Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Rivers can seem very ephemeral, often changing course or drying up entirely. Yet some rivers have persisted for tens or even hundreds of millions of years, even testifying to the breakup of Pangea, the most recent supercontinent, about 200 million years ago. On the one hand, their courses may be determined by tectonic processes, such as the formation of mountain belts. And on the other, they themselves can affect tectonic processes by creating continent-scale features such as giant submarine fans. Martin Gibling has spent a lifetime studying rivers and river sediments around the world. He is emeritus professor in the Department of Earth and Environmental Sciences at Dalhousie University in Halifax, Nova Scotia, in Canada. This episode is the first of two of our conversation about rivers. In this episode we talk about fluvial deposits in the geological record and the impact of the breakup of Pangea on river systems. In the second episode, we talk specifically about the history of rivers of Europe and the Americas, as well as the impact of recent Ice Ages. We end by considering how humans have changed rivers and their deposits throughout human history. Martin Gibling, welcome to Geology Bites.

Martin Gibling: Thanks very much, Oliver. It's a great pleasure to talk with you.

Oliver Strimpel: How far back into the geological record do fluvial deposits go?

Martin Gibling: Go back a surprisingly long way. The oldest fluvial deposits about which we are really certain are in the Moodies Group of southern Africa, and those date back to about 3.2 billion years, and that's a long way back towards the origin of the Earth at 4.6 billion. They may well be older fluvial rocks, but up to now, they've been too altered and deformed for anyone to be certain about. However, it's worth saying that in rocks of Western Australia, researchers have found zircon grains that date back to about 4.4 billion years. That takes us shortly after the formation of the Earth, and, very surprisingly, they yield isotopic evidence for surface temperatures that were very much like those of today. That implies liquid water on the surface of the Earth. It in turn implies the presence of flowing water and rivers. So we have to assume that rivers have been with us since the very earliest days of planet Earth.

Oliver Strimpel: OK, but with those zircons, that's quite a complicated chain of inferences. How do we actually distinguish an intact fluvial deposit in the geological record from other waterborne deposits, such as those laid down on lake beds or on the seashore, and those deposited by wind?

Martin Gibling: Yes, you have to get into the details in the rock record in the strata. And there we're looking at the organization or architecture, as it's called, of the strata, as well as particular sedimentary features and fossils that are present. And, of course, if you're far back in time, you won't have the fossils to guide you. And if you've got very deformed or altered rocks, then you're struggling to make sense of the sedimentary features. But what you can often see in younger strata, is places where river channels have eroded down into the strata below, and you get surfaces that look very much like channel forms filled in with sand and gravel. What's filling them in is often cross beds laid down on the avalanche faces of ripples and dunes, and those allow you to measure the flow directions at the time the water was flowing, and in the case of river channels, they are unidirectional. They're all going in one direction down the channel, which is strong evidence that these were fluvial. And then you can find things like rain prints and mud cracks in the strata which indicate that these were exposures on the terrestrial surface, and then in younger beds you get terrestrial fossils, roots, woody material, freshwater fish, shells and things like trackways made by giant arthropods, and also by dinosaurs, of course. So all these lines of evidence can make you pretty certain that you're dealing with fluvial deposits.

Oliver Strimpel: Wouldn't most of those also possibly appear in wind-borne deposits? Or is it that it's unidirectional with rivers so that rules out wind because it can blow this way and that.

Martin Gibling: It can, but you can of course get systematic directions of wind flow that would generate unidirectional cross strata, but in those cases you wouldn't be seeing the channel bases, sometimes with gravel lining the surfaces as a kind of a lag where the channels eroded in, and that's where the architecture of the strata, and their organization come into play. You would also get mud layers in river channels that you wouldn't be expecting to get in dune systems, for example.

Oliver Strimpel: In that previous episode of Geology Bites, Neil Davies described the impact on the landscape of the first land plants during the mid Ordovician to the mid Devonian. Can you remind us as to how fluvial deposits changed when plants first colonized the land?

Martin Gibling: Yes, Neil has made a wonderful job of setting the stage for this. What happened when the earth was greened — the great Green Landscape revolution? As I think of it, the earliest land plants in the Ordovician are represented by spores, microscopic fragments of plants, and they came from very small plants, with simple, unbranched stems and minimal rooting structures. As we move on through the Silurian, they start to become a bit larger, complex enough that you get a bit of a macro fossil record, and then when you hit the Devonian, things really take off. Large rooting systems evolved forests by the mid Devonian and enough biomass for it to get preserved in the fluvial record, along with things like peat, which indicate that vegetation is really beginning to build up solidly. In the case of rivers, this led to a profound and dramatic increase in the amount of mud that was preserved in fluvial deposits. And Neil and his colleagues have very nicely documented this very, very dramatic trend. What the mud implies is the weathering was intensifying, almost certainly due to the organic acids, and the carbonic acids associated with plants and with the drawdown of CO2 from the atmosphere and groundwater systems. And when you had mud being produced in greater abundance, you also had more of it transported in river systems and then trapped and baffled by the standing vegetation. And so it tends to build up in the river plains and become preserved, rather than just bypassing through to estuary’s motions. The mud would have had the effect of making river sand more cohesive, strengthening the bed of the rivers, strengthening particular the river banks and they would have become more difficult to erode by flowing water. So you start to get more obvious channel banks and channel forms in the preserved record through the later Silurian and Devonian.

