Amazon Basin (Brazil): River Basins
1. Zoom from Bangladesh to the Amazon Basin
2. If you look at Earth, from space you see water. Of all the objects in the solar
system, Earth is the only one that has liquid water at the surface. In fact,
Earth goes a step farther by having all 3 phases of water (liquid, ice, water
vapor) coexist at the temperatures and pressures of Earth’s surface).
a. Most of Earth’s water actually turns out to be located deep within the
earth, within the rock of the mantle, but it doesn’t come into our lives
very much.
b. Most of Earth’s surface water, 97%, is in the ocean. And most of what
is left over is in glaciers (mostly Antarctic); at least for now. During
the age of the dinosaurs the planet was warm enough that there were
no large ice caps. And it may happen again.
c. But a large portion of what is left over is in the Amazon River basin.
The Amazon river, like most rivers is one part of a large drainage
basin, with many smaller tributaries coming together to make one
large river before it enters the sea, as we saw with the Ganges River.
So it makes more sense to talk about the Amazon Basin than just the
Amazon river.
i. Define basin (watershed; drainage area).
ii. The Amazon Basin contains so much water that when the
Amazon drains out to the Atlantic Ocean, which it does on the
north side of Brazil, it contains as much water as the next 7
largest rivers combined!
1. An average of over 200,000 m3/s (next largest is the
ganges, at 40,000 m3/s), but more than 300,000 m3/s
at times of flood.
2. Responsible for 20% of all the world’s freshwater
returned to the sea.
3. The huge amount of vegetation also produces 20% of all
the oxygen of the planet.
4. Makes a huge plume of freshwater that flows 400 km
into the Atlantic ocean!
5. In fact, such a strong flow, combined with tides, that the
Amazon doesn’t have a true delta – the sediment gets
washed far to sea.
6. Its estuary is over 200 miles across; Even 1000 miles
upstream, the river is still over 7 miles wide in places; it
is so deep that ocean liners can sail 2,300 miles inland.
iii. Amazon is big by any other criteria as well
1. Second-Longest river (over 4000 miles) – only the Nile
is longer, by a tiny bit.
2. Largest Basin area (almost twice as large as the Congo
River basin in Central Africa) – 7 million km2
3. Over 1100 different tributaries feed the Amazon; 17 of
these are more than 1000 miles long themselves (a
large portion of the world’s longest rivers are here!)
4. They carry varying amounts of sediment depending
upon where they flow through; an unusual occurrence
is where the Rio Negro (black river) meets the sandy
Rio Solimoes. The water is black because of large
amounts of humic acid from vegetation – like strong tea.
5. Adjacent to the Orinoco River basin; there is actually a
natural canal between them.
3. Why, of all the places in the world, is there so much water here?
a. First of all, to get so much water flowing out the Amazon, we need to
have a lot of rain. And the Amazon Basin receives an enormous
amount of rain.
i. Now, there are some places in the world that average more
rain per area over the course of a year (more than 10 meters
per year!)
1. Mt. Wai’ale’ale, Kauai, Hawaii
2. Mt. Tutendo and Lloro, Colombia
3. Mawsynram and Cherrapunji, Meghalaya, India
ii. But Nothing compares to the total rainfall received over the
whole Amazon region.
b. To understand that, we have to look at why it rains in the first place.
i. Explain Atmospheric Circulation
1. Sunlight at equator
2. Evaporation at the sea surface
3. Thermal convection
4. Condensation/precipitation at colder T (higher)
5. Coriolis effect
6. Westward air currents at equator
7. Andes Mts, blocking and lifting air
8. So, moist warm air comes off the Atlantic, cools as it
travels up across central South America, and rains out.
c. The Andes is the longest mountain range in the world, and the tallest
outside of Asia, so it not only provides an efficient block to the
moisture, but provides a vast amount of sediment, which provides for
fertile soils all across the Amazon basin.
