[♪ INTRO] We live in a watery world, where even dry land is crisscrossed by rivers that move huge volumes of water around the planet every day.

And each of those rivers has a pretty simple role: to carry water downhill, usually toward the ocean. For anyone who’s had much experience with a river, it would seem like the direction of a river’s flow is pretty reliably constant.

But there are things that can make it reverse course. It’s been happening since ancient times. And aside from being weird and surprising, these river reversals often reflect geological changes and can have long-lasting impacts on the surrounding ecosystems.

Today we’re going to be looking at five rivers that flowed backward, either briefly or permanently, and we’ll see what made them dramatically change course. The Amazon River is just about as mighty as a river gets. And while scientists are still debating whether or not it edges out the Nile for the title of the world’s longest river, it is by far the biggest by volume.

Every day it carries water more than 6,000 kilometers east across South America, from the Andes Mountains to the Atlantic Ocean. There, on average, it dumps enough water to fill 68 Olympic swimming pools every second. But it was not always this way.

In 2006, a pair of researchers from the University of North Carolina made a surprising discovery. They’d been studying sedimentary rocks, trying to figure out how quickly the Amazon moves minerals across the continent from the Andes to the Atlantic. And that is when they found ancient mineral grains that appeared to have come from the mountains that lined the eastern coast of South America back in the Cretaceous Period.

Meaning… the minerals had traveled from east to west, opposite the way they flow today. The researchers realized that thanks to those ancient mountains, the Atlantic coast actually sat higher than the land west of it. As a result, when the dinosaurs walked the Earth, water flowed west across northern South America.

Since then, various researchers have tried to piece together the history of the Amazon to decipher what made it turn around and begin flowing east. Of course, the answer is always gravity. But as far as what shaped the land and determined the direction of downhill… that’s less obvious.

One 2014 study used computer models to simulate the geology of this region in the past. The results suggested that when water was flowing west, tens of millions of years ago, it pooled in the low land at the base of the rock that was beginning to rise up to become the Andes, and then drained northward into the Caribbean. Over time, a so-called mega-wetland formed in the northwestern part of the continent.

Then, around 10 million years ago, as the Andes kept rising along the western coast, water started flowing eastward, forming the Amazon River as we know it today. But the models suggested that it wasn’t just the shifting land pushing the water toward the east. The rising mountains also captured more moisture from the Pacific, creating more rain, which led to more erosion.

The erosion helped fill in the lowland areas at the eastern base of the Andes. Eventually, this land was high enough to send water sloping downhill toward the Atlantic. In the end, it may also have been a lot of mundane stuff, like the moving around of sediment on the surface of the Earth that created the modern Amazon River, not just the dramatic creation of the Andes.

For now, this is still the subject of research. But the results suggest that geology and climate can interact in ordinary yet surprising ways to reshape the land and even reverse our planet’s mightiest river. Now, the reversal of a river isn’t always a slow process that happens over millions of years.

In unusual cases, it can happen in almost an instant. And in 1812, residents of New Madrid, Missouri, saw this firsthand. Between December of 1811 and March of 1812, a sequence of earthquakes rattled the central Mississippi Valley.

They included three of the biggest earthquakes in U. S. history. Fortunately, there weren’t a lot of people living near the area that got hit the hardest because the region got devastated.

New Madrid itself got leveled. In the surrounding areas, there were landslides, collapsed riverbanks, and islands that vanished… but one of the strangest things eyewitnesses reported was that, for a short time, the Mississippi River seemed to flow backward. Now, the Mississippi River runs from north to south.

It starts at a glacial lake in Minnesota and empties into the Gulf of Mexico. But right after the February 7 quake, eyewitnesses claimed to see the river run north. One thing we know for sure is that whatever happened did not affect the entire river.

It only affected a short stretch just south of New Madrid. And there were no scientific observations of this, so all the reports are from eyewitnesses. But geological research since then has helped explain what happened.

We know that new, short-lived waterfalls formed along the river as the land cracked and parts of the riverbed jutted upward or sank downward. In particular, a region of land known as the Tiptonville Dome, which is about 150 square kilometers, shot upward in the quake. All of a sudden, water passing through this raised portion sloshed back the way it came until the river settled and found a new route south.

The incident completely changed the Mississippi’s path, leaving behind abandoned channels and new lakes. But, shortly after the upheaval, the river itself carried on toward the south, just like before. Reversals of major rivers like the Amazon or the Mississippi are dramatic, but It doesn’t take a big river to have big consequences.

Much more recently, the reversal of meltwater flowing from Alaska’s Kaskawulsh Glacier completely changed the way water drains in the region, in just a month. Before May of 2016, much of the glacier’s meltwater drained northward along the Slims River, eventually emptying into the Bering Sea. But in May of 2016, the water that used to travel north started draining southward, down the Kaskawulsh River, which empties into the Pacific.