Oliver Strimpel: Are you suggesting that by the mid Devonian, the modern river had formed and that all the different plants that would have had an impact on rivers. How that was all sort of pretty much the way it is today, or — have there been further diversification and have certain regions of the planet been colonized that weren't really fully colonized by the mid Devonian?

Martin Gibling: The mid Devonian was when things really began to take off with a sense of very large forests. Trees that were a metre or even a couple of metres in diameter with very large root systems, and there have been recent discoveries in Upstate New York at Gilboa and elsewhere that have showed, very beautifully, some of these major forest systems. But that was only really the start, and by the Carboniferous, you're starting to get early gymnosperm plants, the ancestors of conifers, of the present day, and those were able to penetrate quite deeply down even into dry land areas. And so you start to stabilize not just the wetland environments, but also relatively dry open plains. And what's happening is that rivers diversify through to about the mid part of the Carboniferous, say the start of the Pennsylvanian, when they are more or less covering the range of things that we see at the present day. So, initially, we had large sand bed rivers, water, typically braided rivers with shifting sandbars and rather weak banks. And those progressively move into being more meandering rivers with muddy banks, where the meanders migrate much more slowly. You start to get abandoned channels. You start to get multi-channel anastomosing river systems, wetlands with black organic stained water. And the whole range of what we now know as river systems today is coming into existence through that period of time, especially with the riparian or bank vegetation that makes the whole system so much more intricate and complex, and where life organisms simply began to evolve at a much more rapid rate in these little niches along the lines of the river corridors.

Oliver Strimpel: Was there any significant impact on the fluvial record when the planet started to be covered by large tracts of tundra and open grassland?

Martin Gibling: This was a major change and people are still trying to work out how much that affected river systems. You have the angiosperm plants, the flowering plants coming in in the Cretaceous. They are typically greedy of water in many cases, and then you have the grasses coming in in the Cenozoic, with their very dense root systems. And the way they bound the floodplain surfaces must have had quite a lot of effect on river systems. It's just a bit difficult to figure out how exactly they affected things, but along with them, you have the rise of gigantic organisms like the dinosaurs and the big herds of ungulates that you might see in the Serengeti at the present day, and the buffalo before they were pretty much exterminated in North America, and those would have trampled through riverbanks and in places like hippos do today. They would have made trails down to rivers, which would have allowed rivers to break out along low points at times, and people have even made a case for Dino Evulsion, so-called, where dinosaurs might have trampled their way through to the rivers and promoted river breakout.

Oliver Strimpel: Wow, that's an incredible image of a dino or a hippo affecting the course of an entire river.

Martin Gibling: Yes. It's quite remarkable, and you mentioned the tundra areas and the rise of permafrost greatly affected Northern River systems eventually in the ice ages and those, of course, are starting to melt away somewhat at the present day, and you're starting to see modifications of rivers as the thermal water begins to erode down into the permafrost. There were complex effects related to the glaciation.

Oliver Strimpel: In your book entitled ‘River Planet,’ you have a wonderful chart showing the longevity of modern rivers. What exactly do you mean when you talk about the age of a river?

Martin Gibling: That's a very good question. Rainwater is always flowing downhill, so in reality the flow paths of water are always evolving, following tiny gullies, little rills moving together into larger channels, and trying to figure out the age of a river is really a bit of a chimera. It's like trying to say how old a rainbow is, for example, but it's helpful to think of the age of a river as the length of time that the river channel or course has been following pretty much the line that it has at present… Wouldn't be exactly the same course, because channels migrate, they break out, but the channel belt as a whole would be occupying within a matter of a few kilometres, probably about the same position. That can be very difficult to determine. You might think it would be obvious, but sometimes rivers cease to exist for a while. Maybe the river plane gets drowned out by the sea or lake, glaciers can overrun it. Lava flows, sand dunes can invade the area. The river may cease to exist for a while, but it then may reestablish itself in the same course, and a good example of this is the Columbia River in the western US. Probably originated about 17 million years ago in the Miocene, as far as anybody can work it out. But, periodically, the river was overrun by lavas and by pyroclastic flows coming off the Columbia Basalt Plateau in the East. And the Cascade Range of volcanoes in the West. And it seems that the river simply ceased to exist in its organized form for periods of time and then it reforged a new course, more or less where it's been before. So, it gets to be a little debatable whether those kind of gaps in the record matter or not, whether you reset the clock if for a little while the river ceases to exist. But in broad terms, I think we could say that with minor gaps there are rivers that have been present for hundreds of millions of years in their courses.

