Tag Archives: Mississippi River delta

The Physical Environment 3: Building on Its Own High Ground

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This is the third in a series of three introductory posts I’m dedicating to the physical environment of deltaic landscapes. The first post looked at land-building and the timing of delta formation across the globe. The second discussed the quirks of topography in river deltas. Today, I’ll conclude the series by revealing what happens when, as is the case on a deltaic plain, a river occupies the landscape’s high ground.

Holding the high ground?

Bird's Foot Delta of the Mississippi River

As the river lays down new land, it also lengthens. Image courtesy of NASA, 2007. Click to enlarge.

The Mississippi River’s deltaic plain begins near Baton Rouge, Louisiana. From here down to the Gulf of Mexico, the region’s high ground is always found closest to the river in the form of its natural levees. That the river is constantly surrounded by, and indeed building upon, its own high ground has some pretty remarkable consequences. As the Mississippi lays down new land, it also lengthens. And as it lengthens, the slope of the river flattens, its waters slow, and sediment starts accumulating in the riverbed. Over time, the river ends up being quite a bit higher in elevation than the surrounding territory, natural levees aside.

Since water always takes the path of least resistance, the only things keeping the river in its channel under these conditions are its natural levees. Given a large enough flood, the river could easily overcome its banks to find a much steeper, much more direct path to lower ground. This kind of event—called an “avulsion”—takes place in deltas with some frequency, both at large and small scales.

Small scale: meander cutoffs, meander scars, and oxbow lakes

NASA - Rio Negro Meander Scars - 2010

Meander scars and oxbow lakes in the floodplain of the Rio Negro, Argentina. Image courtesy of NASA, 2010.

At smaller scales, avulsions work to cut through meanders (creating a “cutoff”) and form new river channels alongside older ones to produce distinctive patterns of meander scars and oxbow lakes.

In 1944, a U.S. Army Corps of Engineers consultant named Harold Fisk used a spectacular aesthetic sensibility to map these patterns as produced by the lower Mississippi over the last 10,000 years. You can download high-quality PDFs of Fisk’s maps from the Army Corps of Engineers here.

Harold Fisk - plate 22-09 - 1944

Plate 22-09 from Harold Fisk's "Geological Investigation of the Alluvial Valley of the Lower Mississippi River," 1944. Click to enlarge.

Large scale: “delta switching”

Meanwhile, large-scale avulsions in the Mississippi River have resulted in much more dramatic landscape transformations. In these events, the river spills over to build an entirely new lobe of land out into the Gulf of Mexico. Also called “delta switching,” this process has occurred about seven times over the last 7000-8000 years.

Delta Switching

The seven deltaic lobes of the Mississippi River. The Balize delta is the current "bird's foot" delta. Image public domain from Wikimedia Commons. Click to enlarge.

In fact, we’re overdue for another. If it weren’t for a serious piece of infrastructure called Old River Control near Simmesport, La, the Mississippi would likely be flowing down to the Gulf through the mouth of Atchafalaya River, over 100 miles west of the current Balize (or “bird’s foot”) delta of the river. Given that the Mississippi River is one of the world’s busiest commercial waterways, imagine the havoc that would cause not only the city of New Orleans, but also the entire United States.1

References

Fisk, Harold. Geological Investigation of the Alluvial Valley of the Lower Mississippi River. Vicksburg, MS: US Army Corps of Engineers, Mississippi River Commission, 1944.

Gupta, Avijit (ed.). Large Rivers: Geomorphology and Management. Hoboken, NJ: Wiley, 2008.

McPhee, John. The Control of Nature. NY: Farrar, Straus, and Giroux, 1989.

Roberts, Harry. “Delta Switching: Early Responses to the Atchafalaya River Diversion.” Journal of Coastal Research 14, 3 (1998): 882-899.

