Tag Archives: the basics

The Physical Environment 3: Building on Its Own High Ground

Posted on by
1

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 2: High Ground, Low Ground

Posted on by
4

This is the second in a series of three introductory posts focused on some of the basics of deltaic physical environments. Last time, I looked at what defines a river delta as well as the timing of coastal delta formation globally around 7000-8000 years ago.

Natural levee and backswamp

A key thing to remember about deltaic landscapes is that the rivers running through them are creating, rather than eroding, land. I know I probably sound like a broken record on that front, but it’s also easy to overlook some of its implications. When we think of a typical riparian landscape, I think we often imagine a river valley, in which the highest ground is furthest from the river.

Thomas Cole - The Oxbow - 1836

A river valley. Note the high ground is furthest from the river. Image: Thomas Cole's "The View from Mount Holyoke, Northampton, Massachusetts, after a Thunderstorm," 1836 (more commonly known as "The Oxbow"). Click to enlarge.

Because of the way rivers deposit sediments in deltas, however, the highest ground is actually closest to the river. During seasonal flooding in these landscapes, sediment-laden water rises above the riverbank. As it spreads over the floodplain, that water suddenly slows, allowing particles of suspended sediment to settle and build on the landscape.

Trudeau - Plan of the City of New Orleans - 1798

Carlos Trudeau's map of New Orleans in 1798. Note that settlement hugs the Mississippi and that lands furthest from the river are marked as "cypress swamp." Click to enlarge.

Since the heaviest (and therefore largest) particles settle first, the high ground in river deltas always forms closest to the river. This embankment is called the natural levee. The further one gets from the main channel of the river, the smaller the particles of sediment that accumulate. The relatively narrow strip of high ground that constitutes the natural levee, then, begins to slope back down until it becomes a low wetland known in Louisiana as “backswamp.”

So, while deltas are typically very flat places, what little elevation there is usually hugs the river. That topography is particularly visible in the patterns of historical settlement in the Mississippi River delta.

Abbot - Approaches to New Orleans Map - 1863

Henry L. Abbot's Civil War map of "Approaches to New Orleans," 1863. Note that 65 years after Carlos Trudeau's map, settlement still hugs the high ground adjacent to the river. Click to enlarge.

 

 

 

While levees, drainage projects, and other extensive alterations to the landscape have allowed people to build homes and expand urban areas into what was once swampy low ground, places like New Orleans first emerged on the natural levee immediately along the river.

But where does all this sediment come from?

River deltas aren’t building new land and high ground from sediment plucked out of thin air. This material all comes from somewhere. And the way to figure out that “somewhere” is to look at a map of a river’s watershed. In the case of the Mississippi, you’ll notice that its waters are supplied by streams and rivers from as far west as the Rockies and as far east as the Appalachians.

Shannon1 - Mississippi River Watershed - Wikimedia Commons

Mississippi River sediments originate as far west as the Rockies and as far east as the Appalachians. Image courtesy of user "Shannon1," Wikimedia Commons. Click to enlarge.

If we think about the distinction between river valleys and river deltas as being one of eroding versus accreting landscapes, we can start to imagine that large river systems are massive sediment redistribution programs. Rain and snowmelt falling higher up in the watershed carry weathered rocks and soils from across the continent down to the mouth of the Mississippi. There, deposited as sediment, these fragments of distant landscapes build the delta. In a way, then, Deltas are the means by which ancient mountains get transformed into new shorelines.

But humans have also radically interfered with that sediment redistribution process in deltas all over the world and the Mississippi River delta is no exception. These days, most sediments are either trapped behind dams throughout the watershed or prevented from spreading over the landscape by the levees lining the river. Land-building in the Mississippi’s deltaic plain has, except in a few locations, practically ceased.

Suspended Sediments 1700 vs 1990 - USGS Circular 1133 - 1995

Sediments reaching the Mississippi River delta have declined significantly since 1700 due to dams and levees. Diagram from Meade (ed.), "Contaminants in the Mississippi River, 1987-92," 1995, pg. 18. Click to enlarge.

References

Campanella, Richard. Bienville’s Dilemma: A Historical Geography of New Orleans. Lafayette, LA: University of Louisiana at Lafayette, 2008. Also be sure to check out Campanella’s website at http://richcampanella.com/.

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

Meade, Robert (ed.). Contaminants in the Mississippi River, 1987-92. Reston, VA: US Geological Survey, Circular 1133, 1995.

The Physical Environment 1: River Deltas and Delta Time

Posted on by
4

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 ↩