Oliver Strimpel: That makes sense. So how old are some of the oldest rivers, broadly speaking, that still run in their ancestral courses?

Martin Gibling: The oldest rivers that anybody has really documented running in ancestral courses go back to the Carboniferous about 300 million years ago, by no means back to the beginning of the Earth. The Earth is an extremely dynamic system, and rivers have been destroyed in the way that almost anything else has on Earth over geological time. But some of the world's oldest rivers are those of Eastern Europe, the Volga, the Don, the Dnieper. They may be as old as the Carboniferous, and they run down into the Black and Caspian Seas, which are the last remnants of the Tethys Ocean. Tethys got squeezed out when Africa came up against Europe. Arabia likewise, and then India against the southern part of Asia. And Tethys was eliminated, except for these minor oceanic remnants. And in that part of Eastern Europe, the deformation has been South of the Black and Caspian Seas in Turkey and the Caucasus. And so the rivers further north have not yet been disrupted by the rise of mountains to the South that affect them. And so in the course of time, they may very well reverse their directions to flow North as other rivers have done, but they haven't done that yet, so they're still in ancestral course, quite remarkably or broadly within them. And if you want an example in North America, there were rivers running along the line of the modern Mississippi west of the Appalachians back into the Carboniferous, about 300 million years ago, and this is a case where the Mississippi for a while in the Triassic and Jurassic ceased to be in its present position due to fault activity and marine flooding. But it's been running along its present line down to the Gulf of Mexico since the Jurassic.

Oliver Strimpel: A lot of these courses take us back to the time we had the supercontinent Pangea. Wouldn't the subsequent breakup of Pangea have completely altered the drainage patterns and therefore the causes of the rivers?

Martin Gibling: Yes, certainly so. The breakup of Pangea into smaller continents that drifted apart had profound effects on virtually all of the world's rivers. When Pangea was in existence, there were rivers of immense length, some of them probably longer than any in existence today. They would have flowed out of mountain chains, like the Appalachians in the Eastern US and the Variscan mountain chains in Europe, and those were equivalent in scale to the modern Andes. So we have to envisage rivers probably on the scale of the Amazon at times, running out across the adjoining plains. And in the case of the southern continents, Gondwana land. There's evidence from age dates, from again zircon grains, that there were very large rivers that flowed out of Antarctica, which was not at that time under ice, and they flowed out across Australia into the Sydney Basin, for example, on the New South Wales coast and then out into the eastern part of India. And these continents, of course, are now thousands of kilometres apart. But at that time, they were part of single, very large river systems that ran through. But once Pangea began to break up, all of those systems were dismembered and began to change.

Oliver Strimpel: Fascinating. So can we track a particular river that used to be contiguous, that now has been separated by thousands of miles of ocean? I mean, do we look at fossils or is it all the detrital zircons? How can we do that matching or is that even possible?

Martin Gibling: It's very difficult, and there aren't many cases where anybody has made a very precise match. These are not, for the most part, rivers that exist at the present day, so they're not rivers with names, in other words, that we apply to actual flowing water. In the case of the Sydney Basin in Australia, it's the grains that give you the age signature of an eroding upland in Antarctica. And the Sydney Basin has the Triassic rocks of the Hawkesbury Sandstone, a very, very big Brahmaputra scale, or nearly that scale of river system. It's flowing in across a fold belt out into what was the ocean at that time, and it has been matched to the Highlands in Antarctica. But it's very difficult to actually track it all the way back there. There are gaps. There are erosion, and so on and so forth. So it's these kinds of lines of evidence bits circumstantial that really build those stories. But I think some of those are the most fascinating stories, that you could pick up a river in one continent and potentially find its continuation in another continent, 5000 kilometres away. I think there's going to be a lot more coming through on that kind of thing in the future.

Oliver Strimpel: Yeah, it's amazing and can you go even further? You said earlier that fluvial deposits were characterized by being unidirectional, so can you infer the direction of current flow and say, OK, well, upstream there, there must have been a mountain belt at that time.

Martin Gibling: Yes, a lot of this comes from where the fluvial deposits are located and their flow directions. What people refer to as paleocurrent flow, the flow directions of the ancient currents. And if you go in the field, you can measure tens or hundreds of cross-bed directions and figure out which way the flows were going, and you can begin to build up patterns regionally of flow directions.

Oliver Strimpel: Did any major rivers actually survive the breakup of Pangea? Or do we have a whole fresh lot of rivers after that happened?

Martin Gibling: There were quite a lot of rivers that are relics. There are places on some of the plateau lands in Africa and in parts of Asia are up on the Tibetan Plateau where there are remnant river patterns. People have mapped them. They appear on satellite images, and they've been inferred to represent remnants of the original Pangean drainage. But they're not now connected to active systems. They've been broken through, and so they seem to be virtually all that we've got in the way of the original drainage, except for those rivers we talked about, particularly like those of Eastern Europe, which haven't yet had the effects of mountain building from collisions.

