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The Amazon basin (or catchment) occupies approximately
7 million square kilometers in the North Central part of South
America.
Most of it is in Brazil, but it also includes parts of Bolivia,
Colombia, Ecuador, Guyana, and small parts of French Guiana,
Surinam and Venezuela. The amazon basin covers 58.5% of the area
of Brazil.
It is estimated that the Amazon discharges between
34 and 121 million liters of water per second and deposits a
daily average
of 3 million tons of sediment near its mouth. The annual outflow
from the river accounts for one-fifth of all the fresh water
that drains into the oceans of the world. The outpouring of water
and sediment is so vast that the salinity and color of the Atlantic
Ocean are altered for a distance of about 320 km from the mouth
of the river.
The major headstreams of the Amazon are the Ucayali
and Maranón
rivers, both of which rise in the permanent snows and glaciers
of the high Andes Mountains and follow parallel courses north
before joining near Nauta, Peru. From this confluence the main
trunk of the Amazon flows in a generally eastern direction to
the Atlantic Ocean. The Amazon enters the Atlantic through a
broad estuary, roughly estimated at 240 km (about 150 mi) in
width. Here delta deposits have formed a maze of islands that
separate the river into branches. The mouth of the main stream
is 80 km (50 mi) wide. This branch, known as the Pará,
is separated from a smaller branch by Marajó Island, which
has an area of more than 36,000 sq km (14,000 sq mi). During
new and full moon a tidal bore, or wave front from the ocean,
sweeps some 650 km (more than 400 mi) upstream at speeds in excess
of 65 km/hr (40 mph). This phenomenon often causes waves up to
5 m (16 ft) in height.
The Amazon
catchment includes the largest and wettest tropical plain in
the world. Heavy rains drench much of the densely forested
lowland region throughout the year but especially between January
and June. The width, rate of flow, and discharge volume of the
river reflect seasonal variation. During the months of maximum
precipitation, broad areas crossed by the Amazon are subject
to severe floods. In Brazil the width of the river ranges between
1.6 and 10 km (1 and 6 miles) at low water but expands to 48
km (30 miles) or more during the annual floods. The rate of flow
ranges between 2.4 and 8 km/hr (1.5 and 5 mph); and the crest
of the water at flood time often rises 15 m (50 ft) above normal.
To drain the vast mass of water, the Amazon has carved a deep
bed in the plain through which it flows. In one section near Óbidos,
the bed is more than 91 metres (300 ft) below the average surface
level of its water.
Steamboat on the Amazon
Because of its
size, annual floods, and navigability, the Amazon is often called
the Ocean River. The total number of its tributaries
is not yet known, but more than 200 are in Brazil alone. Seventeen
of the largest tributaries are more than 1,600 km in length.
The Amazon proper is navigable to ocean liners of virtually
any tonnage for two-thirds of its course. Transatlantic ships
call regularly at Manaus, nearly 1,600 km upstream, and ships
of 3,000 tons can reach Iquitos, Peru, 3,700 km from the river's
mouth, the farthest point from sea of any port serving ocean
traffic. River steamers of more modest tonnage can navigate
on more than 100 of the larger tributaries.
The
Amazon forest contains the largest single reserve of biological
organisms in the world. No one really knows how many species
there are in the Amazon forest, but scientists estimate that
there are between 800,000 and 5 million species living there,
amounting to 15 to 30 percent of all the species in the entire
world. As naturalists catalogue new species of freshwater fish,
their findings suggest that there may be as many as 3,000 kinds
of fish in the Amazon's rivers and lakes.
Piranha
Among the specialized fish found in the area are:
the pirarucu, said to be the largest freshwater fish in the world
with specimens
measuring over 2 meters in length and weighing 125 kilograms;
the tambaqui, which have teeth that can crack seeds as hard as
those of the rubber tree and the jauari palm; and the piranha.
The ferocity of the meat-eating piranha has been exaggerated.
Although it is true that some species in rare circumstances have
killed large animals and even people, their behavior depends
on the state of their habitat. In main river channels and in
larger lakes with plenty of food they generally leave swimmers
unmolested. They appear to become aggressive only when they do
not have enough nourishment.
Vicente Yáñez
Pinzón may have discovered
the delta region of the Amazon in 1500, but exploration did not
begin until 1540-41, when an expedition led by Francisco de Orellana
started down the Napo River, in what is now Ecuador, and reached
the Atlantic Ocean. Pedro Teixeira undertook the first upstream
voyage. Between October 1637 and August 1638 he ascended the
Amazon to the source of the Napo River and crossed the Andes
to Quito, Ecuador. Later, he returned by the same route. In modern
times the river has been explored by many scientific expeditions,
including that led by Theodore Roosevelt and others sponsored
by the National Geographic Society and the government of Brazil
in 1914.
Native American
Some authorities believe that the river
was named after the Amazons, women warriors of Greek mythology
who were thought to reside
in the region; other scholars insist that the name is derived
from the Native American word amassona ("boat destroyer").
Despite centuries of effort to overcome the dominance of nature,
people have made little impact on the Amazon and most of its
vast drainage basin. No bridge spans the river. Except near its
mouth, the Amazon watershed constitutes one of the most thinly
populated regions in the world. Much of the territory drained
by the river system has never been thoroughly explored. One may
fly for hours over the tropical forests that cover much of the
river's floodplain and see no sign of human settlement. In many
valleys, Native American tribes hostile to strangers continue
to live much as they did before the arrival of the Europeans.
Most commerce is narrowly confined to the navigable sections
of the river system. The economy continues to be dominated by
primitive agriculture, hunting and fishing, and the gathering
of various forest products.
Commercial farming, tourism, and industry
play only a minor role in the region, but manufacturing, mining
and lumbering, the principal
economic activities, are increasingly important. Manaus, the
largest settlement, is once again one of Brazil's major cities.
In the 1980's Brazil, under pressure from international conservation
groups, started to ensure that efforts to develop the Amazon
did not irrevocably compromise the nation's forest resources.
Recent discoveries of oil and gas place further pressures.
However, the Brazilian government is active in promoting sustainable
exploitation
of the huge resources in the Amazon. Environmental monitoring
and licensing systems have been set up. National Parks conserve
the flora and fauna. The rights of Indians are now recognized
and their ways of life are protected in reserves.
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