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The earth really only has two forces that shape its landscape. On the one hand, there are volcanic forces that make new rock through the cooling of the magma that resides deep within the earth’s crust, which is occasionally brought to the surface through the action of volcanos and other tectonic forces. On the other hand, there is erosion, which breaks up large rocks and rock surfaces, reducing large pieces of stone into smaller ones. Erosion produces particles of stone, called sediment, that are transported by river systems from their source to their ultimate resting place. Sediment deposits are often turned to stone beneath the earth’s surface and sedimentary rock formations are sometimes lifted into mountains and other raised geological features by the forces of plate tectonics. Sedimentary rock formations can then be eroded, once again producing sediment that is transported by the forces of wind and, especially, water.
The Mississippi River is one of the earth’s great movers of sediment. We could imagine it this way: we could envision that we are looking at a rock surface high up in the Rocky Mountains (or the Ozarks, Appalachians, etc.). Erosion from various weather dynamics (i.e. wind, rain, snow, etc.) causes a large chunk of stone to break loose from the bedrock of the mountains. These chunks of rock then tumble down the slopes of the Rockies, carried by the waters of mountain streams.
As these rocks tumble, the large chunks of rock break up into smaller and smaller pieces. These detached pieces of stone are sediment that is now transported by water and wind. In the energetic streams of the high Rocky Mountains, large pieces of sediment—boulders and pieces of gravel of various size—are carried by the water flowing downhill with great force. In the upper courses of streams, channels tend to be narrow and deeply incised, which promotes very swiftly flowing water capable of moving large particles of sediment.
When we reach the lower elevations of mountains, the incline of the land surface decreases, stream channels widen in relation to their depth, and the force of flowing water decreases. Therefore, streams become less capable of carrying large pieces of sediment. The size of the sediment particles therefore varies according to the force of the water acting on them, which in turn varies in relation to the incline of the stream system, as well as its width relative to its depth. By the time we reach one of the streams on the High Plains—let’s say, the Platte River of Wyoming and Nebraska—the largest sediment particles range between pieces of gravel less than 10cm in size and sand of around 1/10mm in size.
In the lower courses of river systems, the incline becomes very low and the channels of rivers become very wide in relation to their depth. These factors result in much slower water flow. Accordingly, this slow-moving water is only capable of transporting very small pieces of sediment, which are referred to as silts and clays. These kinds of sediment are smaller than 1/10mm in size. Silts and clays tend to remain suspended in moving water, falling out of suspension only when the movement of water stops completely. Thus, silts and clays tend to accumulate on the flood plains of rivers, where flood waters initially rise over bank but then come to a stand-still as flood waters recede. This process leaves behind sediment, which accumulates slowly over the course of thousands of years.
The other circumstance in which silts and clays come to rest is when rivers meet the ocean and their water flow comes to a halt. At that point, the velocity of the water in the river drops to near zero, and any sediment that may be suspended sinks to the ocean floor. In this manner, huge amounts of sediment accumulate at the mouths of major rivers, such as the Mississippi, and this is responsible for the formation of new land surfaces.
When major rivers meet the ocean, they form a special type of alluvial fan called a delta. The Mississippi River delta initially formed around 100 million years ago (Figure 1), as the river began to deposit vast quantities of sediment into the Mississippi Embayment, an erosional depression including the “bootheel” of Southeastern Missouri, as well as parts of Illinois, Arkansas, Kentucky, Tennessee, and Mississippi. This depression was originally filled with water from the Gulf of Mexico and was then subsequently transformed into the delta of the Mississippi River. All of the flat bottomland stretching from Cape Girardeau, Missouri, to the coastline of the modern Gulf of Mexico belongs to this delta feature of the Mississippi River.
Sometime shortly before 10,000 years ago, as the glaciers of the last Ice Age (referred to by geologists as the
Pleistocene epoch) began to melt, global sea level began to rise rapidly. By around 7,000 years ago, global sea level had risen to more-or-less its modern elevation, having gone up by as much as 120 meters from its Pleistocene level. This post-Pleistocene sea level rise inundated a coastal plain that extended more than 100 kilometers beyond the modern coastline and that is now covered by the Gulf of Mexico.
When the modern sea level was achieved around 7,000 years ago (in the middle of the epoch that geologists call the Holocene), the Mississippi River began to spill into the Gulf of Mexico near its current location and it began to build the modern land surfaces of the Mississippi River Delta Gulf Coast. From this point onward, the Mississippi River alternated between a number of channels in the lower courses of its delta.
The alternation of outlets in the Mississippi River delta is caused by the steady accumulation of sediment in the main channel. As sediment accumulates in one channel, it obstructs the flow of water and it also raises the elevation of that channel. Water doesn’t like to flow uphill and streams always follow the path of least resistance. Thus, when enough sediment has accumulated in one outlet of the river system, it will switch channels and flow into the sea via another outlet.
The dumping of sediment by the Mississippi River in combination with this process of channel switching was responsible for forming the modern Mississippi River Delta Gulf Coast coastline, which extends over 400 kilometers from Vermillion Bay to the Chandeleur Sound.
In the early-to-middle Holocene, the Mississippi River emptied into the Gulf Mexico through what is today the Atchafalaya Basin, building up the Salé-Cypremort lobe of the Mississippi River delta. Around 4,600 years ago, the course of the Mississippi shifted far to the east, entering the Gulf near what is today Lake Pontchartrain. This course formed the Cocodrie sediment lobe. Next, around 3,500 years ago, the Mississippi shifted back to the west, cutting across the Atchafalaya Basin and forming the Teche lobe. Around 2,800 years ago, the Mississippi moved back to the east, forming the St. Bernard lobe, which includes the land upon which all of the New Orleans metropolitan area and the South Shore of Lake Pontchatrain is located. Finally, after around a 1,000 years ago, the Mississippi alternated between its modern course, forming the Plaquemine and Balize lobes, and an abandoned course to the west, forming the Lafourche lobe.
Today, the Mississippi flows into the Gulf of Mexico through the Balize lobe, which is extensively reinforced by a system of human-made levees and jetties. These levees and jetties keep the entrances of the Mississippi from the Gulf of Mexico clear of sediment and deep enough for commercial barge traffic, most of which currently goes through the South and Southwest Passes of the so-called “Birdfoot Delta” of the Mississippi River (Figure 5).
In summary, the coastal Mississippi River delta formed over the last 7,000 years or so, as the river dropped sediment when it reached the sea and its water velocity decreased. In alternating between several different river mouths, the Mississippi distributed sediment over a wide swath of land across Southeastern Louisiana, thus forming the modern land surfaces of the coastline in this region.
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