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The Amazon
Introduction
The Amazon, flowing southward across Brazil in the broad equatorial part of South
America, has the world's largest drainage basin, more than 7 million sq km, or nearly 5
percent of the world's total land area. It carries nearly 20 percent of the Earth's total
water discharge to the ocean in a flow so powerful that it perceptibly dilutes the ocean
water of the Atlantic 160 km beyond the coastline. The Amazon has long been
considered the world's second-longest river, after the Nile, with a length of about 6,450
km. In 1994, however, a Peruvian research team declared that the river's true source
was the Ucayli River, not the Maranon, which may make the Amazon the longest river in
the world.
The River's Course and Environment
The Amazon's headstreams form in the Peruvian Andes little more than 160 km from
the Pacific Ocean. In 1541 the Spanish explorer Francisco de Orellana began European
exploration here, descending the river to the Atlantic. He is variously reported to have
imagined, sighted, or been attacked by female warriors. In any event, he gave the river
its name, which refers to the Amazons of Greek mythology. Most of the river's drainage
basin lies east of the Andes. It is composed of low plains less than 150 m (500 ft) above
sea level, strips of floodplain alongside the channels, and broken higher ground in the
upper reaches of its many tributaries, to both the north and the south.
The Amazon's mouth is an estuary, 240 km wide at the coast and studded with low
muddy islands. These represent the beginnings of a delta formed 5,000 years ago when
melting glaciers created an ocean level higher than it is today. A submerged delta built
during periods of glacial maximum and low ocean level stands on the continental shelf.
The estuary's tidal range reaches 5.7 m, and a tidal bore, or wave, occurs from time to
time. Ocean tides are felt as far as Obidos, 960 km inland, where the river's discharge is
an average 180,000 cu m/sec and 283,000 cu m/sec at bank-full stage. This enormous
volume is a result of the humid tropical climate that characterizes most of the basin: the
mean annual temperature is 26 degrees C; precipitation, 2,000 mm. The climate
sustains the world's largest rain forest, or selvas, and promotes intensive land
weathering. The suspended load of silt and clay is 350 million metric tons per year and
resembles that of the midlatitude Mississippi River. Oceangoing ships can travel as far
as Manaus, whereas vessels drawing 6 m can reach Iquitos in Peru, 3,700 km inland.
Of the chief tributaries, the Negro, Japura, Putumayo, Napo, Ucayli, Jurua, and Purus
are also navigable for long distances. Rivers remain the chief means of transport, but
airstrips and highways are opening the Amazon Basin to development.
Jungle
Jungle and rain forest are terms that are often used synonymously but with little
precision. The more meaningful and restrictive of these terms is rain forest, which refers
to the climax or primary forest in regions with high rainfall (greater than 1.8 m/70 in per
year), chiefly but not exclusively found in the tropics. Rain forests are significant for their
valuable timber resources, and in the tropics they afford sites for commercial crops such
as rubber, tea, coffee, bananas, and sugarcane. They also include some of the last
remaining areas of the Earth that are both unexploited economically and inadequately
known scientifically.
The term jungle originally referred to the tangled, brushy vegetation of lowlands in India,
but it has come to be used for any type of tropical forest or woodland. The word is more
meaningful if limited to the dense, scrubby vegetation that develops when primary rain
forest has been degraded by destructive forms of logging or by cultivation followed by
abandonment.
Rainforest Ecology
Rain forests cover less than six percent of the Earth's total land surface, but they are the
home for up to three-fourths of all known species of plants and animals; they also
contain many more species as yet undiscovered. Recent studies suggest that this great
diversity of species is related to the apparently dynamic and unstable nature of rain
forests over geologic time. Despite their appearance of fertile abundance, rain forests
are fragile ecosystems. Their soils can quickly lose the ability to support most forms of
vegetation once the forest cover is removed, and some soils even turn into hard laterite
clay. The effect of forest removal on local climates is also often profound, although the
role of rain forests in world climatic changes is not yet clear.
Types of Rain Forest
Rain forests may be grouped into two major types: tropical and temperate. Tropical rain
forest is characterized by broadleaf evergreen trees that form a closed canopy, below
which is found a zone of vines and epiphytes (plants growing on the trees), a relatively
open forest floor, and a very large number of species of both plant and animal life. The
largest trees have buttressed trunks and emerge above the continuous canopy, while
smaller trees commonly form a layer of more shade-tolerant species beneath the upper
canopy. The maximum height of the upper canopy of tropical rain forests is generally
about 30 to 50 m, with some individual trees rising as high as 60 m above the forest
floor.
The largest areas of tropical rain forest are in the Amazon basin of South America, in
the Congo basin and other lowland equatorial regions of Africa, and on both the
mainland and the islands off Southeast Asia, where they are especially abundant on
Sumatra and New Guinea. Small areas are found in Central America and along the
Queensland coast of Australia.
Temperate rain forests, growing in higher-latitude regions having wet, maritime
climates, are less extensive than those of the tropics but include some of the most
valuable timber in the world. Notable forests in this category are those on the northwest
coast of North America, in southern Chile, in Tasmania, and in parts of southeastern
Australia and New Zealand. These forests contain trees that may exceed in height
those of tropical rain forests, but there is less diversity of species. Conifers such as
redwood and Sitka spruce tend to predominate in North America, while their
counterparts in the southern hemisphere include various species of eucalyptus,
Araucaria, and Nothofagus (Antarctic beech).
Resources and Development
The Amazon Basin is home to more than 2 million insect species, 100,000 plants, 2,000
species of fish, and 600 mammals, many of which are found nowhere else in the world.
The basin also has huge reserves of bauxite, gold, manganese, nickel, copper, tin, and
timber and vast hydroelectric potential. In 1978 the eight Amazon Basin nations signed
the Treaty of Amazon Cooperation (Amazon Pact), agreeing to share in the region's
resources.
In recent years thousands of landless Brazilian peasants have flocked to the sparsely
populated region to build homesteads, leading to clashes between farmers and
ranchers, settlers and large-scale developers, and newcomers and indigenous
aboriginal groups (chiefly slash-and-burn cultivators and fishing communities). Many
scientists fear that unregulated development will have irreversible effects on the fragile
Amazon Basin ecosystem; some contend that the vast fires associated with
development (an area one and one half times that of New York state burned in 1987
alone) are contributing to global warming. Although the rate of deforestation has
declined from the high of 29,992 sq km in 1985, due partly to international pressures on
the Brazilian government, the dangers to the forest are still strong, especially at the
edges. Many experts advocate tourism, mining, and the responsible harvesting of forest
products as alternate means of bringing economic development the region.