The Mississippi is mostly thought of as a superhighway of water, endlessly flowing for some 3,700 kilometers from its source at Lake Itasca—a little south of the Canadian border—to its end in the Gulf of Mexico, into which it discharges about one and a half cubic kilometers of water each day (each average day, that is: In a nationwide drought this can slow to less than half a cubic kilometer, while in flood peaks it can exceed seven cubic kilometers). In the Anthropocene, this superhighway is engineered and polluted, contrasting with the ancient wild river that reaches into, and far beyond, the human peopling of North America that started around the beginning of the Holocene epoch, some 12,000 years ago. Looking at the river through the eyes of geology, though, offers a different perspective, in which that long stretch of water becomes only a minor, if ever-present and fundamental, part of the narrative.

The wider river

The Mississippi River is a formal human-made construct, chosen and named from the thousands of possible tips of the river system, a dense network that branches out and drains from some 3 million square kilometers of land—about 40 percent of the area of the United States of America. Almost any of these tips that reach the outer edge of the drainage basin might have become the “main stem” of the river, rather than being artificially relegated as tributaries. Had the end of the Missouri River (or the end of one of its outermost tributaries) been chosen as the main stem of the Mississippi River system, then this would have resulted in a river about 6,000 kilometers long—the fourth longest in the world (after the Nile, Amazon, and Yangtze), rather than being about the seventeenth longest as the Mississippi is currently defined.

The ancient river

The river is indeed ancient. After all, gravity dictates that rainfall in the central part of the US must first collect and then flow somewhere, and the path taken is constrained to the west by the Rocky Mountains, some 80 million years old, and to the east by the Appalachians, some 400 million years old. The water has therefore been making its way across to the south, to flow into the Gulf of Mexico, since the time of the dinosaurs. The river system itself—including the chosen “main stem” of the Mississippi—has changed its pattern and course, but not its function, innumerable times since then.

Among these changes have been those driven by the fifty or so major glaciations that affected North America in the last 2.6 million years. In the most extensive of these, the Laurentide Ice Sheet periodically extended into the northern part of the Mississippi’s course, effectively bulldozing it out of the way, and leaving this part of the river to find a new course when the ice melted away. Each time the ice sheet melted it formed giant lakes, the water trapped by the remains of ice. Each time an ice dam broke, part of the watercourse made its way south along the Mississippi, to flood into the Gulf of Mexico. There were three major meltwater floods between 15,000 and 8,000 years ago, swelling the flow to an estimated twelve cubic kilometers a day—comfortably exceeding the flood peaks of modern times—which could be sustained for decades or even centuries as these giant lakes drained. Flows on this scale could, and seemingly did, pour enough freshwater into the sea to disrupt the major ocean-current systems, and so affect the global climate.

Once the ice had gone, the Mississippi settled into the, more or less, steady and stable pattern of the Holocene epoch. The water kept flowing more sedately. But the solid part of the river system (that is, the accumulated sediment) had by then grown enormous, dwarfing the thin thread of water that had built it.

The solid Mississippi

In the mid-nineteenth century, the river was already not wholly natural, but it flowed a good deal more freely than it does today. The few scientific measurements that were made then, of the sand and (mostly) mud that the Mississippi carried into the sea were crude, but suggest that some 400 million tons of sediment were being carried annually into the sea, building up, year on year, the Mississippi Delta. This figure is probably representative of the natural river in warm interglacial times; the swollen river of the glacial outbursts would have carried a far greater sediment load.

Not all of the sediment makes it to the sea. Much gets dumped along the way, along the beds of river channels and on the banks and, during floods, is draped as swathes of mud and silt across the floodplain, which locally exceeds a width of a hundred kilometers. In this way, the river has built wide landscapes and also concealed clues to the landscape’s history and ecosystems, in the form of buried animal bones and plant debris—and relics of human culture too—caught up in the sediment layers.

Some of the clues lie in the form of the sediment layers themselves. In times of ice and permafrost, the swollen and sediment-choked river often split into the innumerable shallow and ever-changing shallow channels of a braided river. This is the kind of river still common in the far north of the continent, where ice still lingers in the mountains; on picking their way across, the early gold prospectors cursed them as being “a foot deep and a mile wide.” Fossilized relics of the more extensive Ice Age braidplains can be found as wide, low plateaus around the modern Mississippi. As the river then swept most of the mud it carried far downstream, the sands and gravels that remained are now avidly sought by humans of the Anthropocene in order to build their own quickly growing concrete ecosystems.