And weeks later, the Slims River nearly ran dry. It was an instance of what scientists call river piracy, in which water feeding one stream gets diverted into another. And it’s not unheard of.

Plenty of historical and geological records point to river piracy. But the thing is, it usually happens over geologic timescales. Like, at least thousands of years, not… weeks.

And scientists realized that this unusual case of river piracy happened thanks to something else that is happening faster than it ever has before: climate change. That spring, temperatures rose consistently above freezing a couple of weeks before they usually do. Thanks to these warm temperatures, higher-than-usual levels of meltwater carved a canyon into the thinning glacier that funneled water southward instead of north.

This cut off most water from the Slims River as the Kaskawulsh River swelled. It also completely reorganized water drainage in this region, and that’s likely to have an impact on the ecosystems and human populations surrounding it. One immediate consequence was that the dry floodplain of the Slims River started having almost daily dust storms.

And over time, the new flow of water and sediment is likely to alter habitats and erode and shape the land in unpredictable ways. And as glacier melt accelerates with our warming climate, the river systems that stem from glaciers can be in for an abrupt and dramatic change. Now, it may still be too soon to say what the lasting consequences of the Slims River capture will be, but a more ancient instance of river reversal shows how an event like this can shape ecosystems in the long term.

Today, New Zealand’s Kaituna River travels northward. But, back in a 1970 study, one researcher noted something a little odd. He was looking at a drainage region known as the Marlborough Sounds, which contains the Kaituna River.

And he noticed that the tributaries intersected with the sounds at acute angles that pointed to the south. Typically, the direction of these angles reflects the direction of water flow. So based on geometry alone, the researcher suspected that water once flowed the opposite way, likely through the Kaituna River.

More than 25 years later, a pair of researchers found evidence to back it up: ancient sediment that had clearly traveled from north to south. Researchers figured out that the reversal was caused by sideways motion along the fault line that runs through the northern part of New Zealand’s southern island. Over the last million years, this motion raised the land under the Marlborough Sounds so that it became an uplifted region rather than a sunken one.

And with that, the flow of water reversed toward its new downhill. Early studies couldn’t pinpoint when exactly this happened. But in a 2007 study, researchers narrowed it down to somewhere in the last one-or two-hundred thousand years.

One way they were able to estimate the date of the reversal was through the ecological consequences of this switch: When the new drainage divide created a barrier between members of a fish species so they could no longer reproduce with each other, each group of fish began to evolve separately. Today, there’s a 1.6% genetic difference between the two populations. And scientists were able to use the amount of genetic divergence, among other clues, to estimate how far back the species had split.

Many river reversals caused by geological changes are permanent. But some rivers periodically flow backward and then… switch right back. In general, this happens in sections of rivers that are affected by ocean tides, which can push seawater upstream as they rise.

But there are also some exceptional cases of rivers that periodically reverse their flow. And among the most remarkable is the Qiantang River in China. During especially high tides, the river forms what’s called a tidal bore: basically a large wave of high water that rolls upstream.

And it looks incredible. It’s almost a vertical wall of thundering water with a foamy front that rolls over the flat lower tide. Like other tidal bores, the Qiantang bore forms in an estuary, the funnel-shaped region where the mouth of a river meets the ocean.

How exactly bores form involves some complicated fluid dynamics that we will not get into here. But in the case of the Qiantang River, there are a few main factors that turn extra-high tides into something so exceptional. For one, the estuary rapidly narrows from about 100 kilometers wide to just 20 kilometers wide where it meets the river.

As the high tide squeezes through this narrow region, the water rises dramatically. Then it hits a sandbar, which pushes the water further upward and deforms the wave, ultimately forming the wavefront that becomes the tidal bore. This phenomenon happens to some extent at every high tide, but during the spring tide, the bore can get up to 4 meters high, and it’s like a cliff dropping off to the water below.

It travels more than 100 kilometers upriver, moving about as fast as a bike. Then, when the tide shifts, it rolls right back out. All of these remarkable rivers show that even Earth’s most permanent-seeming features can be turned on their head, whether in an instant or over a geological epoch.

River reversals can be both disastrous and awe-inspiring, and they often hint at the subtler geological phenomena happening around them. Thanks for watching this episode of SciShow. And if you liked learning about these wild rivers, I bet you would love our podcast, SciShow Tangents!

In it, some of the fun people involved in SciShow get together for a lightly competitive knowledge showcase. Every week, they rack up points for teaching the others, and everyone listening at home, the most mind-blowing science facts relating to the week’s theme. Like, there was a whole episode about rivers, for instance!

We talked about how they form, bridges that cross them, and even salmon! And if you love science, laughing, and lighthearted, nerdy competitions, you should check it out! You can find SciShow Tangents anywhere you get your podcasts. [♪ OUTRO]