The Physical Environment 1: River Deltas and Delta Time

In my first post, I gestured at some of the reasons I do research in the greater Mississippi River delta. But what exactly is a river delta 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 exactly is a river delta? 1This 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.

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.

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.” 2While 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 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.

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.

 

References   [ + ]

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.

10 Comments

Join the discussion and tell us your opinion.

– Porous Placesreply
April 01, 2012 at 08:04 PM

[…] Post navigation ← Previous […]

The Physical Environment 2: High Ground, Low Groundreply
April 08, 2012 at 10:04 AM

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.

Porous Places – Mountains and Riversreply
April 15, 2012 at 11:04 AM

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

Deltas and Human History – Porous Placesreply
May 13, 2012 at 11:05 AM

[…] 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 […]

Flood Insurance in a Dying Deltareply
August 20, 2013 at 06:08 PM

[…] is that? As I’ve discussed previously on this blog (see here, here, and here), coastal Louisiana is a very young, very flat, very muddy landscape. It got built […]

DredgeFest Louisiana | Porous Placesreply
January 23, 2014 at 02:01 PM

[…] is more profoundly felt as part of daily life. As I’ve recounted elsewhere on this blog (here and here), southern Louisiana was built up entirely from about 8,000 years of sediments deposited […]

The Physical Environment 3: Building on Its Own High GroundPorous Placesreply
May 01, 2014 at 02:05 PM

[…] 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 […]

Brandon Brooksreply
April 29, 2016 at 05:04 PM

Great article. I love the blog too.

Adam Mandelmanreply
April 29, 2016 at 05:04 PM
– In reply to: Brandon Brooks

Thanks, Brandon!

Douglas J Norrisreply
May 28 at 06:05 AM

Thanks great work and must agree that you can never repeat the basics too many times!
Terrific help for my essay.
Cheers
DJN

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