In the 7,000 years or so since the ice melted, the Mississippi changed into the form of which it became a classic of its kind: The many shallow channels coalesced into the single, sinuous, and deep channel of a meandering river. This formal adjective nicely captures the sense of the active dynamics. A wild river of this kind is forever migrating across its wide floodplain (in excess of 200 kilometers across in the case of the wild Mississippi) as the current erodes one bank, with sediment building up on the opposite bank to compensate. During the larger floods, the channel can avulse—abruptly switching its course to scour out and flow along an entirely different course within the floodplain. And it is common for the more extravagant meander loops to become closed off as the river breaks through to flow along a shorter path. These abandoned meander loops form crescent-shaped oxbow lakes, which become treasure troves of fossilized leaves and pollen, animal skeletons, and other remains that are buried within their soft, muddy sediments. The sandy deposits, neatly stacked side by side by the actively meandering channel, point to a treasure trove of another kind: The Mississippi has become a classic geological model for ancient river deposits that elsewhere—being porous and permeable—commonly contain oil and gas. It is a double-edged kind of treasure, as the exploitation of these hydrocarbon deposits in analogous places like Alberta helps power the changes that are now transforming the Mississippi of the Anthropocene.

The meanders of the Mississippi have ancestors. Here and there, high up on the edges of the modern valley, there are remnants of river strata that date back to before the Ice Ages, to the Pliocene epoch, some 3 million years and more ago. This was a meandering river too, but the fragments of petrified meanders discovered are bigger than those of the Mississippi of Holocene times, and so more water flowed down the older river. Did the river system then have a greater extent, the researchers wondered, perhaps reaching far into Canada? Or, was this simply because there was more rainfall on the North American continent in the Pliocene? It is a significant question. As the Earth’s climate now heats up, we are rapidly approaching a Pliocene epoch kind of a climate (we already have a Pliocene-type atmosphere in terms of carbon dioxide—a greenhouse gas directly linked with climate change—and are waiting for the Earth’s heat balance to reach equilibrium). The Pliocene Mississippi might hold clues as to how an Anthropocene Mississippi could evolve.

The Mississippi meets the sea

The Mississippi River Delta is another geological classic—again with strong hydrocarbon connotations. The landscape that we see now is of some 12,000 square kilometers of wetlands, which now famously hosts New Orleans, a city that will likely fall victim to its Anthropocene predicament (together with much of the rest of the delta) before this century’s end. The entire system, part of which is underwater, extends over more than 30,000 square kilometers, some 12,000 of which are currently above sea level.

Beneath the surface of the delta lie the muds and sands brought in by the river. Over the span of the Holocene, those annual 400 million-ton loads have built into a sediment layer several tens of meters thick, which has been distributed along some 300 kilometers of coastline, as the river has switched to a different outlet to the sea every 1,000 years or so. The currently active Balize Delta, upon which New Orleans is located, was initiated about a millennia ago and now covers an area of about 10,000 square kilometers. Farther west, the Atchafalaya Delta has been developing for the last 400 years and, without the active involvement of the US Army Corps of Engineers, could become the new main route for the Mississippi to the Gulf of Mexico. The latest addition, the nearby Wax Lake Delta, is entirely human generated, resulting from a new artificial channel cut in 1941 and since extended some eight kilometers out to sea.

The Mississippi Delta is a highly dynamic system with a filigree subterranean geometry that has also become a geological classic—another, avidly studied guide to where petroleum might be found stored in ancient examples of these strata. Gigantic fingers of sand mark where the channels flowed out to sea, with even larger ones below marking the shifting positions of “mouth bars” of sand just offshore, as they built out to sea, like a slowly moving one-way conveyor belt. These linear masses of sand are encased in muds so soft and plastic that the sheer weight of sediment can make masses of strata deform and flow deep underground.

