Deltas and Human History

Posted on May 13, 2012 by Adam Mandelman

A few posts back, I flagged an 1856 map showing major mountains and rivers of the world arranged by size. One of the reasons I like that map is that it lays out dozens of river mouths side-by-side. As you scan the lineup, you’re reminded just how many major centers of human history have in fact been located in the watery landscapes of river mouths, whether delta or estuary.

But what exactly is the story behind human occupation of deltas? How far back does it go? And what precipitated it in the first place? Ann Vileisis’s Discovering the Unknown Landscape: A History of America’s Wetlands would suggest that extensive human occupation of deltas, estuaries, and other dynamic, watery places only happened through draining, filling, and otherwise drying these landscapes. Vileisis convincingly argues that, at least for Euro-Americans, “wetlands have long been a landscape on the periphery,” a landscape “long despised and avoided.”

But what about the rest of the world? Euro-Americans only settled in places like southern Louisiana in the last three hundred years. On other continents people have been living in and around wetlands for millennia, and often without attempting to so aggressively dry the landscape. What accounts for this opposing and rich history of human settlement in estuaries and deltaic plains (here’s a particularly striking example) around the globe?

Wilfred Patrick Thesiger, Marsh Arabs, 1950

"Marsh Arabs" photographed by Wilfred Patrick Thesiger in the Tigris-Euphrates marshlands of Iraq, 1950. Photo courtesy of Pitt Rivers Museum, accession number 2004.130.29267.1. Note the livestock in the far-left background of the image. Click to enlarge.

Productivity and sea levels

One of the field of ecology’s most basic measures of an ecosystem is “productivity,” or the rate at which solar energy gets converted into living tissue (or biomass). Primary productivity refers to the rate at which plants convert solar energy directly into biomass. Secondary productivity refers to the rate at which organisms (mostly animals) convert other organisms into biomass. Which is to say, when animals (including humans) eat plants, fungi, and other animals to build tissues, they’re engaged in secondary productivity.

What does this have to do with river deltas? When sea levels began to stabilize about 7,000 years ago after a rapid rise at the end of the last ice age (about 18,000 years ago), they facilitated a huge burst of productivity along the coastal margins of the world’s landmasses. This happened largely because of the particular form continental edges take.

Rising sea levels pushed coastlines several kilometers inland, encroaching on continental shelves. Compared with the steep continental slopes where many coastlines had previously begun, continental shelves have much more gentle gradients. When sea levels stabilized at this higher elevation, they left these shelf areas inundated, creating much larger expanses of shallow water.

Note the steepness of continental slopes as compared with continental shelves. The more gentle gradient of continental shelf areas, inundated by rising sea levels, allowed for much larger areas of shallow waters. Image courtesy of Wikimedia Commons.

Shallow waters allow a great deal more light to reach the ocean floor. Since productivity depends ultimately on the conversion, by plants, of solar energy into biomass, it’s probably no surprise that shallow waters are a great deal more productive than deep ocean.

The productive river mouth

Because rivers deliver soil, nutrients, and organic matter to these areas while also stirring up nutrient-rich bottom waters. Deltas and estuaries, then, by virtue of being the meeting place of shallow ocean waters and nourishing rivers, are some of the most productive ecosystems in the world. Enriched by the combined pulses of seasonal flooding and marine tides, these landscapes rival tropical rainforests in their biological abundance. Indeed, a large portion of the world’s marine fisheries today depend fundamentally on the ecosystem services provided by deltas.

These dramatic increases in productivity along the world’s coasts, particularly wherever rivers encountered the ocean, produced a huge surge in the availability of large, nutrient-rich (think omega-3 fatty acids!) organisms. Without sounding too much like an environmental determinist (after all, human cultural practices and social institutions matter critically here), that abundance of high-quality food provided a set of conditions in which human communities could thrive.

James Tissot, "The Miraculous Draught of Fishes," c.1886-1896. Of course, productivity is hardly a miracle in this sense. Tissot's painting is housed at the Brooklyn Museum.