Oliver Strimpel: Were some rivers born as a direct result of the breakup of Pangea.?

Martin Gibling: Yes, they were. When you have a supercontinent like Pangea, heat begins to accumulate underneath it, and you start to get doming mantle plumes, volcanic activity that begins to generate domes, rifts of various sorts, and these began to affect virtually all the continents in different ways. You have things like the Karoo plume and the Parana plume across southern Africa and South America. These domed-up the crust. And there's evidence from Paleo flow directions that the rivers were radiating in places from those very large domes and then, in particular, once the continents began to separate, new rifts formed, which divided them, widening into oceans, and because the rifts were of relatively low elevation, they drew in the river systems and generated entire new drainage networks, and this is the case with the Niger. Also, with the Rhine very recently, and in the case of the big rivers of peninsular India, many of which occupy Cretaceous rifts. And you would think that then, once a continent is drifting away, nothing much would happen. But in point of fact, continents are drifting over thousands of kilometres of mantle, which is not uniform. It varies in its density, in its temperature, to other properties, and as the continents move slowly across at a few centimetres a year, they rise, they fall. They tilt up and over anomalies in the mantle below, and this is what's called dynamic topography linked to the mantle properties and, of course, rivers are very, very sensitive to elevation and gradient, and so you have good examples. For example in Africa, which is currently very high in the southern areas. It's anomalously high, a super swell as people have called it, and this has affected the history of the Zambezi, Limpopo and Orange Rivers, which have altered their courses and directions to correspond to the elevation of Southern Africa.

Oliver Strimpel: That's fascinating. We actually did have an earlier podcast with Carolina Lithgow-Bertelloni, who talked about dynamic topography, and another one with Becky Flowers, which included some discussions specifically about South Africa and what we can tell about the effect of dynamic topography there. So the ongoing collision of India with Asia that started about 50 million years ago must have had a dramatic effect on the river systems there.

Martin Gibling: Yes, this is one of the big stories of river modification through the breakup of Pangea, and it's absolutely fascinating. India is drifting northwards at quite a rapid rate, separated in turn from Australia, Antarctica, Africa, elsewhere, and by about 60 million years ago in the Paleocene, India is approaching the southern shores of Asia. The Tethys Ocean is narrowing down to a narrow seaway, and then you have the start of collision. And everything changes. In fact, there are not many rivers in this part of Asia that go back more than about 20 million years. And there are constant changes of direction and river capture, tributary systems that affect them all. And if you think of Himalaya and the Tibetan Plateau, there are rivers radiating out in all directions. It's called the water tower of Asia, and the thrust sheets are continuing to move southwards, and indeed India has pushed probably a couple of thousand kilometres into the body of Asia. Why wouldn't it have an enormous effect? And the kind of effects go a bit like this. Probably the oldest river in the system is still the Indus, which actually runs along the line of the collision westwards, and then cuts through the Himalaya. That's the Western Syntaxis, where Asia curls around the edge of India as it indented into the continent. And so it curls around and runs down into the Arabian Sea. And initially, the Ganges occupied the foreland basin, so-called on the south side of the Himalaya, running westwards into the Indus. So it was initially an Indus tributary, and then as India moved progressively northwards, an old body of rock in the Aravalli Hills South of Delhi began to impinge on the river basins and separated them off into 2 catchments. And the Ganges was diverted eastwards about 15 million years ago in the Miocene along its present line down to the Bay of Bengal. Also, up in the Himalaya flowing eastwards on the line of collision was the Yarling Sangpo River, and that initially flowed on into the Irrawaddy and out through Myanmar, and that is known because of fish populations that are relatively similar in the two rivers, but don't match elsewhere. But the Brahmaputra, rather steep, eroding headwords into the Himalaya cut through and captured the line of the Sangpo about 18 million years ago and brought it down into the Bay of Bengal. Those are massive changes in river and drainage systems. And give us time, and there will be more to come because there are several rivers like the Arun and the Gandak that actually cut through the Himalaya and are now within striking distance of capturing parts of the Yarling Sangpo System. They may eventually divert flow down into the Ganges and out to the Bay of Bengal by a different route. It's really fascinating.

Oliver Strimpel: Wow, that's incredible!

Martin Gibling: Meanwhile, down in Peninsula India, the rivers have not yet been affected, and you get big rivers still lying within Jurassic and Cretaceous rifts, as they have been since the break up, when India separated from other continents. But as time passes, and we're talking millions of years, they're going to come closer and closer to the mountain front and the thrust front. And so, as time passes, they are going to be modified, drawn under, with subduction, they're going to become tributaries to other systems or whatever. So there's a lot more still to come. And when we go further east into Asia, the big rivers of the Yangtze, the Mekong, the Salween, all began life as little coastal rivers on the edge of the rising Tibet, and they then eroded headward and started to capture the Red River, which may have been the biggest drainage system. It's thought that the middle part of the Yangtze was originally a tributary of the Red River that reversed its course, cut the three gorges in the Miocene probably, and then was locked into place, so there's been a jockeying of rivers all very close together as the tectonics of the Himalaya and Tibetan Plateau have squeezed them, modified them structurally, changed them, and these changes are still ongoing.