  1. John McPhee told this story with unparalleled skill in a 1987 issue of The New Yorker. You can also read it in his collection, The Control of Nature. ↩

The Physical Environment 1: River Deltas and Delta Time

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In my first post, I gestured at some of the reasons I do research in the greater Mississippi River delta. But what is a river delta anyway and what distinguishes this delta in particular? Over the next three posts I’ll answer those questions by covering some of the fundamentals of delta physical geography. This stuff will probably be a little basic for many physical scientists and perhaps a little far afield for most humanists. But to both camps: please stick with me. The “basics” always bear repeating, especially given how extraordinary deltaic landscapes actually are. And besides, the physical-landscape side of things matters deeply for the cultural and historical work I do throughout my project.

So, with that said…

What is a river delta anyway?1

Bird's Foot Delta of the Mississippi River

The "bird's foot" delta of the Mississippi River. Note the fingers of sediment extending new land into the Gulf of Mexico. Image courtesy of NASA, 2007. Click to enlarge.

Most simply, deltas are landforms that develop wherever a river enters a large body of water—whether ocean, lagoon, or even lake (yep, there are inland deltas)—and deposits sediment more rapidly than can be eroded. Over time, the accumulation of sediment—usually sand, silt, and clay—cause the shoreline to advance, effectively building new land.

Deltas get classified according to the various forces that dominate that land-building process. Which is to say, deltas are defined by whether waves, tides, or the sediment load of the river itself most highly influence the shape of the landform. The active part of the Mississippi delta is classified as fluvial (i.e., river) dominated because of its high sediment loads compared with wave and tide action. Fluvial dominated deltas tend to stretch long fingers or large, broad lobes of shoreline into whatever body of water they encounter.

The Nile River Delta

The term "delta" comes from the Greek capital letter "D," which was thought to resemble the triangular shape at the mouth of the Nile River. Base image courtesy of NASA, 2000. Click to enlarge.

Oh, and why “delta?” The Ancient Greeks believed that the triangular island of sediment at the mouth of the Nile looked much like their triangular letter “D,” or “delta.”2

Delta time

Deltas are fundamentally more dynamic than most other large landscape features. Not only do land-building and erosion take place in what is geologically just a blink of an eye, but the deltas that exist on the planet today are also just really young landforms. It’s fitting, then, that in mathematics and the sciences, “delta” stands for change.

Dboutte - Coastal Change Diagram of Southeastern Louisiana - Wikimedia Commons

The land-building that began at the mouth of the Mississippi River around 7000 years ago was also beginning at coastal deltas around the globe. Image courtesy of user "Dboutte," Wikimedia Commons. Click to enlarge.

Now, one of the most surprising things I’ve learned in trying to get some basic delta geomorphology under my belt is that all of the world’s coastal deltas are about the same age.

Basically, when the last ice age ended about 18,000 years ago, sea levels rose so rapidly that shorelines around the world were pushed several miles inland. Once sea levels began to stabilize about 7,000 years ago, river sediments were able to accumulate (or “accrete”) to form the deltaic landscapes seen around the globe today. So, just as the Mississippi River began building land out into the Gulf of Mexico, so too the Ganges, Yangtze, and Nile rivers (to name just a few) were forming new landscapes in the Bay of Bengal, the East China Sea, and the Mediterranean.

 

  1. This post draws on: Avijit Gupta, “Introduction,” in Large Rivers: Geomorphology and Management, edited by Avijit Gupta (Hoboken, NJ: Wiley, 2008), 1-5; Sampat Tandon and Rajiv Sinha, “Geology of Large River Systems,” in Large Rivers: Geomorphology and Management, edited by Avijit Gupta (Hoboken, NJ: Wiley, 2008), 7-28 ↩
  2. While Herodotus often gets credit for introducing the technical term in the 5th century, BCE, Francis Celoria argues he only used the word as a proper place-name (i.e., Delta, capital “D”) for the mouth of the Nile and that that name had already been in use for as much as several centuries. Only much later in around 200 CE, claims Celoria, did the word take on the general, technical significance it has today. Francis Celoria, “Delta as a Geographical Concept in Greek Literature,” Isis 57, no. 3 (1966): 385-88 ↩

The First Post…

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Hi there.