The Holocene-period strata are just a veneer upon the surface of a mass of sediment of some fifteen kilometers thick, poured in by the Mississippi, which has been building up since the Jurassic period. Three of those vertical kilometers relate to the Quaternary period—when the ice sheets accelerated the sweeping of sediment off the North American continent. The weight of that enormous sediment mass has depressed the Earth’s crust, in effect to create space for its own deposition, and thus the surface of the delta throughout that time has stayed at or near sea level. When the sea level rises quickly, much of its surface is drowned; when the sea level falls or remains steady, the delta wetlands build out seawards.

The deeper layers of strata that the Mississippi has formed by the ocean’s edge include highly productive oil- and gas fields. And the hydrocarbon connections stretch farther and deeper. Some 300 million years ago, in the Carboniferous period, an even larger and similarly long-lived subsiding delta system produced—by a quirk of geological conditions—much of the world’s deep coal deposits. Formed from the remains of forests drowned by successive sea-level rises and then buried under sediment layers, the area of these coal deposits extend today from the American Midwest and East, and across Western and Eastern Europe. The Mississippi Delta is a prime model for geologists seeking rich coal seams.

This Mississippi River and Delta system has been functioning and creating landscapes and strata since the Jurassic period, and rivers and deltas in general have been behaving along these general patterns for 400 million years, since the planetary revolution that took place when forests began to spread across the Earth’s land surface. Now, in the emerging Anthropocene, another revolution is transforming the form and function of river systems. The Mississippi is a major example of this change.

The Mississippi today

The human impact on river systems, including the Mississippi, did not start in the Anthropocene. The Mississippi was likely changed in some way by the indigenous North Americans through variable deforestation and the introduction of agriculture, with that change accelerated by the early European settlers through agriculture, mining, and urbanization. But the acceleration of change since the mid-twentieth century has been dramatic, even if we are only just beginning to grasp the nature, scale, and implications of this transformation. We only have space here to sketch out a few of its main features.

More than 40,000 dams in excess of six meters in height have been built along the Mississippi and its many tributaries, storing water for hydroelectric energy and irrigation—and, for leisure, in order to build lakes for yachting and powerboating. While these dams only briefly stop the inexorable flow of water, they trap much of the sediment, as much as 20 million tons per year, which would otherwise build up the floodplains downstream, and ultimately construct the entire Mississippi Delta. This dam-building process, which is largely an Anthropocene phenomenon, has taken place along virtually all of the world’s major rivers. It followed a late Holocene phenomenon in which a burst of sediment reached the river after forests were cut down for agriculture, making the surrounding landscapes easily erodible. Thus, while substantial Anthropocene strata are rapidly building up behind dams, the Mississippi Delta is now sediment-starved, with estimates through 1970 to 1990 of only 220 million tons of sediment reaching the ocean, nearly half the natural rates. As a result, the delta is no longer growing at its normal rate, and so is yet more vulnerable to drowning through sea-level rise. Indeed, over the last century, a yearly average of about forty-five square kilometers of land on the delta plain is lost through submergence, which is now on average about 0.6 meters lower in elevation than at the start of the Anthropocene. This represents about 80 percent of coastal land loss in the US.

The meanders of the Mississippi, which have moved freely since (at least) the Mesozoic times, are now straightened and confined within concrete walls, notably thanks to the work of the US Army Corps of Engineers from the 1920s onward. The lower Mississippi River floodplain is now about 10 percent of its natural width. This protection of agricultural land and real estate has a range of consequences. Perhaps most serious is that floodwaters, prevented from taking the natural escape onto extensive floodplains, attain higher flood heights and water-flow rates, exacerbating the potential impact of flooding in sensitive urban areas. Levee construction also prevents the river sediments from depositing onto river floodplains and delta tops, further increasing the risk of drowning and reducing the natural fertility of the soils. In the delta, levees also cut off the supply of freshwater to the marshlands which are increasingly threatened by intrusion of saltwater, with the potential to have a serious impact on this threatened ecosystem. One consequence that will be worth exploring in future research is the change in how and where the Mississippi sediments now build up to form the distinctive river-strata of the Anthropocene. It will be highly instructive to see how the classic geological model of Mississippi strata—as used by generations of geologists as an example of how such deposits are formed (not least with regard to the search for underground oil and gas)—can be modified by the construction of a sedimentary model consistent with the times and processes of the Anthropocene.