Oliver Strimpel: One thing that really struck me as you were giving all these examples is how you can have a river course that can be relatively unchanged for geological time, and then it can get, more or less in a geological blink of an eye, captured by another river and then flow to somewhere completely different on the planet, essentially.

Martin Gibling: Yes, yes, and you can get rivers that are captured and then diverted literally from 1 ocean into another. For example, some thousands of kilometres apart. In the case of the Yangtze Mekong and Salween, they actually all run in a series of gorges within about 100 kilometres of each other, and they all end up diverging into completely different oceans. They've been likened to a bundle of javelins in the hands of Zeus, or ancient pictures, where you have the javelins held by Zeus. But their tips are radiating out in different directions. A good example of this, by the way, is the Niger in Africa, Western Africa, which just began in a rift, and it was quite small, and then it took over an inland drainage very suddenly to the West, and then it went further west, when a river that was running out into the Sahara got blocked by sand dunes. And diverted into the upper part of the Niger. So very suddenly, you had a river that rises very close to the coast. Runs north, swings round in the Big Bend at Timbuctoo, and then runs for some 4000 kilometres and runs into the Atlantic on down the coast. In the original position. Absolutely fascinating.

Oliver Strimpel: The continuation of my conversation with Martin Gibling is in the next episode of Geology Bites. We'll be talking about the rivers of Europe and the Americas, as well as the impact of the Ice Ages and humans on river courses. For more about Geology Bytes, as well as pictures and illustrations that support this podcast, go to geologybites.com.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. This episode is the second of two of my conversations with Martin Gibling. In the first episode, we discussed fluvial deposits in the geological record, and we traced the effect that the breakup of Pangea around 200 million years ago had on river systems. In this episode, we address the history of Rivers of Europe and the Americas, as well as the impact of recent Ice Ages on today's rivers. We end by considering how humans have changed rivers and their deposits throughout the history of mankind. Martin Gibling has spent a lifetime studying rivers and river sediments around the world. He's emeritus professor in the Department of Earth and Environmental Sciences at Dalhousie University in Halifax, Nova Scotia, in Canada.

What about the rivers of Europe? You've already touched on a few. What is their history?

Martin Gibling: The European story is dominated by the collision of Africa along with micro continents against southern Europe. And, of course, this generates the Alps, in particular, 30 million years ago; the Carpathians is a great bulge of the Alpine system, something like 15 million years ago. And complications with the Pyrenees as well in Iberia. And the Alps in particular has really dominated the scene. There would have been rivers flowing down across northern Europe into Tethys, but once the Alps began to rise, those were all effectively destroyed or turned into tributaries of other systems. So Europe, at least western Europe, is largely a story of Three Rivers: the Rhone, the Rhine, and the Danube. The Rhone may have been the original major river running around the western side of the Alps, which would initially have just been an island in the Tethys, but gradually became a dominating mountain chain, and progressively through time after about, say, 10 to 15 million years ago, it was the Danube that was dominating, flowing around eastwards right round the fringe of the Alps as they extended, and then running down into the Pannonian Basin in Hungary through Vienna and out into the Hungarian plains. For quite a long time, it flowed into a very large lake in the Pannonian Basin, but had advanced with a major delta filled in the lake with Alpine sediment cut through ranges and got down to the Black Sea about four million years ago.

Oliver Strimpel: What is the Pannonian Basin?

Martin Gibling: Yes, the Pannonian Basin is the very big horse plains of Hungary, where the Magyars raised their horses, famous horsemen, and those are still present as very big flat plains, but underneath there are lake deposits, and this is part of what sometimes is called the lagomare in European terms: very large lakes connected to Tethys, partly brackish, partly fresh, that ran all across this tract of country. But what's been happening lately is that the Rhine has started to come into its own. It occupies a Rift Valley where there have been volcanics, quite recent volcanics in places, and for quite a long time, the Rhine was a series of disconnected drainages, most particularly the Moselle coming in from eastern France. But by about 3 million years ago, these drainages connected up, and Alpine sediment was now making its way right through to the North Sea. And because the Rhine is occupying a Rift Valley at a low elevation, it's beginning to suck out the headwaters of the Danube. And, in places in the Swabian Alps, there are cave systems where water is draining underground out of the Danube and into the headwaters of the Rhine through a series of springs. It's a river capture in progress, moving more or less underground. It's incredible stuff.

Oliver Strimpel: Can you talk about the history of South American rivers, and especially that of today's largest river, the Amazon?