So, it’s not exactly clear to me what the first post is supposed to look like. My friends over at Scapegoats and Panaces, The Contemplative Mammoth, and Curious Terrain seem to have just launched right in, as if their blogs had suddenly started broadcasting posts midstream. Meanwhile, it looks like AZ, a dear friend over at Dining and Opining, decided to acknowledge her entry into the blogosphere.

Not that it really matters. If I’m realistic about this at all, very few people are going to actually read this first post. Blogging is, after all, ephemeral and what I write here won’t have much currency five posts from now.

Nonetheless, thanks in part to my adviser’s consistently “meta” approach to mentoring (classic questions include: Why do we give exams? What are the attributes of a good presentation? What should a table of contents achieve?), I’ve decided there are a handful of questions a first post should at least try to answer.

What is this blog for?

I’ve started writing here as a companion project to my dissertation (more on that in a moment). I figured a blog provides the space to do a number of things that might help shepherd me through the long, often frustrating process of completing a doctoral degree.

First, in as much as all writing is also thinking, posting here inevitably provides grist for the mill as I think through knotty questions of narrative, argument, and evidence.

Second, Porous Places provides a venue for writing in a much more playful register compared with the confines of a dissertation. I plan to write here about my research in ways that (I hope!) might appeal to a much broader audience than the one scholarly work typically addresses.

Third, I think too often the tortuous (and frequently torturous) process that goes into producing research gets obscured by the slick coherence of the finished product. Research is messy, provisional, and constantly subject to revision for far longer than most people imagine. I’m hoping that Porous Places will serve as a useful place for both acknowledging that messiness and to invite comment and conversation on my work in progress.

Fourth, this is also a place for what I (in another nod to my adviser) would like to call “cool stuff.” While my dissertation needs to fulfill certain criteria in terms of analytical rigor and scholarly contribution, I’d also like to think that it’s going to contain a whole bunch of surprising, quirky, interesting, thought-provoking stuff, stuff that I hope people stumbling across this blog will find just as compelling as I do. Porous Places will catalogue and discuss some of the most striking finds of my dissertation research. It will also be a place for me to write about the anecdotes, events, documents, characters, and so on that, while perhaps really “cool,” just can’t fit within the limits of a dissertation. Some things just don’t make the final cut in terms of narrative or analytical coherence. And some things, while perhaps conceptually related to my work, just don’t fit the focus of the project. Porous Places gives me a way to at least sink my teeth into some of that material instead of casting it aside entirely.

Which reminds me: this fourth point isn’t just about sharing “cool stuff.” I’m hoping that the blog will also serve as a valuable reminder for me of all the things I love about this project. Like I said, writing a dissertation is a long, difficult process. Anything that can keep up a graduate student’s enthusiasm for their subject in the dark days is a valuable thing indeed.

Ok, so I’ve made it clear that this blog is a sort of companion piece to my dissertation project…

So what is my project about anyway?

My dissertation is a historical geography of water in the Mississippi River delta from about 1850 to the present. Now, I realize that’s a pretty opaque description. For one, what the hell is “historical geography”? While my next few posts will do a lot more to unpack that sentence, for now it’s probably enough to elaborate on the project’s main features.

MODIS image of Mississippi River Delta Region and Sediment Plume

True-color MODIS image of the Mississippi River delta region, acquired March 5, 2001. Image courtesy of NASA and Liam Glumley. Note the sediment plume emerging from the active delta in the middle-right portion of the photo.

First, I should emphasize that the delta I’m talking about is a landform. That is, I’m referring to the greater river delta of the Mississippi and not the agricultural “delta” 200-300 miles to the north. That Mississippi Delta isn’t really a delta at all, but rather an alluvial plain lying between the Yazoo and Mississippi Rivers (I’ll be writing more on this distinction in the future).

Shannon1 - Mississippi River Watershed - Wikimedia Commons

Extent of the Mississippi River watershed. Image courtesy of user "Shannon1," Wikimedia Commons.

