Saviez-vous que certains fleuves peuvent en fait changer de direction ? Cela peut sembler incroyable, mais cela se produit lorsque de grands événements géologiques comme des tremblements de terre, des glissements de terrain ou même d'immenses inondations déplacent la terre autour d'eux. Un fleuve majeur à risque de cela est le Brahmapoutre en Inde—c’est un fleuve puissant, mais aussi une bombe à retardement. Les experts disent qu'un seul fort tremblement de terre pourrait le faire changer complètement de cours, entraînant des inondations dévastatrices et des destructions dans la région. Cela est dû au fait que le fleuve traverse une zone sismiquement active, où la croûte terrestre est en mouvement constant. C'est un rappel de la puissance et de l'imprévisibilité de la nature ! Animation créée par Sympa.
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FunTranscript
00:00About 2,500 years ago, a earthquake radically changed the course of one of the most illustrious rivers on the planet, the Ganges.
00:08It is in the nature of watercourses not to remain immutable, so that if you live near one of them, it could one day evaporate without leaving a trace or, on the contrary, submerge an entire city.
00:20In the case of the Ganges, it is an earthquake of colossal magnitude that caused this spectacular metamorphosis.
00:27Researchers estimate that such an event could happen again, which would be a catastrophe for the millions of people whose existence depends on it.
00:36Born in the form of modest streams in the Himalayas, the Ganges traverses a distance of more than 2,500 kilometers.
00:43Throughout its journey, it merges with other large watercourses, including the Brahmaputra and the Meghna, before all these waters overflow into the Gulf of Bengal.
00:54Of all the rivers in the world, the Ganges has the largest delta.
00:58Like many other rivers in the region, its path evolves slowly over the centuries.
01:03When a river modifies its course by eroding its banks, these transformations can extend for several decades.
01:09Scientists have shown that the river system of the Ganges-Brahmaputra had undergone several shifts over the last six millennia.
01:17By examining the satellite images of the Ganges Delta, researchers made an intriguing discovery.
01:23They identified a depression in an arc of a circle about 50 kilometers from the current bed of the river.
01:29This formation, which extended over several kilometers and reached more than 1,000 meters wide,
01:34seemed to correspond to an ancient main arm of the Ganges, which had disappeared for centuries.
01:39In 2018, a team of researchers went there to analyze the region and collect samples to deepen their investigations.
01:47On the way back, they saw an excavation where a worker was digging to build a pond.
01:52The next day, this cavity was to be filled with water.
01:55But by examining the displaced earth, scientists spotted an unusual element.
02:01We could observe vertical strata of light sand mixed with darker mud.
02:05These particular formations are witnesses of the past.
02:08They reveal that an ancient earthquake caused a sudden rise in sand and water from the depths,
02:14creating sand volcanoes frozen in time.
02:17By analyzing these deposits of sand and mud, researchers have established that these earthquakes occurred about 2,500 years ago,
02:24and that they were of remarkable intensity, reaching magnitudes between 7 and 8.
02:30Although the epicenter was located more than 180 kilometers from the main course,
02:35the power of the phenomenon was enough to change the course.
02:39Such a disaster could have affected nearly 140 million people.
02:43Located at the junction of tectonic plates, the Ganges-Brahmaputra system is frequently subjected to earthquakes.
02:49No imminent danger has been identified,
02:52but experts estimate that another large-scale earthquake could occur in the next centuries or millennia.
02:58Such an event would threaten up to 170 million people,
03:02equivalent to half the population of the United States.
03:05However, it remains impossible to accurately predict the date and intensity of a future earthquake.
03:12The study of the past movements of watercourses following an earthquake
03:15allows researchers to identify trends and anticipate, to a certain extent, similar events.
03:21This knowledge could be valuable for strengthening preparation and setting up prevention strategies
03:26before a major disaster occurs.
03:29The Ganges is not the only river exposed to this type of threat.
03:33The Yellow River, the Irrawaddy, as well as several American watercourses,
03:37such as the Klamath and the San Joaquin, are also affected by these risks.
03:42The powerful Mississippi River has also changed course several times over the last seven millennia.
03:48Today, a vast network of dams and dikes, known as the Water Regulation Structures,
03:54has been erected in the center of Louisiana to contain these overflows
03:58and prevent it from merging with a neighboring river, the Atchafalaya.
04:02However, experts believe that this device could not withstand large-scale floods.
