Geographic Description: Amazonia is the largest tropical rain
forest in the world. Originally, more than 2 million square miles of
the region were covered by dense tropical forest. For centuries, its
vastness and inaccessibility have protected the preponderant part of
the forest, and still do so today. However, over the past 30 years,
government sponsored road building projects, colonization
schemes, and industrial developments have transformed large areas
of Amazonia from pristine forest to polluted factory sites and
sprawling settlements. This may be an ominous harbinger of things
to come as the heavily exploited rain forests of Africa and Southeast
Asia run out.
The River Basin:
The largest of the world's rivers in terms of volume of water
discharged into the sea is the Amazon. This mightiest of rivers
forms a network of water channels that permeates nearly half the
continent of South America. The main river is some 6500 kilometres
long, second only to the Nile in length. It is fed by more than 1,000
tributaries, including seven that are more than 1600 kilometres
long, and it drains more than half of Brazil, as well as parts of
Bolivia, Peru, Ecuador, Colombia and Venezuela. Its total drainage
basin of some 7,000,000 square kilometres and encompasses about
one-third of South America,an area more than 10 times the size of
Texas and nearly as large as the entire contiguous United States.
Over most of this vast region the climate is very warm and humid.
Rain falls about 200 days each year, and total rainfall exceeds 2
metres per year. One result of so much rain is that Amazonia is
covered by the largest tropical rain forest in the world. Another
result is that the river carries by far the largest volume of water of
any river in the world. On the average, some 112 billion litres per
minute (6 billion cumecs) flow into the sea, about 10 times the flow
of the Mississippi. The discharge is so great that it noticeably dilutes
the salinity of the Atlantic's waters for more than 100 miles
offshore.
The great river begins as hundreds of tiny streams high in the
Peruvian Andes, some of them within 100 miles of the Pacific
Ocean. Rushing down the slopes, stream after stream continues to
merge to form larger and larger rivers. Near Iquitos in eastern Peru,
the northeastward flowing Ucayali and the Rio Maranon, the two
main headwaters of the Amazon, unite to form a truly major river.
Iquitos is the point farthest upstream that shallow draft-freighters
and passenger vessels can penetrate (deep-water ships can reach
as far as Manaus in central Brazil).
Beyond Iquitos the river changes both its character and course.
Turning abruptly eastward, it more or less parallels the equator as it
meanders over lowland plains. At Iquitos the river also changes its
name. Locally it is known as the Solimoes from Iquitos to its
junction with the Rio Negro at Manaus, and is called the Amazon
only from Manaus to the sea.
Crossing the low interior basin of Brazil, the Amazon flows along a
very gentle gradient: Only about 5cm per kilometre. Sluggish now,
it branches into numerous secondary channels, which are separated
by very densely forested islands. Beyond the riverbanks are broad,
swampy floodplains dotted with lakes and covered with lush,
periodically flooded forests.
All along the course of the river there are seasonal floods.
Tributaries flowing from the south tend to reach their highest stages
from February to April, while those coming from the north tend to
crest in June and July.
On its long journey to the sea, the Amazon also varies in color.
Some of its tributaries are called "white" rivers, through their color
is more often a murky yellow or tan. Others are known as "black"
rivers, their waters dark but crystal clear. The white rivers rise in
the Andes, and their turbidity results from the heavy loads of mud
and silt they carry. The black rivers, in contrast, rise in areas of
ancient basement rock where little sediment remains to be washed
away; only dissolved organic matter stains their clarity.
Clearly the most dramatic union of a black-water stream and a
white one occurs at Manaus, where the Rio Negro flows into the
muddy Amazon. For many miles the black and white waters flow
side by side in separate, clearly defined streams before they finally
intermingle. About 600 miles from the coast, at Obidos, the ocean
begins to affect the river. Tides are able to penetrate this far
upstream because of the extremely gentle slope of the land.
Beyond the point where the Zingu flows in from the south, the
Amazon splits up into a maze of channels clogged by larger and
larger islands. (Marajo, the biggest island in the delta, is about the
size of Vermont and New Hampshire combined.) Finally, beyond its
several mouths, the river merges with the sea where a powerful
ocean current bends it northwestward along the coast. Upon mixing
with salt water and depositing brown silt along the continental shelf,
the Amazon slowly loses its tan and becomes part of the much more
massive South Equatorial Current. Particles of clay smaller than 4
microns, still held in suspension, tinge the current gray until it
passes beyond the Guianas (French Guiana, Suriname, and
Guyana). And, even after the waters clarify, dissolved nutrients
from Amazonia travel as far as the Grand Banks of Newfoundland.
Cycles of Flow: The Amazon River pulsates once a year. From
November through May. the volume of the mainstream swells. For
example, on June 1, 1989, the level of the river at Manaus, 900
miles from the ocean, had been 45 feet above low water, nearly
reaching the 1953 all-time high-water mark on the flood gauge. The
Amazon's volume in that month far exceeded the combined flow of
the next eight largest rivers on Earth, as it does by the end of every
May, even in years of normal flow.
During the second half of the year, the flow diminishes. By way of
another example, in November 1990, around Manaus, stretches of
white beaches and sandbars were exposed to the sun for the first
time in living memory. The river had fallen 50 feet to its lowest level
on record in this century.
The only official fluviometric studies of the main stream flow were
done in 1963 and 1964 (years estimated of lesser than average
rainfall) by the U.S. Geological Survey. Measurements were made
at Obidos, 960 kilometres inland, where the Amazon squeezes
through a single channel little more than a mile wide. Findings gave
the average minimum discharge at 340000 cumecs (cubic metres
per second) while the average maximum reached 940000 cumecs.
For comparison, the Mississippi at Vicksburg averages 69,000
cumecs. It has been suggested that the Amazon's average annual
discharge equals 20 percent of the total continental runoff of all
rivers on Earth. Note, this does not mean that the Amazon system
holds one-fifth of all the world's fresh water, as some books have
interpreted this data. In fact, all Amazonia's waterways hold less
than one ten-thousandth of the world's fresh water, most of which
is locked up in polar ice.
Settlements: Although there are a few sizable cities along the
river's banks and scattered settlements inland, Amazonia is largely
uninhabited. Here and there, plantations have been cleared in the
jungles, and natives ply the streams in search of latex and Brazil
nuts. But mostly the great green luxuriant rain forest is still pristine
wilderness, one of the few large areas left on Earth where nature's
creation remains more or less unspoiled and intact.
Flora and Fauna: The flora and fauna of Amazonia are not all
known to science, since so much of them are native to the least
explored parts of the rain forest. No one knows exactly how many
species of fish there are in this river-sea, with estimates reported of
more than 2,000. Among these are some of the biggest fish outside
the ocean. (The arapaima reaches 5 metres in length and can weigh
as much as 180 kilograms.) Here, too, is the electric eel and the
notorious piranha. And the biggest of all snakes, the anaconda, is at
home here, more often in the water than out. The salt sea has
contributed many inhabitants to this freshwater sea, including a
dolphin, a manatee and stingrays that nestle in the sandbars high
up the tributaries where they rush out of the Andes. Only a tiny
percentage of Amazonia's millions of species of plants and animals
are known to science, but those few that have been studied have
already yielded valuable foods, medicines, and commercial
products.