Wetlands as Borderlands: Where Land and Water Meet

Posted on June 1, 2012 by Adam Mandelman

Moving water connects these places, weaving the threads of the landscape together. The places where water and land combine—the riparian zones—mediate these connections, and what happens in these zones affects areas far beyond their boundaries.
— Nancy Langston, Where Land and Water Meet

Although it only describes riparian wetlands, this passage from Nancy Langston applies to pretty much any soggy ecosystem. For me, it has long suggested that wetlands might be usefully thought of as borderlands, albeit not in the conventional terms of state or political boundaries. Rather, wetlands are borderlands in that they are fundamentally “in-between” places—whether in terms of ecology, geography, or even culture and society—that have wide-ranging impacts.

Where land and water meet: physical borderlands

Wetlands exist simultaneously as both boundary and intersection. Which is to say, they mark the edges of terrestrial and aquatic ecosystems while also embodying thorough hybrids of land and water. As such ”liminal” (or borderline) places, they gain a truly unique identity of their own.

A home in the marshes along Bayou Manchac, Louisiana. Photo by author, October, 2011.

For both geographic and temporal reasons, the in-betweenness of wetlands also makes them a kind of borderland that can be particularly difficult to consistently define.

Geographically, wetlands rarely have distinct edges and exist instead as part of a continuous gradient between dry land and open water. How far into an estuary must I venture before I’ve left the wetlands at its edges? And if I find floating mats of grass and other plants scattered throughout that open water, what then? Meanwhile, depending on changes in topography, hydrology, climate, and resident animal and plant communities, the wettest parts of a wetland can even shift across a given landscape (the Tijuana Estuary’s river mouth, for example).

Temporally, wetlands might be continuously, seasonally, or only infrequently wet. How often and how persistently must a place flood (or drain) to be defined a wetland (or not)? Similarly, wetlands can come and go quite easily over time, emerging or receding into the terrestrial and aquatic worlds at their edges through sinking soils, ecological succession, climate change, or even through biotic activity (the building or collapse of beaver dams, for example).

A bald cypress wetland near Weston, North Carolina during the dry season. Note the high-water marks on the trees running midway through the image. Photo by Duane Burdick, June19, 2010.

Now, it’s worth noting that if the “moving water” of Langston’s passage doesn’t seem to apply to the so-called stagnant swamps or bogs of the imagination, well that’s only because our imaginations have wrongly equated wetlands with stagnation. Though its movement might be so sluggish as to be imperceptible, water in even the boggiest of bogs is always moving, whether percolating through underlying soils, migrating in surface flows, or even simply evaporating into the air.

While our language and cultural references often suggest otherwise—think: “bogged down”; being “swamped with work”; the doldrums scene in the animated version of The Phantom Tollbooth; or the Swamps of Sadness in the Neverending Story—wetlands, in their borderland in-betweenness, are in fact very dynamic landscapes.

What makes them so dynamic? Well, all that slow-moving water makes for a host of very distinctive ecosystem functions. For a start, wetlands serve as critical habitat for a variety of mammals, fish and shellfish, amphibians, and birds, many of which might be endangered or threatened. It also probably goes without saying that wetlands harbor very distinctive plant communities, from spanish-moss-draped bald cypress and tupelo gums to carnivores like venus flytraps and pitcher plants.

Bald cypress, spanish moss, and water lilies in a swamp near Breaux Bridge, Louisiana. Photo by author, June 21, 2010.

Additionally, in gathering and retaining slow-moving water, they help recharge aquifers and provide crucial buffers for flood and storm-surge mitigation. At much larger regional and global scales, wetlands perform valuable ecosystem services by acting as sites for carbon sequestration, de-ntirification of agricultural runoff, and balancing global sulfur cycles. In a related fashion, wetlands, often described as the “kidneys” of watersheds, play important filtering roles for the waters that, no matter how sluggishly, move through them. Wetland biology, topography, geochemistry, and hydrology all uniquely combine to render these landscapes crucial sinks for excess nutrients and toxic chemicals.

