Hi, I’m Peter Clift, Professor of Geosciences at Louisiana State University.
In earlier videos I talked to you about the Asian monsoon,
why it exists, how it has changed through time and how it is linked to
uplift of the Himalayas. Today I want to introduce you to the great rivers of Asia
and explain how they are tied up with both the changing climate and uplift of mountains in Asia.
Why are the rivers of Asia special?
Southeast Asia is home to some of the biggest
rivers on Earth and these rivers deliver more sediment into the ocean than any other rivers.
There are eight great rivers that originate along the margins of the Tibetan plateau.
these are, from West to East , the Indus, the Ganges, the Brahmaputra, the Irrawaddy, the Salween, the Mekong,
the Yangtze and the Yellow. On their way to the sea these and other rivers of SE Asia
provide nutrients and water to floodplains where food to feed almost 3 billion people is grown.
The Mekong delta in particular is known as the rice bowl of Vietnam.
The Yangtze River, the longest river in Asia, plays the same role in China. The importance
of the rivers to humans dates back to the oldest civilizations, including those in China and along
the Indus Valley in India and Pakistan. The supply of water to these areas was critical
for developing the agriculture that allowed these populations to grow.
These powerful rivers continue to be important for the nations they flow through.
This importance is shown spectacularly by the Three Gorges Dam on the Yangtze River of China.
The waters behind this dam produce more electricity
than any other power plant on Earth.
So, why are the rivers so close together?
Although Asia is not the only place in the world with large rivers,
Southeast Asia is unique in having several large rivers that flow very close together. This is seen
in the southeastern corner of the Tibetan plateau where the Salween, Mekong, Red and Yangtze rivers
flow parallel to each other in deep gorges. The Salween’s drainage basin is especially unusual.
This 3300 km long river flows have an unusually narrow drainage basin, squeezed between the
Irrawaddy and Mekong drainage basins, which are also curiously narrow. In addition,
some rivers have unusual courses such as the hair-pin like “first bend” of the Yangtze.
Big rivers on other continents, like the Mississippi River of North America
have broad drainage basins with regular branching patterns
where tributaries flow towards the sea, progressively coming together as they do so.
The Mississippi also flows far from the other big rivers of North America.
In contrast, the great rivers of SE Asia have drainage basins that are remarkably narrow
because of the strong tectonic activity of this region. The Yangtze River example provides some
useful insights into how tectonics can affect river courses. Upstream, the Yangtze flows out
of the Tibetan Plateau towards the southeast, apparently heading to the South China Sea.
However, in SW China the river changes its mind and heads off towards the northeast
finally entering the East China Sea near Shanghai. This 90° course change suggests
strong tectonic control on river courses around the India-Asia collision zone.
How does this the first bend of the Yangtze relate to the India-Asia collision?
Uplift of the Tibetan Plateau began when India started colliding with the rest of Asia
about 50 million years ago. Uplift disrupted the old south-flowing river system,
splitting the various tributaries into the independent rivers we see today.
Early in the collision, what is today the middle Yangtze River flowed southwest. As the collision
continued, Tibet rose up, strengthening the monsoon and its rains, then fed these giant rivers.
Around 25 million years ago as the uplift continued, the upper Yangtze joined the middle Yangtze and its
old connection to the South China Sea was lost. The middle stretches of the river would have
instantly reversed direction! The Yangtze went its own way. The leftovers of what would have
been the Mississippi of South-East Asia became the Red River, flowing into the Gulf of Tonkin.
Rivers and the monsoon
After the major reorganization of South and
East Asian river patterns around 25 million years ago the network has been pretty stable, because
these rivers are deeply entrenched in their canyons. The amount of water and sediment carried
by these rivers has varied over time because the strength of the summer monsoon rains has changed
since the monsoon first strengthened in Paleogene time, that is more than 23 million years ago.
As Earth’s climate cooled, starting around 15 million years ago, and especially after
8 million years ago, the rains weakened and the rivers washed less sediment into the ocean.
After the northern hemisphere ice age began about 2.5 million years ago, the Asian monsoon
and its rains strengthened when the climate was warmer, and drier as Earth’s climate cooled
and the glaciers advanced. Monsoon strength is also linked to other global climate phenomena
such as El Niño, being weaker during strong El Nino, when the Eastern Pacific
is warmer than usual and stronger during La Nina, when it is cooler.
More recent warming caused by anthropogenic emissions of CO2 may make the rains stronger.
Global warming has also caused Himalaya glaciers to shrink.
Their spring melting is a significant source of river water, a supply that is now threatened.
I hope you've enjoyed learning how the great rivers of Asia and how they come to look the way they do
How they changed with the changing Asian Monsoon
and how they allow people to live and prosper across this
great continent. Thanks for watching along with me.
Now keep an eye out for the next video about how Monsoons and people have changed through time.