00:00It is about 2,500 years ago, a crisis has radically transformed the course of the most famous fleas of
00:06the planet.
00:07The Ganges.
00:08It is in the nature of the water to not be immutable.
00:11If you live near the one of them,
00:14it could somehow evaporate without letting trace,
00:17or, at the same time, submerge a whole city.
00:20In the case of Ganges,
00:22it is a crisis of colossal height which has caused this spectacular metamorphosis.
00:27The researchers think that such an event could be reproduced,
00:30which would be a catastrophe for millions of people whose existence depends.
00:36Prenant naissance sous forme de modest ruisseaux dans l'Himalaya,
00:40the Ganges parcourt a trajectory of more than 2,500 km.
00:43At the time of its arrival,
00:45it conflue with other large pools of water,
00:47such as the Brahmaputra and the Meghna,
00:50before all these eaux ne se déversent into the Gulf of Bengal.
00:53Of all the rivers in the world,
00:55the Ganges has the most vast delta.
00:58Like many other rivers in the region,
01:00its traces evolve slowly over the centuries.
01:03When a river changes its course in eroding its berges,
01:06its transformations can extend over several decades.
01:09The scientists have put in evidence
01:11that the fluvial system of the Ganges Brahmaputra
01:13had known several movements over the last millennium.
01:17In examining the images satellites of the delta du Ganges,
01:20the researchers have made a intriguing discovery.
01:22They identified a depression in arcs of circle
01:26at about 50 km of the actual lake.
01:29This formation,
01:30which extended on several kilometers
01:31and reached more than 1,000 meters wide,
01:34seemed correspond to a ancient branch principal du Ganges,
01:37disparu depuis centuries.
01:38In 2018,
01:40a team of researchers went to the place
01:42to analyze the region
01:43and collect the materials
01:44to deepen their investigation.
01:46On the path of return,
01:48they saw a excavation
01:49where a worker was digging
01:50to manage an étang.
01:52The next day,
01:53this cavity was filled with water.
01:55But in examining the Earth,
01:57the scientists found a habit habitable.
02:00We could observe
02:01the vertical strates of sable clear
02:03but more sombres.
02:05These particular formations are
02:06witnesses of the past.
02:07They revealed
02:08that a ancient crisis
02:09has led to a sudden surge
02:11of sand and water
02:12from the depths,
02:13creating the sand of sand
02:15and sand.
02:16In analyzing these
02:17sand and sand,
02:19the researchers have established
02:20that these crises
02:21were produced
02:22there about 2,500 years
02:24and that they were
02:25of a remarkable intensity
02:26atteignant
02:27the magnitude
02:28between 7 and 8.
02:30Even that the epicenter
02:31is located at more than 180 km
02:33from the main course
02:34the power of the phenomenon
02:36has enough to modify the trace.
02:38A such catastrophe
02:39would have affected
02:40nearly 140 millions
02:42of people.
02:43Located at the junction
02:43of the tectonic plates,
02:45the system of the Ganges
02:46Brahmaputra
02:46is frequently submitted
02:47to secousses.
02:49Aucun danger imminent
02:50has been identified
02:51but the experts
02:53think that
02:53another large mass
02:54could arrive
02:55in the next centuries
02:57or millennia.
02:58A such event
02:59would threaten
02:59up to 170 millions
03:01of people
03:02or the equivalent
03:03of half of the population
03:04of the United States.
03:04However,
03:06it is impossible
03:07to predict
03:08the date
03:09and the intensity
03:10of a future
03:10tremblement
03:11of the earth.
03:12The study
03:12of the events
03:13passed
03:13along the sea
03:14of the researchers
03:15identify
03:16and anticipate
03:18the events
03:20similar.
03:21This knowledge
03:22could be very precious
03:23to strengthen
03:23the preparation
03:24and put in place
03:25strategies
03:26before a major catastrophe
03:28to arrive.
03:29The Ganges
03:30is not the only
03:31river exposed
03:31to this type
03:32of threat.
03:33The river
03:33and the river
03:34and the river
03:36American river
03:37such as the Clamath
03:38and the San Joaquin
03:39are also concerned
03:40by these risks.
03:42The powerful
03:43river Mississippi
03:44has also changed
03:45the course
03:45several times
03:46over the last millennia.
03:49Today,
03:49a vast network
03:50of river
03:50and digues
03:52known as the
03:53structure
03:53of the water
03:54has been built
03:55in the center
03:56of the Louisiana
03:56to contain its湾
03:58and prevent
03:59it to fusion
03:59with a nearby
04:00river the Tchafalaya.
04:03However, the specialists claim that this
04:05device could
04:06not resist
04:07to a large flood
04:08so, the
04:10Mississippi could again
04:11fall
04:11into its
04:12bed and
04:12change course,
04:13which would cause terrible
04:15consequences,
04:16particularly in
04:16the south of
04:17Louisiana. One
04:18of the recent most
04:19remarkable catastrophe caused
04:21by the brutal change
04:22of a river
04:24is that of the Koh-Zi
04:25in India.
04:26In August 2008,
04:28in the space
04:29of a few days,
04:30the river
04:30has deviated
04:31nearly 100 km
04:32a huge impact
04:34of a huge impact.
04:35If the state
04:36of the Bihar
04:36is regularly struck
04:37by the inundations,
04:39a phenomenon
04:40of such a large
04:40height
04:41has not been produced
04:41for more than half a century.