Martin Gibling: That is one of the most fantastic river stories of all. There's the Amazon. It's the world's largest river by a very large margin. It's bigger than something like the eight next rivers all combined, and it is only about 9,000,000 years old in its present position. What happened was that South America began to drift away from Africa, and there were rivers flowing into the Pacific on the far side, one of which would have been a proto Amazon. It is sometimes referred to as the Sano Zama, which is Amazonas spelt backwards -- if you wanted a name for a river now flowing in the opposite direction that it now occupies. But in the Miocene, about 25 million years ago, the Andes started to rise, and drainage began to flow eastwards, and so the Amazon went through a reversion. And initially it terminated as a series of major wetlands on the South American continent, blocked off by a rock arch, so it didn't get to the Atlantic. Eventually it broke through the arch and then ran into a rift that runs along the lower line of the Amazon and reached the Atlantic Ocean. That was about 9,000,000 years ago. And that's when Andean sediment first begins to appear in the deep sea fan of the Amazon off the South American coast. Meanwhile, the Amazon has picked up a lot of the drainage from the Orinoco in the northern part of the continent, and there's a very bizarre circumstance called the Casiquiare Channel, which is a place which actually connects the two rivers and where the Amazon is draining off flow from the Orinoco, but they've not separated into separate catchments as yet. It's a river capture that's actually in progress. And Alexander von Humboldt took a canoe through it and showed in the early 1800s that these were actually connected. Armchair observers In Europe had claimed that it couldn't possibly be true.

Oliver Strimpel: Can you pick out a couple of examples that represent the diverse histories of the rivers of North America?

Martin Gibling: Yes, I could talk forever about this. There are so many rivers, their history is so interesting. We've talked a bit about the Mississippi, and that goes back to about 300,000,000 years following the line of the foreland basin along the western side of the Appalachians. That was a mountain chain of Andean scale. But once Pangea was assembled along that line, then very big rivers would have run out across the continent, and some of them ran out into the Boreal Sea in northwestern Canada. So you had continental-scale rivers that no longer exist, running out from the Appalachians. The Mississippi continued to run south, and in the Cretaceous in particular the Cordillera began to rise in West and North America, and then the entire drainage system of North America was altered. The Gulf of Mexico had come into existence in the Jurassic, and you had rivers running down off the Cordillera, rivers that we know today. At the Platte, the Arkansas, the Red, Canadian Pecos rivers, and so on, and these all ran down to the Gulf, where they competed, if you like, with the Mississippi for access to the Gulf waters. So the Mississippi at times was reduced to quite a small river. But as time passed, the Missouri fed into it, and so the Mississippi has regrown into a very major drainage that captures a lot of what comes off the Western Plains. So, it's managed to maintain its place. The river that you have to talk about, though, with North America, is the Colorado River. And I couldn't possibly talk to you about this without getting into the issues of the Grand Canyon. And in the Cretaceous, the Colorado headed eastwards into the Western Interior Seaway across the Midwest. But then dynamic topography came in, as the continent drifted over the Pacific. The Colorado Plateau rose, and the basin and range systems of rift valleys lowered the topography in the very western part of the continent. And all of this resulted initially and rather disconnected drainage systems, but eventually a through flowing river system of the Colorado that connected drainages ran into a series of lake basins and then, about four million years ago or so in the Pliocene, made its way down to the Pacific and cut right through. And the Grand Canyon, most of it, was cut, apparently within the past six million years. It's incredibly big. Have you been there?

Oliver Strimpel: Yeah, I have yeah, yeah.

Martin Gibling: Absolutely an impressive thing to see, and it's very young. The lowered base level when it got through to the ocean meant that the river just cut a massive gorge as other rivers have done. The Volga has done that. The Nile has done that. And these will eventually fill up with sediment, probably as the other ones have done, but at the moment it's standing there as the most famous canyon on Earth.

Oliver Strimpel: What about the Ice Ages, particularly the more recent ones? Have they had a significant impact on rivers?