04:09Thus, the Mississippi could again cross its bed and change course,
04:13which would have terrible consequences, especially in southern Louisiana.
04:18One of the most striking recent examples of a disaster caused by the sudden change in the course of a river
04:24is that of the Kosi in India.
04:26In August 2008, in the space of a few days, the river deviated nearly 100 kilometers,
04:33a considerable upheaval.
04:35If the state of the Bihar is regularly hit by floods,
04:39a phenomenon of such magnitude had not occurred for more than half a century.
04:44This sudden crash affected nearly 1,000 villages and 3 million inhabitants,
04:48forcing the evacuation of a million of them to safer areas.
04:52Many of these populations, having not experienced such floods for decades,
04:56were not prepared to face them, which aggravated material and human losses.
05:01For decades, scientists have been trying to understand the precise mechanisms
05:05that drive a river to change its course abruptly.
05:09A recent study highlights the interaction of two main factors
05:13at the origin of this phenomenon, known as avulsion.
05:17First of all, a preparation is put in place,
05:20similar to an accumulation of tension before a sudden break.
05:24Over time, the sediments, sand, earth and other materials transported by the current
05:30accumulate in certain places.
05:32This accumulation can either increase the flow of water compared to the surrounding lands,
05:37or accentuate the slope of an adjacent area.
05:40Then comes the trigger, the decisive element that precipitates the change of course.
05:45It can be an exceptional creek, an earthquake,
05:49or even a simple floating pile of wood obstructing the flow of water.
05:53Researchers have analyzed satellite images covering 170 cases of avulsions
05:57over the last 50 years.
05:59They found that these events occurred most often near floodgates
06:03and mountainous reliefs.
06:05In these areas, rivers have shown a 3 times higher probability of changing course
06:10than in the intermediate plains.
06:12In addition, by studying in detail 58 river channels using high-resolution maps,
06:17they measured the elevation of watercourses compared to neighboring lands.
06:21They also evaluated the inclination of a possible new route compared to the current course,
06:26and found that near the mountains, the massive accumulation of sediments
06:30ended up elevating the bed of the rivers, causing overflows.
06:34But near floodgates, especially in the deltas,
06:37a marked slope is necessary to allow the rivers to pierce the mud and open a new passage.
06:44These two mechanisms interact like a swing.
06:47If one intensifies, the other can be less pronounced while allowing an avulsion.
06:52In order to predict the movement of a river in the event of a change of course,
06:56researchers have developed an advanced algorithm
06:59capable of analyzing the slope of the terrain and the river flow.
07:02Tested on 10 cases of reported avulsions,
07:05it has systematically anticipated with precision the path adopted.
07:09Scientists hope that these advances will improve the protection of river populations
07:14against sudden floods.
07:16The changes in river courses have played a decisive role
07:19in the emergence of one of the first organized administrative systems.
07:23Scientists have found clues in Mesopotamia,
07:27whose name means the land between two rivers,
07:30in reference to the Tigris and the Euphrates.
07:33The societies that thrived there depended on these rivers for their water supply,
07:37their agriculture and their trade.
07:39In order to understand the impact of sudden changes in the river network on these ancient populations,
07:44researchers relied on two types of maps.
07:47Archaeological maps retrace the expansion of cities over time
07:52and inform us about the types of buildings built.
07:55Some buildings, such as monumental palaces or fortified enclosures,
07:59testify to the emergence of a centralized administration.
08:02The second type of map allows us to reconstruct the ancient Tigris and Euphrates rivers,
08:07whose course has varied,
08:09sometimes slowly by describing a sinuous path,
08:12this is the meandering,
08:14sometimes brutally and spectacularly,
08:16this is the avulsion.
08:18Around 5000 BC,
08:21the climate of Mesopotamia gradually dried up,
08:24complicating agricultural practices.
08:26To survive,
08:27farmers had to settle near the rivers
08:30in order to take part in seasonal crops
08:33or to dig irrigation canals
08:35allowing water to flow to their crops.
08:37The vital importance of agriculture
08:39favored the growth of villages
08:41and cities along the rivers,
08:43thus guaranteeing the populations a constant access to their resources.
08:47But when a river suddenly changed course,
08:50the populations did not give up.
08:52They organized themselves to dig large canals
08:55to restore irrigation
08:57by capturing water from the new river bed.
08:59The emergence of infrastructure
09:01and administrative buildings
09:03testifies to their ability to organize collectively
09:06to meet all these challenges.