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.

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.

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.

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.