4. The Amazon has a wet season and a dry season. Varies by region, but
generally Jan-June is wet and July-Dec is dry. Water levels throughout the
Amazon can then rise and fall a lot.
a. Very strange feature – flooding forests. Many forests are totally
flooded for part of the year.
b. Giant otters and manatees and 15-ft long arapaima fish and electric
eels and schools of piranhas (some of which, the red-bellied piranhas,
really will attack and kill humans and other large mammals) and all
c.
d.
e.
f.
sorts of other animals will swim around the flooded forests, only to
have these habitats then disappear for part of the year.
There are relatively few people who live in the amazon basin because
of how wet it is. Some villages are built on stilts, and get around by
boat for part of the year. There are even some villages, like in Iquitos,
Peru, that float. All the houses are built on rafts that float about.
One fascination region: Anavilhanas Archipelago – the world’s largest
freshwater archipelago, with over 400 river islands.
Another remarkable region is the Pantanal, which is the world’s
largest single wetlands region – about 75,000 sq miles (on Brazil,
Paraguay, and Bolivia). About 80% of it is totally flooded during the
rainy season. It is home to a remarkable diversity of species of plants,
animals, and all other life forms.
Rainforests around the world occupy about 2% of Earth’s surface, but
contain about 50% of its species. Ironically, this remarkable
biodiversity exists because rainforests have some of the worst
geologic conditions for life, so there is an intense competition for
resources, which leads to extreme diversity (explain).
i. The vast amount of rain has rendered the soils there worthless.
All nutrients of any value have long since washed down into
the ground – soils can be many hundreds of feet thick.
(explain)
ii. Life has evolved to keep what nutrients are available above
ground. (canopies of plants above ground; leaves that fall; in
some places little rain actually reaches the ground)
iii. Incredibly fortunate for us.
1. 80% of the world’s diet originated in rainforests – fruits
like avocados, coconuts, figs, oranges, lemons,
grapefruit, bananas, guavas, pineapples, mangos and
tomatoes; vegetables including corn, potatoes, rice,
winter squash and yams; spices like black pepper,
cayenne, chocolate, cinnamon, cloves, ginger, sugar
cane, turmeric, coffee and vanilla and nuts including
Brazil nuts and cashews.
2. Over 3000 different fruits grown in rainforests. Natives
eat about 2000. Only about 300 have made it yet to the
Western World
3. Amazon rainforest is also a great place to find
medicines – it would be impossible to start from scratch
and come up with drugs to resist all bacteria and fungi;
why bother? Plants and animals have done through
evolution, developing all kinds of immunities and
resistances.
4. About 25% of western drugs are made from rainforest
products, even though far less than 1% of rainforest
trees and plants have yet been tested.
5. The U.S. National Cancer Institute has identified 3000
plants that are active against cancer cells – 70% of these
plants are found in rainforests.
6. Some of the biggest advocates for preserving rainforests
are the pharmaceutical companies – this is where our
future medicines will come from.
5. Top 5:
a. Congo River (Democratic Republic of the Congo, Central African
Republic, Angola, Republic of the Congo, Tanzania, Cameroon, Zambia,
Burundi, Rwanda) – second largest in basin size and water outflow,
only to the Amazon.
b. Nile (Ethiopia, Eritrea, Sudan, Uganda, Tanzania, Kenya, Rwanda,
Burundi, Egypt, Democratic Republic of the Congo, South Sudan); all of
Egypt was green 5000-7000 years ago; the Nile is the only water from
Sudan to the Mediterranean
c. Mississippi (changed mouth many times since last ice age) – show
map of Mississippi changes; Army Corps Engineers project to keep
Mississippi in its current channel may be biggest engineering project
in history
d. Yenisey (largest river system that drains to the Arctic; its headwaters
include Lake Baikal, which is the world’s deepest (over a mile – the
bottom is almost 4000 feet below sea level), oldest (30 million years),
and largest freshwater lake (20% of the world’s surface freshwater).
Questions:
1. What would the Amazon basin be like if there were no
Andes Mountains?
2. The island of Barbados, in the Caribbean, is at a
subduction zone but is not a volcano, like most other
islands there. It is made of sediments. Find it on a map;
why do you think it is there?