Martin Gibling: They certainly have, and they've had an impact at a far more rapid rate than the plate tectonic effects, which are in the millions, tens of millions of years; the Ice Age has been operating on scales really of thousands or hundreds of thousands of years, and the changes are absolutely radical. All the ice across North America and Eurasia came a long way down, down into as far as Nebraska in North America, and this is ice 3 kilometres thick. It does extraordinary things. It erodes river valleys. It spreads huge volumes of glacial debris everywhere. It depresses the crust. It does everything. And rivers, of course, cease to exist when the ice of that magnitude covers them. So let me give you a few examples. The Yukon River was initially a relatively small drainage that rose near the Pacific Ocean and ran down into the Pacific. But its exit got blocked by an ice lobe and it reversed, and so at the present day, it rises within 50 kilometres of the Pacific and then runs 3000 kilometres round to the Bering Strait. Quite incredible. And the Mackenzie, one of the big rivers of North America, occupies a glacially eroded through that didn't previously exist. There was no drainage along its present course, and the Mackenzie has originated only since the Ice Age, something like 12,000 years ago, and it must be among the youngest of all the big rivers of Earth, entirely as a result of the glacial effects. Other things that happened, in particular in North America, the weight of the ice depressed the land, and once the ice sheets melted back, which they did very quickly, enormous glacial lakes formed, including glacial Lake Agassiz in North America, as well as very Big Lakes in Siberia. And these were held in place by lobes of ice that was melting, as well as glacial debris. Natural dams which failed and released a number of catastrophic outburst floods and melt water that rolled down rivers to the ocean, down the Mississippi system as far as the Gulf of Mexico out through the Saint Lawrence and Hudson to the Atlantic and out along the Columbia River to the Pacific. And it's the Columbia River floods that really brought all of this to light through the work of J Harlen Bretz in the 1920s, who interpreted the landscape of the Channeled Scablands in Washington state as due to catastrophic floods. Geomorphologists didn't like this. This was catastrophism. This smacked of old creationist thought, and they laid into him. It couldn't be right. There must be other explanations. But in the course of time, Bretz was proved to be correct, and the estimates now are that for short periods of time, the outbursts that ran down the Colombia rated at 20 million cubic metres per second. And that's equivalent to 50 river Amazons at peak flow. Absolutely stunning. They generated waterfalls on the scale of Niagara which are now completely dry. If you've ever been across that extraordinary landscape, and they carried sediment out for 1000 kilometres down into the Pacific with something like 200 metres of sand in deep water off the Pacific that resulted from all of this.

Oliver Strimpel: Did the Ice Ages also affect the courses of European rivers?

Martin Gibling: Yes, there's the Channel River, which initially connected the Rhine, the Thames, the Seine, the Somme into one large river system that drained most of western Europe, ran right down to the Bay of Biscay, and when the ice sheets melted, the sea level rise simply drowned out the lowest 800 kilometres of the Channel River system. And the rivers were all left as disconnected remnants. The Rhine, the Thames, the Somme, and, of course, in terms of human geography, this has affected everything running right through from the Stone Age right through to modern day Brexit. It's an ongoing story of how a river and glacial dynamics have affected human-political history

Oliver Strimpel: Speaking of recent effects, humans are now having a massive impact on rivers, what with all the dams we've constructed and the confines we've placed on rivers for agricultural purposes and to protect cities from flooding. How far back can the impact of humans be seen in the record of fluvial deposits?

Martin Gibling: That's a really very fundamental question with a lot of importance at the present day. Hominins arise something like 7,000,000 years ago in Africa. And I asked Gail Ashley, who'd worked at Olduvai Gorge, whether she could document any trace of what early humans did. We're talking perhaps two or three million years ago, and her response, which I've always remembered, is that the early hominins trod lightly on the Earth, leaving only stone tools and footprints. And so there's nothing that she could see where you could make a link between river dynamics and humans. Once fire was used, which goes back about a million and a half years in Africa, there must have been some effect on alluvial landscapes with the burning of forests, the release of sediment, and so on. But so far, I haven't come across anything that really links early fire records to rivers, at least not as far back as that. When humans really began to have an effect is once they start to domesticate plants and animals, and that is as recently as about 11,000 years ago. Prior to that, there were fisheries along rivers. There was food processing by rivers, but settlements were only transient, and people were hunter gatherers, moving through the landscape and, as people say in northern Canada, following the wealth of the land. Their livelihoods depended on their mobility. But once plants were domesticated, wheat, barley, beans, onions, et cetera in the Near East, and animals, cattle, sheep, goats, pigs by about 10,000 years ago, then things began to change. Something like nearly 10,000 years ago, people recognized agriculture as an organized system, and within a few thousand years of that, it's reckoned that virtually everyone in the Near East was a farmer, probably within little more than 3,000 years. And around the same time, you're getting rice domesticated in China, just inland from Hong Kong. Something like 9,000 years ago. And all sorts of water systems. Irrigation goes along with that. You get staples like maize and squash in Central America. Domesticated 9 to 10 thousand years ago. And, oddly, bananas in New Guinea going back something like, I think, 7000 years or thereabouts. The Amazon Basin, long thought to have been a kind of human backwater by people who probably never went there, has emerged as being a major centre for plant domestication of all kinds. There’s a lot of research going on into what happened there. And we know about these things because you can document the changes in animal bone structure, size, and so on, and in plant genetics that go along with humans deliberately engineering plants and animals for their own purposes. And over the next few 1,000 years, you have donkeys domesticated in Egypt, horses on the Russian steppe, yaks in Eastern Asia, water buffalo, probably in India or China, and then camels in Arabia. And all of these were domesticated over a period of three or four thousand years. And why was this important for rivers? Well, if you're going to have crop lands, you have to deforest the land. Make way for the arable products. You need pasture for grazing animals, and these all affect the stability of river systems and the sediment supply coming down to the rivers. But, very importantly, in places like Mesopotamia, you have got to have irrigation if you're going to have crops. And this goes along with the rise of permanent settlements in places, particularly like Jericho some 10,000 years ago, in the Rift Valley of the Jordan, and then the cities of Mesopotamia along the Euphrates and the Tigris. In Egypt along the Nile, and with the Indus Valley Civilization along the Indus and its tributaries. And if you've got cities with big numbers of people, you need water supply. You need sewage systems. You need infrastructure along the rivers. Wharfs, bridges, pathways. You generate large depressions which have been documented by archaeology on floodplains to make bricks. These cities were brick, mud brick manufacture, and all that came out of the floodplain plain. And so by 6,500 years ago, you have the rise of irrigation systems in Mesopotamia and the first big dam in Egypt. The dam of the pagans nearly 5,000 years ago were 14 metres high with the facing of stone blocks, and so people are really beginning to modify the river systems. And I have to mention the Yellow River in China, where emperors going back several thousand years constructed enormous embankments, relief channels, and kept the river in its place for the benefits of agriculture and of navigation. And finally, the Romans, masters at water management as they were at pretty much anything else in the engineering line, aqueducts 100 kilometres long and 45 large dams built in the Near East to supply the Roman legions with water and food. So, humans over the past 10,000 years have really been affecting river systems.

