This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. At various times in Earth history, flooding events have taken place that are qualitatively different from the weather-related flooding we hear about regularly in the news. They are different not only in the water volumes and flow energies involved, but in the effects they imprint on the landscape.

Victor Baker has been studying megafloods for over 50 years. He is a professor of hydrology and water resources, geosciences and planetary sciences at the University of Arizona. Victor Baker, welcome to Geology Bites.

Vic Baker: Thank you very much.

Oliver Strimpel: What exactly are megafloods?

Vic Baker: Well, the term megaflood has been around for a while, referring to really big floods: mega. But in the last decade or two, it has been more narrowly confined to floods where the volume of water per unit time, called the discharge, is 1 million cubic meters per second or more. Hence mega. And also, these floods have immense energies, many orders of magnitude, that is, factors of 10, greater than anything that can be produced by rainfall-type flooding.

Oliver Strimpel: What events do we think are responsible for triggering a megaflood?

Vic Baker: Well, probably the easiest kind of event to understand would be the failure of an immense dam. If you are blocking water, let's say in a river valley or some other kind of trough on the landscape, the water will build up to great depth, and then the dam could fail, and this water can be hundreds of meters deep. So all of that is the potential energy for the megaflood. But there are other mechanisms as well, because associated with large masses of glacial ice, you can have meltwater that can build up to high energy levels as well. More mysterious, however, are some megafloods that emerge from underground, and we're still trying to understand how those occur.

Oliver Strimpel: What effects do megafloods have on the landscape?

Vic Baker: Well, this is part of what led to their recognition. The effects are spectacular. They are way beyond what a normal river could do. Normal rivers do erosion and sedimentation, but in megafloods everything is on a magnified scale. Whereas in normal rivers you think of moving sand, gravel, maybe sometimes some small boulders, megafloods can move objects tens of meters in diameter. So that is just one measure of the energy. They also produce what we would see in a normal river as small ripples on the bed of the river, or even larger forms that may be a meter high and several meters in length. The ripple-like forms we get in megafloods can be as long as a football field in individual spacing, they can be 10 or 20 meters in height, and they will have boulders on their surfaces up to a couple of meters in diameter.

Also, the erosion features are spectacular. They include giant rock basins, which we would call potholes in a normal bedrock river. But these rock basins can be tens of meters, maybe even as much as 100 meters, across. And there are also giant cliff-type cataracts, which we would think of in a normal river as a waterfall. But in the case of megafloods, the water flowing over the cliff is as deep as the cliff is high, and some of these giant megaflood cliffs are hundreds of meters high.

Oliver Strimpel: We're going to get into some examples of where we see these phenomena in a moment, but do we understand the mechanisms that cause a megaflood to rip apart the landscape like this?

Vic Baker: Some very odd processes that are not often viewed as important in normal water flow become much more important in megafloods. One of these is a strange phenomenon called cavitation, where the water velocity is so great that, according to Bernoulli's theorem, the pressure in the water drops to a very low level and little bubbles of vapor form within the water. It's kind of like what happens when we put energy into water on our stove and cause it to boil, but this energy is put in by the actual motion of the water itself. These bubbles, when they come to a flow boundary, will implode with immense energy. That energy is then imparted to the boundary, and it can be at levels of tens of thousands of bars of pressure. Cavitation can erode high-speed steel propeller blades on certain kinds of boats.

Another process that is poorly understood, because of its rarity among phenomena that occur on the Earth, is macroturbulence, which is the water equivalent of what a tornado is in air. The secondary flow velocities are so great that the water pressure drops and you produce a lift effect, and this lift effect can entrain very large boulders off the bed. So in megafloods, the high-energy portions can put boulders many meters in diameter into suspension. This is totally unlike what occurs in normal rainfall-generated river flows.

Oliver Strimpel: Do we think these mechanisms are able to explain these high cliffs that you see in some of these situations?

Vic Baker: This is something that we work out from our observations of megaflood landscapes in different parts of the world, and we see that there are characteristic associations of landforms. We've seen this in many different places, and one of the clear associations is where the flood flow may initially be carving grooves and rock basins on the surface of a pre-flood landscape. This gets organized into what's called an inner channel, which is a deeper part of the eroded surface. That inner channel grows because there's greater energy expenditure when you have a narrow cross section, and this deep channel will erode headward into the source area of where the flood is coming from. This creates a subaqueous cataract that recedes backwards. So that's where these high cliffs occur, and this produces a distinctive kind of landscape that, as I say, we see in many parts of the world.

Oliver Strimpel: No megafloods have occurred within recorded human history. Was there resistance within the scientific community to the idea of catastrophic events such as megafloods?

