00:00Imagine that you crossed a bridge by car to visit a friend.
00:04The next day, you get back in your car to go home,
00:07but there is no longer a river under the bridge you crossed the day before.
00:11Believe it or not, that's exactly what happened to the inhabitants of southern Honduras
00:15when Hurricane Mitch, of Category 5, hit the region in October 1998.
00:22The Choluteca Bridge, which the local authorities had rebuilt earlier that year,
00:26found itself dry in a way, because the river had shifted and no longer passed under it.
00:31The structure quickly became famous on the internet under the name of Bridge to Nowhere.
00:36The course of this waterway changed overnight.
00:38In general, it takes a little longer.
00:42A river will stop flowing in a straight line at any change in the landscape around it.
00:47Even such a tiny thing as a mouse can weaken the banks of a river.
00:51Imagine that a family of mice decide to settle somewhere on the riverbank
00:55and dig a comfortable hole.
00:57Over time, the water enters this hole and begins to carry the furniture floor.
01:03After a few centuries, the river will make an end to this place.
01:07And if you are worried about the family of mice, do not worry,
01:11because it will have moved long before the water floods its house.
01:16Things happen even faster for small streams of water,
01:19because their bed is also small.
01:22The current breaks the stones.
01:24The ground absorbs them.
01:26And this is how a riverbed is formed.
01:29It is in the middle of the watercourse that the current is the strongest,
01:32and its power shapes its bed directly.
01:35The water that flows in rivers and rivers is loaded with earth and other particles.
01:40All these materials go from the source to the mouth,
01:42then throw themselves into the oceans.
01:45In some cases, these particles can also accumulate.
01:48And so new beaches,
01:51among which probably your favorite beach,
01:53are born.
01:55On the other side of the watercourse,
01:57the current is much faster and comes to hit the nearby bank.
02:01So, the erosion process starts again.
02:08Fluvial erosion occurs when the water that flows carries the ground.
02:11And I must tell you that things go very fast in this case.
02:15The water is so powerful that it can even dig into the rock and create gorges.
02:20If the banks of a river are made up of furniture materials,
02:23like humus,
02:24i.e. the earth in which flowers grow,
02:27it is easier for the water in motion to erode them.
02:30In the plains, the rivers are often wide and slow,
02:32because the ground around them offers little resistance.
02:35So you don't have to worry about the waves rocking your boat on a plain river.
02:40In the mountains, the watercourses quickly carry everything that is on their way,
02:45thus revealing the rocky substratum.
02:48Mountain watercourses are generally frozen in the stone.
02:51This rock at the bottom does not move an inch.
02:54Erosion is therefore responsible for the modifications of the watercourse,
02:58but there are also mountains and hills.
03:00When a watercourse reaches a mountain or simply a rock,
03:04it has two possibilities,
03:05bypass it or cross it.
03:08In the plains, the rivers have no natural obstacles
03:11and therefore choose a winding path.
03:13All these turns and detours increase the length of the river,
03:16from the source to the mouth.
03:20The powerful Mississippi River in the United States is an example of this.
03:24It presents many turns and curves,
03:26which are called meanders.
03:28Only in the last century and a half
03:30has the Mississippi changed course several times.
03:33The last major case dates back to 1876,
03:37when it left the historic city of Vicksburg dry.
03:40Two years later,
03:41a team of American engineers came to the region
03:44to build the Yasu Derivation Canal,
03:46which took 25 years to complete.
03:48Thanks to him, the boats could sail again to Vicksburg,
03:52which relaunched the city's economy.
03:55What happened in this city in the south of the United States in the 19th century
03:59is a natural consequence of river erosion.
04:02The curves of a watercourse are accentuated over time,
04:05which accelerates the flow.
04:07The erosion is increased
04:09and the meanders slowly approach each other,
04:11until they merge.
04:13When this happens,
04:15the river becomes straight again the next day,
04:17leaving a lake in the shape of an iron horse
04:19at the place where the elbow was.