This diagram illustrates some of the "kidney" functions of wetlands in a watershed. Image courtesy of the City of Caspar, Wyoming.

And again, that sluggish water is really important. It’s not only the critical condition for sustaining these water/land hybrids, it also transforms wetlands into a very particular kind of node in a watershed’s network of streams. I’ve often thought of watersheds as collapsing distance. Which is to say, whatever happens upstream often leaves its signature somewhere downstream. Wetlands—as places in the watershed where land and water are in close, prolonged contact—are where that upstream signature often gets rendered a little more visible. Whether in the form of eroded upland sediments, fertilizers from agricultural runoff, industrial toxics, or even pharmaceuticals in urban wastewater, upstream detritus accumulates in wetlands thanks to the combined forces of slack water, ecology, and topography. As places so thoroughly in between land and water, I imagine wetlands to be almost like the connective tissues of watersheds.

Landscapes on the periphery?

Riparian meadows and backswamps, estuaries and deltas, coastal marshes and inland bogs, and so on aren’t just dynamic ecological and geographical borderlands, however, they’re also fundamentally cultural and social ones as well. I’ve mentioned before that Ann Vileisis’s watershed (see what I did there?) book, Discovering the Unknown Landscape, highlights the ways the Europeans and their descendants that occupied North America have long scorned watery landscapes. Her work is revelatory for its insights around the intersecting ways culture, politics, and ecology have played out in the history of American wetlands, frequently with depressingly destructive results. Indeed, Vileisis points to what seems to be a trend wherever western modernity has taken root: the draining and dredging of watery landscapes. Wetlands around the world are increasingly threatened, if not disappearing entirely.

Udo Keppler, "If you want to get rid of mosquitos, drain the swamp that breeds them," 1909. Though actually a political cartoon commenting on early twentieth-century U.S. economic policy, it illustrates the widespread Euro-American perception of wetlands as dire nuisances. Image courtesy of Library of Congress, call number: AP101.P7 1909 (Case X) P&P. Click to enlarge.

Yet, as I’ve also mentioned before, Vilesis’s narrative, aside from discussing Native American use and occupation of wetlands, also obscures some of the fundamentally human histories of these places. At the very least, though wetlands may have been anathema to Euro-Americans, that still leaves the long, if not always intensive, histories of human occupation in other watery landscapes around the world.

Marsh Arabs photographed in 1950 by Wilfred Patrick Thesiger. 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.

Landscapes of encounter

But, even if we were to focus only on North America (as my research does), I don’t think a narrative of derision, resource extraction, and destruction—followed by snippets of redemption in the environmentalist late-twentieth century—tells the entire story. For one, even if the majority of Euro-Americans shunned wetlands, watery landscapes often acted as refuges for marginalized peoples in the New World, from displaced Native Americans and Cajuns to escaped slaves and other fugitives. Though these histories might on the surface justify the claim that North American wetlands were “landscapes on the periphery,” that phrase doesn’t do much to reveal the ways such landscapes also served as sites of encounter between the margins and the core.

Imagining wetlands as borderlands instead of peripheries—that is, as places that imply encounter and exchange, rather than marginality and obscurity—allows much more room for moving them to the center of historical and geographical questions about nature and society. Rather than simply being wild places on the edges, edges that have receded with ever greater human transformations of the environment, wetlands as borderlands become busy places full of deeply human questions.

For example, what was life in Louisiana swamp or marsh communities actually like and how did places like New Orleans, or the reclamation efforts of planters and other entrepreneurs, intrude on their lives? Or, how did wetlands facilitate the establishment of fugitive “maroon” societies of escaped slaves? How did these colonies participate in clandestine slave communication networks that tied “civilized” plantations to unruly marsh and backswamp? Similarly, how did they manage encounters with bounty hunters, loggers, and other emissaries of of the metropolitan core?

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

African Americans hide in a Louisiana swamp after the Colfax Massacre of 1873. White militias had killed dozens of freedmen in the wake of a contested Louisiana gubernatorial election. James L. Langridge, "Negroes Hiding in the Swamps of Louisiana," 1873, "Harper's Weekly," May 10, 1873, pg. 396. Click to enlarge.