Vic Baker: Yeah, this definitely was a big issue. Geology has a fundamental problem, in that the things we study that are most interesting to us occurred in the deep past, and we don't have access to them directly. This made geologists somewhat concerned as to how they could achieve a kind of certainty about what was going on when they couldn't observe it directly. And so a principle developed called uniformitarianism, which argued that the validity of the science would depend on our ability to observe the kind of process that we were trying to study, observe it today, and therefore know that such a process occurred. Unfortunately, this idea has a fundamental flaw, and it was actually pointed out by the man who invented the word uniformitarianism, William Whewell, a Cambridge University professor in the early part of the 19th century. He pointed out that in science, it is not for us to say in advance what we could find or what rules it should follow. It's nature that tells us. And this led to a lack of study of megafloods by geologists until really the later part of the 20th century.

The man who transformed this, I was fortunate enough to get involved with when I began my major studies of megafloods. He was J Harlen Bretz, a University of Chicago professor who proposed the megaflood origin of a region in eastern Washington called the Channeled Scabland. His megaflood hypothesis was attacked, and he ran into huge resistance from geologists for decades. But eventually that idea prevailed because of the field evidence. The actual stuff that you go out and study in the field could not be explained in any other way, and eventually the community came to understand that. And today, many kinds of catastrophic phenomena are widely understood to be important in geology. A good example is impact cratering, which was resisted by geologists and even other scientists for centuries, even though the phenomena had been observed.

Oliver Strimpel: Just extending this discussion a little bit beyond science, many stories of creation and religion invoke some kind of a megaflood, Noah's flood being perhaps the best known in our culture. But do we think any of those have any historical inspiration, or is it just something that we put down to religious belief?

Vic Baker: I wouldn't say "just put down to religious belief." As an explanation for a scientific phenomenon, a supposed religious view that God did it is not very productive in terms of understanding what was going on. There is an alternative religious view, and it was actually held by the geologists who developed the whole geological timescale. That is that we have a document of what God did. In religion it's called creation, but a scientist would refer to it as reality, and reality is a teacher. It presents information to us through the rocks, through the evidence of past floods. Whether you ascribe that to a religious purpose or not, it is still a teacher. We learn from it, and early people always paid attention to their environment. The Aboriginal people in Australia understood their environment well enough to survive in the most difficult place on the planet, a desert continent. So they understood the world and they observed the world. There is no question that some of those people were impacted even by megafloods in the geological past. For them, the whole world would have been destroyed, and ascribing that to a special force of nature would seem to be obvious. So we can't dismiss those stories, and in fact we can learn from them.

Even the Bible, if one reads it carefully: the Noachian debacle, and this has been worked out by biblical scholars, contains two incompatible stories of the great flood. One of the stories is that it rained 40 days and 40 nights. The other story is that the waters upwelled from the deep. They are incompatible. If you're a theologian, you would say, well, God is perfect; God doesn't do two opposite things. So there may be stories about waters upwelling from the deep and stories about water raining from the sky. Interestingly, this is an issue in regard to the origin of megafloods.

Oliver Strimpel: You mentioned earlier, as part of the definition of a megaflood, a certain flow rate in cubic feet per second. So how do we know what the parameters of megafloods were: the volumes and depths of the lakes that drained suddenly, the energies involved and so on?

Vic Baker: This is one of the discoveries that I found remarkable. With relatively small flows, we have to use instruments to measure them and try to figure out how fast the water is moving. But what I discovered when I was doing my PhD, on what is essentially the biggest megaflood to have occurred in the United States, about 16,000 years ago, is that because of their energy level, these floods leave an incredible amount of evidence on the landscape. It's like what a volcanic explosion does, or what a meteor impact does. When a large bolide strikes the Earth, it leaves an immense crater. We have one of these in northern Arizona called Meteor Crater, and that lasts for long periods of time, and its dimensions and characteristics reflect that energy. Well, the same thing happens with megafloods. The water is flowing on the surface; it erodes, it leaves sediment, it erodes up to a certain level, it leaves a channel of a certain size. The gradient on the surface eroded by the highest water is proportional to the energy expenditure of the flow, because it is the slope, and the water moves under the influence of gravity. So it isn't a big deal to use quantitative equations to calculate the flow velocity. You multiply the flow velocity by the cross-sectional area of the channel, and you get the discharge. And for really big floods, the error bar is relatively small, because the evidence is just preserved on the landscape.

So this was kind of a revelation to me. And as an interesting sideline, I started to think, well, maybe we could do this for smaller high-energy floods that are produced by rainfall and the like. It turns out that in certain circumstances that is the case. You can actually get excellent data on the magnitudes of extreme floods in certain situations, and I even named this as a new science, called paleoflood hydrology. This science is now used to evaluate the safety of dams, nuclear power plants and other kinds of things where very rare but high-magnitude floods pose a significant risk.

Oliver Strimpel: Can you give some examples of megafloods that have occurred on Earth and what the evidence is for each of them?