04:44This sudden
04:44has affected
04:45nearly hundreds of villages
04:46and 3 millions
04:47of inhabitants
04:48forcing the evacuation
04:49of millions
04:50of them
04:50more sure.
04:52The number
04:52of these populations
04:53had not known
04:54such as
04:55inundations
04:55since decades,
04:56were not prepared
04:57to face
04:58which has aggravated
04:59material and humans.
05:01Since decades,
05:02scientists
05:03seek to understand
05:04the precise mechanisms
05:05which lead
05:06to a river
05:06to brutally change
05:08its course.
05:09A recent study
05:10met in light
05:11the interaction
05:12of two main factors
05:13of this phenomenon
05:14known
05:15by the name
05:16of the water.
05:17First of all,
05:18a preparation
05:18is put in place
05:19semblable
05:20to an accumulation
05:21of tension
05:22before a sudden rupture
05:25of the time.
05:26The sediment,
05:27sable,
05:27terre
05:28and other materials
05:29transported
05:29by the current
05:30are accumulating
05:31at some places.
05:32This accumulation
05:33can either
05:34raise the water
05:35or accentuate
05:38the pence
05:38of an adjacent zone
05:39of the sea.
05:41the decisive element
05:43which precipitate
05:44the changement
05:44of course.
05:45It may be
05:46an exception
05:47or even
05:49a simple
05:50amas of wood
05:50flotting
05:51of the accumulation
05:52of the water.
05:53researchers have analyzed
05:54images satellites
05:55covering 170
05:56cases of avulsion
05:57over the last 50 years.
05:58They have noticed
05:59that these events
06:00were often produced
06:01at the most often
06:02and the reliefs
06:04montagnes.
06:04In these areas,
06:06the rivers
06:06have demonstrated
06:06three times higher
06:08to change
06:09the trajectory
06:09that in the intermediate
06:10areas.
06:11In other,
06:12in studying
06:13in detail 58
06:14chenots fluviaux
06:15at the aid of
06:15cartes
06:16haute resolution,
06:17they have measured
06:17the sur-élévation
06:18of the cours
06:18of the water
06:19compared to the
06:19terrains.
06:20They have also evaluated
06:22the inclination
06:22of an eventual
06:23new trace
06:24from the current
06:25level
06:25of the river
06:31to cause
06:32the drought
06:33but at the
06:34approach
06:34especially
06:35the delta,
06:38the water
06:38is necessary to
06:39allow the river
06:40to break
06:41the river
06:41and to open
06:42a new passage.
06:44These two
06:44mechanisms
06:45interagissent
06:46like a balance.
06:47If one
06:47s'intensify,
06:48the other
06:49could be
06:49less pronounced
06:50to allow an avulsion.
06:52To
06:53change
06:53the
06:56to
06:57analyze
06:57the
06:57terrain
07:00and
07:01the
07:01flow
07:01flow
07:01of
07:02the
07:02sea
07:03of
07:03the
07:03sea
07:03of
07:03the
07:04repertoriées, il a systématiquement anticipé avec précision le tracé adopté. Les scientifiques
07:10espèrent que ces avancées permettront d'améliorer la protection des populations riveraines face aux
07:15crues soudaines. Les changements de cours des rivières ont joué un rôle déterminant dans
07:19l'émergence de l'un des premiers systèmes administratifs organisés. Les scientifiques
07:24en ont trouvé des indices en Mésopotamie, dont le nom signifie « la terre entre deux fleuves »,
07:30en référence aux tigres et à l'Euphrate. Les sociétés qui y prospéraient dépendèrent de
07:34ces fleuves pour leur approvisionnement en eau, leur agriculture et leurs échanges. Afin de
07:39comprendre l'impact des modifications soudaines du réseau fluvial sur ces populations anciennes,
07:44les chercheurs se sont appuyés sur deux types de cartes. Les cartes archéologiques retracent
07:49l'expansion des villes au fil du temps et nous renseignent sur les types de construction érigées.
07:54Certains édifices, comme les palais monumentaux ou les enceintes fortifiées, témoignent de
07:59l'émergence d'une administration centralisée. Le second type de carte permet de reconstituer
08:04l'ancien tracé des fleuves Tigre et Euphrate, dont le cours a varié, tantôt lentement en
08:10décrivant un tracé sinueux, c'est le méandrage, tantôt brutalement et de façon spectaculaire,
08:16il s'agit de l'avulsion. Aux environs de 5000 avant notre ère, le climat de la Mésopotamie
08:22s'est progressivement asséché, compliquant les pratiques agricoles. Pour survivre, les cultivateurs
08:28devaient s'installer à proximité des rivières afin de tirer parti des crues saisonnières ou de
08:33creuser des canaux d'irrigation permettant d'acheminer l'eau jusqu'à leur culture. L'importance
08:38vitale de l'agriculture a favorisé l'essor des villages et des cités le long des fleuves,
08:43garantissant ainsi aux populations un accès constant à leurs ressources. Mais lorsqu'un fleuve changeait
08:48brusquement de cours, les populations ne se résignaient pas. Elles s'organisaient pour creuser
08:53d'imposants canaux destinés à rétablir l'irrigation en captant l'eau du nouveau lit
08:58fluvial. L'essor des infrastructures et des bâtiments administratifs témoignent de leur
09:03capacité à s'organiser collectivement pour relever tous ces défis.
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