All of this is to say nothing of the succeeding waves of colonization—each accompanied by their own explorers, resource harvesters, and traders—that spread over southern Louisiana. From French to Spanish, back to French, and finally to the Americans, the watery landscapes of the Mississippi River’s lowest reaches were fundamentally sites of knowledge exchange, cultural encounter, and both conflict and accommodation.

Of course, the region was distinguished by political boundaries for some time, making Louisiana a much more conventional borderland. But the cultural and economic exchanges that distinguished the colonial period also persisted long after those political boundaries dissolved. Those succeeding waves of colonial occupation and their creole (or hybrid) cultural legacies took place largely because of the Mississippi River watershed’s role as a vital transportation network that spanned over two-thirds of the continent.

Given that crucial role of the river delta, it’s hard for me to imagine southern Louisiana’s borderlands history (in the more conventional sense of the term) wasn’t just as profoundly shaped in some way by its wetlands as by its shifting national boundaries.

Wetland frontiers: natural resources, land reclamation, and human bodies

Besides being borderlands of encounter between the marginal and the powerful or between vastly different cultures and economies, wetlands in my research area were also important resource frontiers. Logging in Louisiana swamps at one time counted for a majority of cypress lumber extracted in North America, while logged-over swamps and treeless marshes also saw a variety of aggressive attempts at being “reclaimed” or otherwise developed for agriculture, oil and gas extraction, and even residential communities. Certainly, scholars like Ann Vileisis have told these stories of extraction and drainage in great detail. Yet the ways logging, oil and gas development, and wetland reclamation also represented an ever-shifting borderland between wild and improved land receives little attention.

A floating sawmill in a Louisiana swamp. Photo by George François Mugnier, taken sometime between 1880 and 1920. Image courtesy of the Collections of the Louisiana State Museum.

How did these industries and their laborers adapt to working in these watery landscapes? What happened culturally, economically, environmentally when a new logging town sprung up in the midst of a Louisiana swamp only to be abandoned and recede back into “wilderness” (albeit a dramatically altered one)? What new technologies did logging or petroleum industries bring to wetlands that enabled them to do their extractive work and how did those new technologies transform wetland ecosystems into new, part-humanized, part-wild landscapes? These are just a handful of questions that arise when we consider wetland resource frontiers as landscapes of adaptation and hybridity rather than simply as sites for disappearing wilderness.

Finally, and I might be pushing the borderland metaphor here, all of these stories necessitated deeply visceral encounters between human bodies and an inhospitable environment. People are, by nature, not so well-adapted to watery places. To state the painfully obvious, we lack the right kind of eyes and skin, to say nothing of fins, webbed appendages, or gills to comfortably navigate and inhabit soggy places. Swamps and marshes present some of the most challenging environmental conditions for human beings. Though they might have rarely risked starvation, people living and working in wetlands faced water and vector-borne diseases, hostile wildlife, exposure, and even drowning on a daily basis. Compound those dangers with the facts of being a hunted fugitive or the risks of operating temperamental floating sawmills, logging equipment, or dredges and other machinery, and wetlands inevitably became dramatic sites of confrontation between a fragile human body and a decidedly “other” landscape.

Laying a Standard Oil pipeline in Louisiana swamp.

Laying a Standard Oil pipeline in Louisiana's swampy environment was a challenge for both industry and its laborers. Image courtesy of "Fuel Oil Journal," December, 1914. Click to enlarge.

Porous places: boundary crossings in watery places

In bringing this post to a close, a disclaimer/apology is in order: I’ve tended toward more abstract, imprecise concepts and hand-waving than I would like. Of course, that fulfills one of the original goals of this blog: a means of thinking (out loud) through dissertation ideas and questions as they evolve. And this particularly idea is definitely one in progress.