Vic Baker: Once we started looking for megafloods, and people started doing that in the later part of the 20th century, and it's just accelerating today, we found lots of examples of megafloods in the geological record, and they've been caused by different sorts of things. The Missoula floods were caused by ice-dammed lake failures. There were ice-dammed lake failures in Central Asia, and in South America associated with the Patagonian ice sheet. But there were also other megaflood processes. One that occurs today, but probably has not produced a megaflood in modern times, is where a volcano erupts underneath an ice sheet. This occurs in Iceland, where you have some big ice masses. The largest glacial mass in Europe is Vatnajökull, in the southeastern part of Iceland, and there are volcanoes under it. When they erupt, they produce water that will rise up, and when the water gets near the height of the glacier, the pressure of the water, which is denser than the ice, will cause the water to squirt out from underneath the glacier. It's an immense flood. These have occurred in the geological past, and some have been megafloods.

Another example, which people didn't realize and which has only come to light in more recent years, is that a huge lake was formed in what is now the North Sea, because in the last Ice Age and previous glaciations, ice went from Scandinavia across into Scotland. Then you had large flows of water coming around the southern margin of the ice sheet that was covering Scandinavia, filling a lake basin that was impounded at its southwestern end by the Cretaceous chalk that went from France into England, between what's now Calais and what's now Dover. This barrier failed, it's thought, about 600,000 years ago, and that created what's now called the English Channel, through catastrophic megaflooding.

The biggest example, though, is during the Miocene period. About 5 million years ago, you had a land barrier between what is now Morocco and southern Spain, and that was basically breached by rising sea level in the world ocean, and water spilled over that divide, creating what is now the Strait of Gibraltar. At that time, the Mediterranean Sea did not exist. Instead, there was a large basin into which that water flowed, and that produced what seems to have been the largest megaflood on Earth for which we have good documentation: at least about 50 million cubic meters per second, lasting possibly as much as a couple of years.

Oliver Strimpel: When we spoke earlier, you mentioned the Mediterranean being divided into two parts, with a subsequent flooding event in the east.

Vic Baker: Yes, it is actually two separate basins, a western basin and an eastern basin, and they were separated by a kind of sill or high area. That is where Sicily is located, so between Sicily and Africa, Tunisia, and also between Sicily and the rest of Italy. That barrier had to be breached by the rising water, and it also has cataclysmic flood features on its floor. The Strait of Gibraltar has an excellent example of an inner channel and cataract, and there are also analogous features in that barrier between the two parts of the Mediterranean.

Oliver Strimpel: Was the connection between the Black Sea and the Mediterranean thought to have involved a megaflood? Do we see the kind of inner channel and cliff features that give the clues to that?

Vic Baker: Very definitely. There was a group of oceanographers who recognized the evidence, as had been done for the Mediterranean, that you had a history of, in this case, the filling of the basin where the Black Sea is located today, and a spilling through the area of northwestern Turkey, where you have the straits of the Bosphorus and the Dardanelles that lead into the Aegean, during the last Ice Age peak, which was maybe around 20,000 years ago. What we today call the Black Sea, because it has a kind of separation from the world ocean and it is basically anoxic in the deeper part of its water, leading to the so-called black color, was actually not salt water at all. It was all freshwater, and it was fed by the glacial runoff coming into rivers from the surrounding parts of Asia. And that may have spilled through those straits. But once it didn't have that meltwater coming into the Black Sea, and with the rising sea level after the Ice Age, seawater moved into what is the Black Sea today.

And there clearly was an event. There are actually deltaic deposits and the like that show that there was a major spilling about 10,000 or 11,000 years ago. The interesting thing there is that there were people living around this Black Sea, and there's actually evidence of dwellings that were flooded by this rise of seawater. And there was a suggestion that this may have been one of the stories that led to the Noachian debacle: that the people who were displaced by this migrated to the Fertile Crescent area, in what is now Iraq, and that at the time of the transfer of the Israelites to that area by the Babylonian conquest, they were exposed to the story, and that led to the biblical account.

Oliver Strimpel: You mentioned Lake Missoula and the flooding that created the Channeled Scablands, which are in eastern Washington State. Was that kind of ice dam failure associated with the end of an Ice Age? You mentioned 16,000 years. Does that tally with the end of the Ice Age, or an Ice Age?

Vic Baker: It's more complicated. The peak of the glaciation was associated with a large ice mass that was covering mostly the Canadian Rockies but extending down into the northwestern states of the US: Washington State, Idaho, the western part of Montana. The peak cold period of the Pleistocene would have been about 20,000, maybe as early as 22,000, years ago. And that peak probably resulted in a blockage of the streams in the western part of Montana. The streams were rapidly filled up behind an ice dam that rose to impound water 800 meters deep. What we think happened was that as the ice was advancing, it created an initial seal that wasn't as stable. The water built up, and it had a huge failure that destroyed the dam at around that early point, maybe about 20,000 years ago.