04:21This new watercourse is called a dead arm.
04:23These stagnant lakes dry up,
04:26because they are no longer fed by a source,
04:28or turn into swamps.
04:30In some cases,
04:32man uses them as wet meadows for agriculture.
04:38Lake Chico in Arkansas
04:39is the largest dead arm in North America,
04:42formed by the Mississippi several centuries ago.
04:46The Mississippi takes its source in a Minnesota lake.
04:49But a river can also be born from the merger of two other watercourses,
04:53or from the simple rise of underground water.
04:56Even the melting of the snow can feed a river,
04:59like the Amazon that draws its water into the Andes.
05:02Determining the source of a watercourse can be difficult,
05:04which is why the debate on whether the Amazon is longer than the Nile is still open.
05:10What scientists know for sure
05:12is that the Amazon carries more water than any other river.
05:16A fifth of all the fresh water that penetrates the oceans of the earth
05:20comes from this river in South America.
05:22When you look at the Amazon River on a map,
05:25you notice that it flows from Peru to Brazil,
05:28that is, from the west to the east.
05:30This direction may seem strange
05:32if we compare it to that of the Mississippi,
05:34which flows south on the map.
05:36It's confusing because you probably know that rivers flow down,
05:41under the effect of gravity,
05:43but the bottom is not necessarily synonymous with the south.
05:45The water that flows tries to find the easiest way,
05:48so there are no rules in terms of cardinal direction.
05:53The Nile is the best example,
05:55since it flows north, into the Mediterranean Sea.
05:59The north of Egypt, where all the pyramids are located,
06:02is on a land lower than Sudan.
06:04This is where the Nile is formed,
06:06at the meeting point of the Blue Nile and the White Nile.
06:10The Nile, in northern Asia,
06:11and the Mackenzie River, in Canada,
06:14both flow north-west,
06:15while the Yellow River, in eastern Asia,
06:18flows in the same direction as the Amazon,
06:20to the east, to the Pacific Ocean.
06:23These great rivers are among the best proofs
06:25that a stream of water does not necessarily have to flow from north to south.
06:29Gravity is the only important factor
06:31that determines the direction of a stream of water,
06:34because running water is always pulled down.
06:37The flow of water is faster on steep slopes.
06:40When there is a sudden break in the flow,
06:43waterfalls form.
06:45The Salto Angel, in Venezuela,
06:47is the highest waterfall on the planet.
06:50The water falls from a height of 800 meters,
06:52so be careful where you put your feet.
06:55In a waterfall, the water that falls
06:57can easily reach the speed of a Thomson gazelle,
07:00but when a river approaches the end of its course,
07:03it slows down considerably.
07:06The stream of water does not have enough energy
07:07to start the neighboring lands.
07:09All the sediments it has collected along the way,
07:11such as the sandstone, slow it down.
07:15Where a river meets the ocean or a lake,
07:18it gets rid of its sediments
07:19and a delta is created,
07:21a rich and fertile land.
07:23The Ganges, in India,
07:25has the largest delta in the world,
07:27several hundred kilometers wide
07:30and visible from space.
07:32The Amazon, on the other hand,
07:33does not have a real delta
07:34because the strong currents of the Atlantic Ocean
07:36carry everything that the torrent brings it.
07:40The deepest part of the bed of a river,
07:42which is in its middle,
07:43is called a chenal.
07:45This is where the current is strongest,
07:47and it is therefore on these channels
07:49that the ships circulate.
07:51When engineers want to enlarge a navigable route,
07:54they just need to dig a deeper chenal.
07:57And there you go,
07:58the big ships can pass.
08:00This is one of the ways
08:01that man modifies watercourses.
08:03We also often reinforce the banks of a river
08:05to prevent erosion,
08:06or we build dams
08:08that stop or divert the flow of water.
08:10The oldest operational dam on the planet
08:13dates back to the time of the Pharaohs of Egypt.
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