I came to focus on this particular region precisely for its sogginess. Rainfall from 41% of the lower 48 United States gets funneled down to the river delta where it spreads over the landscape in a network of swamps, rivers, and bayous. As a landscape built up by millennia of continental sediments deposited by the meandering Mississippi, the delta is relentlessly flat and wet.

Dboutte - Coastal Change Diagram of Southeastern Louisiana - Wikimedia Commons

Coastal change diagram of Southeastern Louisiana. Note that almost all of Southeastern Louisiana was built up over about 6000-7000 years of sediment deposition. Image courtesy of user "Dboutte," Wikimedia Commons.

It’s in that geographic context that I’m trying to understand the ways people have negotiated watery landscapes in different ways at different times. How did they approach living in such soggy territory? What were their relationships to wetlands, the Mississippi River and its distributaries, seasonal flooding, waterborne illness, etc. and how have those relationships changed over time? Likewise, how did people’s attempts to live in the delta transform not only the landscape, but also their own livelihoods, cultural values, and social systems? In many ways, the dissertation is an exploration of the changing cultures of water that have shaped both the region’s landscapes and the people living in them.

James L. Langridge, "Negroes Hiding in the Swamps of Louisiana," 1873

An 1873 engraving depicting African Americans hiding in a Louisiana swamp. James L. Langridge, "Negroes Hiding in the Swamps of Louisiana," 1873, "Harper's Weekly," May 10, 1873, pg. 396. Click to enlarge.

To try and answer some of these big, messy questions, I begin with rice agriculture and hydrology on mid-1800s Louisiana river plantations and (likely) end with hurricane Katrina. Along the way, I’ll be examining bald cypress timber harvesting, oil and gas extraction, and petrochemical production, three industries that both deeply depended on, and deeply transformed, the ecological and geological history of the delta.

As I proceed with this research, there’s also one overarching conceptual theme that guides the questions I pose and the evidence I gather: porosity. Though it sounds quite abstract on the surface, it mainly means I’m interested in stories that suggest the historical fluidity of environmental boundaries that humans have taken for granted in different ways at different times. Examples would include:

  • Levees separating river from nineteenth-century plantation
  • Gradients between salt and freshwater
  • Boundaries between urban center and sinking or eroding marshes
  • The conceptual and ecological boundaries between swampy wilderness and reclaimed (or “civilized”) agricultural land
  • The skin that separates human biology from waterborne diseases or toxins
Bell Crevasse Detail

Detail from "The Bell Crevasse," supplement to "The Picayune," May 16, 1858. Click to enlarge.

Now, again, these are big, messy questions, but, this is also what a work in progress looks like.

Hopefully that’s gone some way toward clarifying not only the purpose of the blog, but also some broad strokes regarding the research that will be fueling its content. But I’d also like to offer some further clarification by concluding with the answer to just one more question:

What kinds of things will I be posting?

Well, I alluded earlier to the fact that I plan to create at least a few posts further elaborating on some of the vaguer sounding territory of my project description. There will absolutely be a post on the field and practice of historical geography.

I also plan to devote a more extended piece to the physical geography of the Mississippi River delta (with some more information on why it’s really important not to confuse it with the agricultural region to the north). Indeed, there will probably be a whole post devoted to the peculiarities of deltaic landscapes. Other examples would include: the Mekong River delta of Southeast Asia, Egypt’s Nile delta, and the Rhine-Meuse-Scheldt delta in the Netherlands and Belgium.

It goes without saying that I’ll be writing posts based on bits and pieces of dissertation chapters as they get produced. I’ll also, however, be creating posts commenting on current events or other content from around the web that either pertains directly to the delta region or some of the conceptual themes that run throughout the project. Anything to do with watery places and porosity/permeability is absolutely fair game.

I’ll be posting every Sunday, so feel free to follow me on Twitter, subscribe to my RSS feed, or subscribe by email (in the sidebar to the right).

Thanks for reading!