But after that failure the ice readvanced, and it kept advancing, so that the water would rise against it, and then the water pressure would actually work under the dam and lift the ice, because of the higher density of the water, if you got the water level high enough. The flooding would burst out from underneath the ice, and then, as the water level dropped in the lake, the dam would collapse back. So you had repeated flooding outbursts. The result of this is that you had megafloods of different sizes: probably something pretty massive early on, maybe as much as 20,000 years ago. That flood could have been as great as 20 million cubic meters per second. And then after that you had a series of smaller megafloods, some would say as many as 100, although not all of those would have been megafloods. Some of them would have been much smaller, produced by some kind of repeating mechanism within the lake basin.

Oliver Strimpel: You've also studied Mars. It appears to be dry today, but its surface shows lots of features that can only have been created by water. Do we see evidence of megafloods there?

Vic Baker: In the early '70s, there was a Mars mission called Mariner 9, which was the first to make a global imaging of the surface of the planet. In those images, I recognized the catastrophic flood channel features that I had been studying in the Scablands. But the features on Mars are a little different from what we see in the Scablands. We think that's mainly because of the lower Martian gravity, which affects the physics of the flows, the ability of the flows to erode, that sort of thing. But the whole assemblage of things is very similar. How did this happen on a planet that today is incredibly dry? It does not show evidence of water in its atmosphere. It does, however, and this is from more recent measurements, have evidence of lots of water ice. A lot of that water ice is buried, but some of it is actually present at the Martian poles. The water on Mars that is in evidence today, and I mean it's been directly measured by different tools, if we spread it over the surface of Mars and made it a liquid, would cover the entire surface of Mars to a depth of at least 20 meters. So there is a lot of water near the surface of Mars today, and that's not well known, because a lot of this is recent data.

Most of the water on Mars, though, is deep underground. That has only come to be recognized fairly recently. It was thought that in the history of the planet there may have been a great deal of water. In fact, it may have constituted a kind of ocean more than 3 billion years ago. But that quantity of water spread over the surface of the planet would be hundreds of meters deep. So it was commonly thought that that water would have been lost to space: water from a body of water on the surface would evaporate into the atmosphere and dissociate into hydrogen and oxygen. The oxygen would combine with the surface rocks. Mars, known as the Red Planet, is highly oxidized; the iron has oxidized on the surface. The hydrogen, being a very light gas, with the relatively low Martian gravity and the effects of the incoming solar wind and other phenomena, would escape from the atmosphere and be lost. But what we now know about the water on the surface is that it never stayed on the surface very long. The ocean appeared and went away completely, unlike Earth, where we've had an ocean on the surface of the planet since very early in its history. So something very odd was going on on Mars.

This water, we think, went into the subsurface. Mars has a high infiltration capacity compared to the Earth. The types of rocks will allow the water to go very deep, and the low Martian gravity means that the rocks at depth in Mars are not as compressed together as they are on Earth. That leaves more open pore space for that water to migrate into. This suggests that even today, Mars has lots of water. The mystery is how that water gets to the surface. Today we'd have to drill for it, and that's not going to happen, technologically and economically. It's completely unfeasible to have drill rigs on Mars to get the deep water. But it did happen in the past, and we're still trying to figure that out. We think it may have been associated with some kind of huge energy input into that subsurface water. In my view, there are two candidates. One is volcanism, which, through heat, could cause the water to migrate up to the surface. The trouble with that, though, is that these produce megafloods, with huge amounts of water bursting out onto the surface. How do you get that to happen? So the other idea is that the water may have contained gas under pressure, similar to what happens when you uncork a champagne bottle: you get a huge flow of the liquid, with gas emerging at the same time.

Oliver Strimpel: So I guess this is what you were hinting at when you said that the biblical story of waters upwelling from the deep may have some relevance to the origin of megafloods, both in the case of the deep subsurface water on Mars and also for water flooding out from beneath ice on Earth. It's interesting that one thinks of the paradox: why is the surface of Mars all dry? But you're saying we sort of asked the wrong question, in a way, when we asked where all the water disappeared to. Really, more appropriate is: given how porous the rocks are and how much storage capacity Mars has, how come the water ever got to the surface in the first place? And we see all these signs of water everywhere on the surface.

Vic Baker: Science is about asking the right questions. It's not so much about answering the questions; it's always looking for the better question. We're instinctively trying to do the best we can to investigate, discover new stuff as we go along, and let those discoveries keep propelling us in new directions. The whole business of understanding megafloods has actually operated in this way.

Oliver Strimpel: Victor Baker, thank you very much.

Vic Baker: OK. Thank you.

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