The Anthropocene strata of the Mississippi, as well as assuming a fundamentally different geometry, now include a range of new components, different from those of all preceding epochs. A precursor to this Anthropocene record of pollution is the impact of the mining of the Mississippi Valley Type ore deposits—a globally recognized name given to a type of carbonate-hosted lead-zinc ore deposits, first recognized in southeastern Missouri, northeastern Iowa, southwestern Wisconsin, and northwestern Illinois. Lead mining first commenced in the mid-seventeenth century and became widespread in the mid-nineteenth century. As lead extraction decreased after a peak of about 25,000 metric tons per year in the 1840s, zinc became the main metal mined from the region, with nearly 60,000 tons per year worked in the first decade of the nineteenth century; the last mine closed in 1979. The tailings from these mines have inevitably found their way into the rivers and accumulated in the sediment load transported by the river.

The subsequent Anthropocene array of human-generated materials evident in river sediments is remarkably diverse, including a range of plastic polymers, metals such as aluminum and titanium, concrete, and novel ceramics, with much of these fashioned into distinctive technofossils such as bottles and bricks. These objects, and fragments of them, will be carried and deposited together with the river sediment, having been sorted by the running water into size and shape and hydrodynamic equivalence. At the microscopic scale, there is now an influx of microplastics, fly ash particles, glass microbeads, and novel chemicals—persistent organic pollutants such as pesticides and dioxins, which are not only dissolved in the water but also cling to sediment particles such as clays. The study of how these objects and compounds travel through, and are successively buried and exhumed in strata of the river system sensu lato—and how a portion of this novel and infinitely varied cornucopia eventually makes its way into the sea—is in its infancy.

A vast increase in nitrogen and phosphorous entering the Mississippi River system during the Anthropocene results from the runoff of fertilizers, livestock waste, and soil erosion from the major farming states of the Midwest, and human sewage from large urban population centers. Such nutrient-enrichment causes algal blooms that ultimately deplete the oxygen content of waters, especially those entering the Gulf of Mexico at the Mississippi Delta. This creates a “dead zone” that extends westwards from the delta; the extent varies seasonally and has been recorded as up to 22,700 square kilometers (summer 2017), extending as far west as Texas.

The biology (which can also be thought of as future paleontology) of the Mississippi is also changing, as the accompanying essay on “neobiota” (invasive species) makes clear.

The future of the Mississippi in the Anthropocene

What of the future? As the world warms and the sea level rises from the buildup of greenhouse gases, much of the Mississippi Delta (and most of the other sea-facing deltas in the world) will likely disappear over the next century or two, inundated by the sea. A new delta will begin to form, perhaps somewhere near where Baton Rouge is currently located. Even that, though, will likely step farther backwards upon the North American continent as sea levels continue to rise over centuries and millennia, as the Earth slowly adjusts to its new heat balance.

The river, now shortened by losing its seaward portion, will likely adjust in other ways. Quite how its corset of concrete, brick, and steel will develop will depend on how the local population adapts to the trauma and disruption of losing the towns and cities of the delta. All other things being equal, the displaced people will have to move inland and rebuild—and so add to the concrete, brick, and steel that holds the river tight. The abandoned infrastructure of New Orleans and its neighborhoods, meanwhile, will either be degraded and eroded (those parts above the sea surface) or be in the early stages of burial and future fossilization (those parts at and beneath the newly extended shallow sea floor). Other scenarios might be envisaged, though, in which the technosphere does not repair its physical fabric so effectively, if its human components lose or weaken their cooperative function.

The upstream Mississippi will warm as the Earth enters its new, hotter climate state. There will be some physical and chemical effects (water that becomes fractionally less viscous, an increase in the rate of chemical weathering), though the most obvious effects will be in an emerging new community of plants and animals that are better adapted to the heat. Here, the array of invasive species may prove crucial, as among the thousands of invaders will be some from hot climates elsewhere in the world, and they will be the ones best placed to take advantage of this new thermal landscape. One might consider this a positive side effect of the Anthropocene (better, perhaps—some kind of local biosphere more functional than the maladapted remnants of a former, cooler climate state).

The Mississippi will then be a new river in a new Anthropocene world.