Martin Gibling on Rivers in the Geological Record – part 2

Transcript

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.