Oliver Strimpel: In an earlier podcast, I talked with Phil Gibbard about whether and how we might establish a new geological period called the Anthropocene, defined by human impact on the planet. If we do establish such a period, we would need to mark the start of it with something laid down in the record that happened more or less simultaneously around the globe. Can our impact on rivers help us out here?

Martin Gibling: I think it can help us out quite a lot. I've liked the approach of Phil Gibbard and his colleagues that you need to consider the Anthropocene to be a very broad-based event, going back a very long way in human history. And I've moved away from thinking of a narrowly defined global spike. I mean what we've been talking about with human effects on rivers means that they go back about 10,000 years with domestication of animals and plants. And you can pick that up with what people call legacy sediments, the enormous run-offs of sediments into rivers along with cereal pollen, the bones of domesticated animals, techno fossils such as pottery and bits of masonry. And you can show that humans were affecting the landscapes very prominently far back in time. And once you get writing, say 5000 years ago, you begin to get written records of irrigation development. Mesopotamia and the documentations in China in particular, associated with the attempts of the emperors to control the Yellow River. So there's no question at all that human effects go back a long way. The attempt to put a golden spike in, to mark the base of the Anthropocene, is focusing on about 1950, linked to nuclear fallout from the bombs and bomb tests. I'm uncomfortable with using nuclear fallout in this way. These are destructive systems. They cause huge deaths of all sorts, but, really, my line of thought is more that we need a broad all-encompassing term that takes us a long way back in Earth history to understand what we've done.

Oliver Strimpel: And our widespread impact on rivers, where would you put that then? Would it be the rise of the modern engineered dam where pretty much every major river on the planet has been significantly dammed? Or would you put it further back in antiquity?

Martin Gibling: There's no question that the rise of the big dams has hugely affected virtually all modern rivers, but it doesn't just begin there. For example, by the time of the Doomsday book in 1087 or thereabouts, when William the Conqueror invaded Britain, he recorded a water mill for every 250 inhabitants of Britain. And those water mills had small weirs. They had little dams. They had mill ponds. And records, particularly out of monasteries, show that the stocks of salmon and other migrating fish were cut back through the medieval period. We've been doing this for hundreds of years, even in that kind of setting. And by the 1840s, in the northeastern US alone, there were 65,000 small dams and mill ponds. All the rivers had been retooled long, long before the era of the big dams. We have to understand the human impact on the land as being very long-ranging.

Oliver Strimpel: You've studied just about every major river on the planet. Do you have a favorite river?

Martin Gibling: I wish I had studied every major river on the planet. I've got a long way to go. But of all the rivers that I think about, I would have to place the Channel Country of Australia at the top of my list. I spent a long time with Australian colleagues out there in the red centre of Australia. We drove along the old droving roads through Queensland. Rivers of the Channel Country like the Diamantina, Cooper Creek, the Georgina and these run down through the sand dunes down to Lake Eyre with the salt flats. And they are absolutely fascinating. It's very hot work, often into the 40 Celsius range, but you find that life has found a way to quote Jurassic Park. And there are Kangaroos and emus, crayfish burrows, Eucalyptus trees growing along the side, flocks of parrots, budgies, and these are multi-channel and anastomosing rivers dry for many months of the year and then flooding down to Lake Eyre. And it's absolutely fascinating walking through the creeks. These are about as near to a pristine river as you're likely to get. They're just wonderful places.

Oliver Strimpel: Martin Gibling, thank you very much.

Martin Gibling: Oliver, thank you very much for talking with me. I'm delighted to talk about rivers anytime.

Oliver Strimpel: For more about Geology Bites, as well as pictures and illustrations that support this podcast, go to geologybites.com.