00:00Around 2,500 years ago, an earthquake completely changed the course of one of the largest and
00:06most famous rivers on the planet, the Ganges.
00:09Now, it's normal for rivers not to sit in one place, so if you live next to one, it
00:15could also just disappear without a trace or flood the whole city one day.
00:21In the case of the Ganges River, a supermassive earthquake became the reason for this dramatic
00:26change.
00:27Scientists say it might happen again, and it would be a huge problem for millions of
00:31people who depend on this river.
00:36The Ganges River starts as small streams in the Himalayas and travels a long way, over
00:421,500 miles.
00:44Along its journey, it meets other big rivers before all the water flows into the Bay of
00:48Bengal.
00:50The Ganges has the largest delta in the world among all rivers.
00:54Just like many other rivers in the region, it slowly changes its path over time.
00:59These changes, if they happen when a river wears away its banks, usually take a long
01:04time, sometimes decades.
01:06Scientists have discovered that the Ganges Brahmaputra River system has shifted several
01:11times over the last 6,000 years.
01:14When they looked at satellite images of the Ganges Delta, they noticed something interesting.
01:19There was a crescent-shaped dip in the ground about 28 miles away from where the river is
01:24flowing now.
01:25This depression was over 1 mile wide and stretched for many miles.
01:30They thought it might've been an old main channel of the Ganges River a long time ago.
01:35In 2018, the team decided to visit the area to check it out for themselves and take samples
01:41to learn more about it.
01:43While driving back home, they noticed a pit where someone was digging dirt to create a
01:47pond.
01:48The next day, they planned to fill it with water.
01:51But the researchers spotted something strange in the dirt.
01:54There were vertical bands of light-colored sand mixed into the darker mud.
01:59These special layers are clues from the past.
02:02They show that an ancient earthquake made sand and water shoot up from below the ground
02:07and created sand volcanoes that were trapped in time.
02:13The scientists studied the sand and mud and figured out that these earthquakes happened
02:17around 2,500 years ago and were strong, with magnitudes of 7 to 8.
02:23The epicenter was over 110 miles away from the main stem of the river, but it was powerful
02:29enough to displace it.
02:31It could've affected around 140 million people.
02:34The Ganges-Brahmaputra system is close to spots where Earth's tectonic plates meet,
02:40so earthquakes happen here often.
02:44There's no danger right now, but scientists think that another massive earthquake could
02:49happen in the next few hundred or thousand years.
02:52It could threaten up to 170 million people.
02:55That's about half the population of the U.S.
02:58No one can predict exactly when an earthquake will happen or how strong it will be.
03:04Learning how earthquakes in the past moved rivers helps scientists figure out patterns
03:08and make guesses about similar events in the future.
03:11This knowledge could help us prepare better and create safety plans before another big
03:16earthquake strikes.
03:18The Ganges isn't the only river at risk.
03:21The Yellow River, the Irrawaddy River, and rivers in the United States like the Klamath
03:26and San Joaquin all face similar dangers.
03:29The mighty Mississippi River has also switched its path many times over the last 7,000 years.
03:35Right now, there's a huge system of dams and barriers called a control structure in
03:40central Louisiana.
03:41This structure's job is to stop the Mississippi from spilling over its banks and joining a
03:46nearby river.
03:48But scientists think that, in theory, these barriers wouldn't handle mega floods.
03:53So the Mississippi could break free and change its course again.
03:57This would cause massive problems, especially in southern Louisiana.
04:02One of the recent examples when a river changing its course caused disaster was the Kosi River
04:07in India.
04:09In August 2008, over just a few days, the river's path shifted by almost 60 miles,
04:15which is a huge change.
04:18Floods happen in some parts of the state rather often, but they hadn't seen such strong
04:22ones in over 50 years.
04:24They affected around 1,000 villages and 3 million people, and about 1 million people
04:29had to be evacuated to safety.
04:32Since many of these people hadn't faced floods in so long, they weren't ready to
04:36handle it quickly.
04:37It caused even more damage to their homes and lives.
04:42Scientists have been trying for decades to figure out what exactly makes rivers suddenly
04:46change their course.
04:48A new study explains that two things work together to make a river to do it.
04:53This process is called avulsion.
04:56First there's the setup, which is like building up tension before something big happens.
05:00Over time, sediment, sand, dirt, and other stuff carried by the river gets deposited
05:06in certain places.
05:07It either raises the river above the land around it, or makes a nearby slope steeper
05:12than the river's current path.
05:15Then there's the trigger, which is like the final straw that makes everything happen.
05:19This trigger could be a big flood, an earthquake, or even something like a pile of logs blocking
05:25the water.
05:27Researchers looked at satellite images of about 170 cases of avulsion from the past
05:3350 years.
05:35They noticed these events happen most often near river mouths and near mountains.
05:40Rivers were about 3 times more likely to change course in these places than in the areas between
05:46them.
05:47They also studied 58 river channels in detail, using super-clear maps to measure super-elevation
05:53– that's how high the river is compared to the land around it.
05:57They also measured slope advantage – that's how steep a new path might be compared to
06:01the river's current one.
06:03They found that near mountains, rivers often pile up so much sediment that they become
06:08super-elevated and spill over.
06:11But near river mouths, like in deltas, rivers need a steep slope advantage to break through
06:16the mud and form a new path.
06:19These two factors work together like a seesaw – if one is stronger, the other doesn't
06:24need to be as strong for an avulsion to happen.
06:27To figure out where a river might go if it changes course, the researchers created a
06:32smart computer algorithm.
06:35This tool uses maps to measure the terrain's steepness and the river's flow to predict
06:40where the river might move.
06:42They tested it on 10 past avulsions, and it correctly guessed the path every single time.
06:48Scientists hope they could use these findings to protect people living near rivers from
06:52sudden floods.
06:55Rivers changing course played a big role in creating one of the first organized administration
07:00systems in the world.
07:02Scientists found clues for this in the territory of ancient Mesopotamia.
07:07This name means the land between two rivers, and it was between the rivers Tigris and Euphrates.
07:14People living there relied on these rivers for water, food, and transportation.
07:18Scientists wanted to figure out how these sudden changes in these rivers affected ancient
07:23people.
07:24To do this, they used two kinds of maps.
07:27The archaeological maps show how ancient cities grew over time, including what types
07:32of buildings people built.
07:34Certain buildings, like big palaces or city walls, could show when each city first had
07:39an organized administration.
07:41The second kind of map showed where the Tigris and Euphrates rivers flowed long ago.
07:46They often shifted, sometimes slowly, and sometimes very quickly and dramatically.
07:52Around the year 5000 BCE, Mesopotamia started to become drier, and farming became harder.
08:00Farmers had to grow their crops near rivers, where they could use the natural floods to
08:04water their fields or dig little channels to bring river water to their plants.
08:09Because farming was so important, towns and cities grew near the rivers too, so people
08:14could stay close to their food.
08:17But when a river suddenly changed its course, people didn't give up.
08:21They worked together to dig long canals to bring water from the new river path back to
08:25their fields.
08:27The canals helped them get water again, and the administration buildings showed they were
08:31organizing as a community to solve big problems together.
08:38That's it for today!
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