So, yes, wetlands are clearly redolent of many kinds of historical boundary crossings, though I’ve only suggested a handful here: aquatic/terrestrial, periphery/core, marginal/powerful, other/familiar, body/environment. But there are also onceptual risks in gathering these boundary-crossings under one wetland roof, from Jay Taylor’s concern over imprecision (see his “Boundary Terminology” article listed below for more on this), to producing unhelpful dualisms.

One of our capacities as creative, thinking creatures is analogy and, really, it’s possible to analogize pretty much any one thing to any other. If I start seeing borderlands everywhere, they cease to become useful explanatory metaphors. The same goes for other concepts that have been bubbling away as I undertake my research: permeability, porosity, membranes, etc..

That said, I’ve also already been rewarded a few times in the archives by keeping these concepts at the forefront of my thinking (more on those rewards in future posts). The trick will ultimately be to make sure that—whether I’m speaking of borderlands, porous places, or permeable membranes—each of my arguments actually gains analytical traction from those metaphors, rather than just conceptual window-dressing.

So, if you have thoughts on wetlands as borderlands, or on thinking about non-national, non-political borderlands in environmental history or geographical scholarship, drop me a comment. And thanks for reading.

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?

"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. ↩

Mountains and Rivers

Posted on April 15, 2012 by Adam Mandelman

I’m delaying the third post on deltaic physical environments (the other two are here and here) to briefly write about a map I recently came across at Big Map Blog (which you absolutely should follow, by the way).

George W. Colton, "Mountains and Rivers," 1856. Image courtesy of the David Rumsey Map Collection. Click for high-resolution, zoomable image.

Produced by George Colton in 1856, the map depicts, left to right, several dozen major world rivers decreasing in length and several hundred mountains increasing in size.

Be sure to click the image above for a super high-resolution, deep-zoom version.

Although the map completely abstracts individual rivers from their particular uplands, I still love the way its arrangement of features implies the massive redistribution of sediment that takes place in large watersheds.

Of course, not all the rivers shown here end in deltas. Sometimes tides, waves, sediment loads, and the physical features of both the river and the body of water into which it drains all combine to prevent land from accreting at a river mouth, forming an estuary instead.

But even though such kinds of conditions might not allow for land-building, rest assured that each river depicted on this map is diligently transporting bits and pieces of its uplands down to its mouth. The Alabama/Mobile/Tombigbee, for example, ends in an estuary rather than a delta, but it still discharges 4.5 million tons of sediment into Mobile Bay each year. And it’s one of the smaller rivers show here (in the right third of the image).

Additionally, Colton’s map documents not only the diversity of deltaic landforms around the world, but also suggests just how many deltas have in fact been historically important sites of human occupation. Moving from left to right on the map, the Mississippi, Yangtze, Ganges and Brahmaputra (“Burrampooter” according to Colton), Niger, Tigris and Euphrates, Indus, and Rhine river deltas—to name just a handful—have all supported major population centers, some of which are thousands of years old.

And finally, I’d say that the map is just great for exploring. Zoom right in and you’ll find glaciers, active volcanos, major architectural and natural landmarks, and even some traces of Alexander von Humboldt’s Latin American expedition.

There are a lot more wonderful, super high-resolution thematic maps like this one available through both Big Map Blog and David Rumsey’s Map Collection. Be sure to check both sites out for similar treasures, regardless of your field/subject of interest.

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: river deltas

Wetlands as Borderlands: Where Land and Water Meet

Where land and water meet: physical borderlands

Landscapes on the periphery?

Landscapes of encounter

Wetland frontiers: natural resources, land reclamation, and human bodies

Porous places: boundary crossings in watery places

Further Reading

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

Mountains and Rivers

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

Where land and water meet: physical borderlands

Landscapes on the periphery?

Landscapes of encounter

Wetland frontiers: natural resources, land reclamation, and human bodies

Porous places: boundary crossings in watery places

Further Reading

Share this:

Productivity and sea levels

The productive river mouth

Deltas and society

Further Reading

Share this:

Holding the high ground?

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

Large scale: “delta switching”

References

Share this:

Share this:

Natural levee and backswamp

References

Share this:

What is a river delta anyway?1

Delta time

Share this:

What is a river delta anyway?¹