf coast from Yucatan to Florida disclose no indications of any such elevations. This statement conforms with my understanding that global sea level rise has only recently begun to accelerate.
Number 4 represents the heart of the article, spookily similar to current discussions on coastal protection vs. restoration.
4) Short term levee benefits vs. long term costs:
The conditions are very different now from those existing prior to the existence of levees. There are at present no annual accretions of sedimentary matters from the periodical overflows of the river. These accretions formerly were a little more than equal to the annual subsidence of the lands…The effect of the withholding by the levees from the great areas of the delta of the annual contributions of sedimentary matters, and the steady, though slow, subsidence of these areas, is one which should be taken into account in deciding the important question of how to protect the people from the flood waters of the river (now Gulf).
No doubt the great benefit to the present and two or three following generations accruing from a complete system of absolutely protective levees, excluding the flood waters entirely from the lower delta country, far outweighs the disadvantages to future generations from the subsidence of the Gulf delta lands below the level of the sea and their gradual abandonment due to this cause. While it would be generally conceded that the present generation should not be selfish yet is safe to say that the development of the delta country during the twentieth century by a fully protective levee system, at whatever cost to the riparian states and the federal government will be so remarkable that people of the whole United States can well afford, when the time comes, to build a protective levee against the Gulf waters, as the City of New Orleans has done on a small scale against the sea waters of Lake Pontchartrain and as Holland has done for centuries…
Do these comments from 112 years ago not sound familiar? Is this not another example of what I like to call Deja Voodoo?
*Based on my scrutiny of those familiar Mercator world maps that hung in many classrooms during the 1950′s, the shapes of Africa and South America fit together much too well to be coincidental. Thus, with all due respect to my geologist friends I have long harbored a prejudice against earth science, based on its refusal to accept continental drift until the late 1970′s! Check out this animation from Wikipedia of Pangea breaking into separate continents.
**It had long been my impression that 1926 was the earliest date on which the fundamental need for the periodic flooding of the delta that is south Louisiana was specifically described in print. The coastal hero who wrote about this will be featured in honor of the first year anniversary of LaCoastPost around October 15 2009.