Deltas and society

And indeed, in some places, they did. In one of my first posts, I described how stabilizing sea levels around 7,000 years also facilitated the formation of deltas and estuaries at river mouths. These new coastal landscapes were sites for some of the highest increases in productivity I’ve been describing. Within about 1000 years of sea levels stabilizing, early urban societies had begun to take hold in these new, highly productive environments all around the world. In the case of the Nile and Yangtze rivers, it only took about 500 years after sea levels stabilized for humans to expand settlement onto newly forming deltaic plains. River deltas globally became important sites for new developments in agriculture and hydraulic engineering, not to mention social organization and cultural exchange.

All that may seem like ancient history. But today deltas (to say nothing of estuaries) are still home to over half a billion people, including almost the entire population of Bangladesh. More than 200 million people live in the Ganges, Nile, and Mekong deltas alone. Meanwhile, several Asian megacities—Shanghai, Karachi, Kolkata (formerly Calcutta), Bangkok, and Ho Chi Minh City, to name just a few—are located on deltaic plains.

Lower Ganges-Brahmaputra Delta in Bangladesh

Almost all of Bangladesh's 150 million inhabitants live in the Ganges-Brahmaputra Delta. Pictured here is the very lowest part of the delta, which contains several large cities of over 1 million people. Offscreen to the north is the Bangladesh capital, Dhaka, also located on the delta. Note Kolkata, India lies on the delta to the east. Image courtesy of Google Earth. Click to enlarge.

That half a billion, then, though a fraction of the globe’s population, isn’t insignificant. After all, the planet’s combined coastal zones—of which deltas are just a tiny portion—make up only 5% of the earth’s land surface.

Which brings me to my main point. Deltas and estuaries aren’t just highly complex ecosystems. They’re also the epitome of porous places. Ann Vileisis may be right that at least Euro-Americans, having long despised wetlands, relegated them to the margins of cultural and social life. But I’d also say that story obscures the fundamentally human histories of these particular wetlands elsewhere in the world.

Can Tho Floating Market, Vietnam, October 15 2009

Cần Thơ Floating Market, Vietnam. Cần Thơ is the largest city in the Mekong Delta. The region has been extensively inhabited for over two thousand years and is home to around 17 million people today. Click to enlarge.

After all, complex societies emerged on (geographically) and alongside (temporally) these landscapes. Moreover, by being located at the intersections of vast watersheds and even vaster oceans, the communities that arose around deltas and estuaries became critical hubs for the exchange of goods, knowledge, and culture. Deltaic and estuarine wetlands may indeed be historically marginal places, but not in the sense of the word that conveys the insignificant or the peripheral. Rather, deltas are margins of encounter. They mark borderlands not only between diverse cultures, but also between earth and water, saline and fresh, nature and society. They are borderlands in which all of these things get muddled in highly productive exchanges (and I’m no longer speaking just ecologically).

Yes, today human-occupied deltas may be facing massive ecological challenges, from rising sea levels and subsiding landscapes, to aquatic dead zones. They have been marked, perhaps indelibly, by human institutions that have sought to stabilize and control what are fundamentally dynamic environments.

But these places also suggest long, deep histories of accommodation and negotiation, of give and take across fluid, porous boundaries between land and water, human and environment.

The Physical Environment 3: Building on Its Own High Ground

Posted on April 23, 2012 by Adam Mandelman

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

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.

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.

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.

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 April 8, 2012 by Adam Mandelman

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.

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 March 25, 2012 by Adam Mandelman

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

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 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.

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 ↩
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 ↩

Porous Places

A blog about watery landscapes

Tag Archives: land-building

Deltas and Human History

Productivity and sea levels

The productive river mouth

Deltas and society

Further Reading

The Physical Environment 3: Building on Its Own High Ground

Holding the high ground?

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

Large scale: “delta switching”

References

The Physical Environment 2: High Ground, Low Ground

Natural levee and backswamp

References

The Physical Environment 1: River Deltas and Delta Time

What is a river delta anyway?¹

Delta time

Productivity and sea levels

The productive river mouth

Deltas and society

Further Reading

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Holding the high ground?

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

Large scale: “delta switching”

References

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Natural levee and backswamp

References

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What is a river delta anyway?1

Delta time

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What is a river delta anyway?¹