The Amazon

If ever a river were defined by extremes, surely it is the Amazon. Though
its channel may be second in length to the Nile, by every other measure
the Amazon is the world’s greatest river. This grandiose waterway
is defined by superlatives (widest, fastest, etc.), as is its watershed. The
Amazon basin gives rise to biological riches found nowhere else on Earth.
Interspersed among the river’s vast tributaries sit rich deposits of minerals
and oils. Indigenous peoples still live in the basin’s upper reaches,
unassimilated into the Western world. Many of the Amazon’s tributaries
flow through lush, impenetrable jungle. Over millions of years, abundant
rainfall, consistent warmth, and an enormous drainage basin have created
one of the most wondrous habitats on Earth.
Consider the superlatives to which the Amazon lays claim:
• The Amazon discharges more water into the ocean than any
other river. Some 5 million cubic feet (142,000 m³) of Amazon
water enter the Atlantic every second. This is five times the
rate of its closest competitor, the Congo, and 12 times that
of the mighty Mississippi. Overall, the Amazon contributes almost
one-fifth of all river water that empties into the oceans
each year. Along with water, it dumps 3 million tons of sediment
a day into the Atlantic.
• The Amazon draws water from more square miles of land than
any other river, with a drainage basin that easily dwarfs all others.
This one river pulls water from an area of 2.6 million square
miles (6.8 million km²), or more than one-third the continent
of South America. In fact, the Amazon’s watershed dwarfs its
nearest competitor, the Congo, by a factor of two.
• The Amazon gives rise to more branches than any other waterway—
so many its tributaries have never been fully counted.
Estimates exceed 1,000.
• This monstrous river system has more miles of tributaries than
any other river in the world. Three of the Amazon’s branches
are longer than 1,850 miles (3,000 km) and major waterways
in their own right. A full seven of its tributaries span more than
1,000 miles (1,600 km). Overall, its tributaries are so intricate
and numerous that hydrologists cannot even agree whether the
Amazon is a 12th- or 13th-order stream.
• The Amazon draws water from both hemispheres, a feat no
other major river can claim.
The Amazon River drains from a bigger area and has more miles of tributaries than any other river. From its source in
the Peruvian Andes to its discharge in the Atlantic Ocean, the mighty river travels some 4 ,000 miles (6,437 km).
26 G Rivers
• The river passes through the largest remaining tropical rain forest
on Earth.
This great South American waterway is extraordinary in other ways.
To begin with, its full length—from source to sea—has never been verified.
While some accounts give the river a length of 3,900 miles (6,400
km), others say it could be 4,200 miles (6,759 km) long, which—if
true—would make it slightly longer than the Nile.
The Amazon certainly carries more water than the Nile: 60 times
as much, in fact. When it comes to width, the Amazon also emerges as
the hands-down champion. Flowing through soft sediments on its way
to the ocean, the channel becomes so broad in places that it resembles
a great lake more than a river. For a full 250 miles (402 km) before
reaching the Atlantic, the Amazon runs 50 miles (80 km) wide; at its
mouth, it expands to some 200 miles (322 km). Unlike most rivers,
which are either wide (and slow-moving) or deep (and fast-moving),
the Amazon is both. Its depth reaches as much as 120 feet (37 m) in
places, and overall, the channel is much deeper than the narrower and
faster Mississippi.
Such a navigable channel makes the Amazon welcoming to large
ships and freighters. Oceangoing vessels easily sail upstream as far as
Iquitos, a town some 2,300 miles (2,700 km) from the coast. In this regard,
the Amazon is also unparalleled; no other river in the world allows
large (3,000-ton) seafaring boats to go so far upriver.
Also remarkable is that the Amazon remains reasonably unspoiled,
a fact no doubt related to its enormity. Not only are its waters among
the cleanest of the world’s major rivers but its main stem remains undammed.
Origins of the Amazon
Hundreds of millions of years ago, when South America and Africa were
one landmass, the Amazon emptied, not into the Atlantic as it does today,
but into the Pacific. Over the next 100 million years, as a result of continental
drift, the continents moved apart, and the river became smaller.
But the waterway still flowed westward to the Pacific. Then, about 7
million years ago, a sudden burst of tectonic activity pushed the Andean
mountains upward, preventing the river from reaching the sea. The uplift
was akin to building a dam, and a large, inland lake formed east of the
Andes. Eventually, the lake drained eastward, working its way over the
course of many millennia through ancient rock known as the Brazilian and
Guiana Shields. Eventually, the draining water carved a channel to the
Atlantic, giving birth to what is today the Amazon River.
The Amazon G 27
For all its eventual power and size, today’s Amazon has surprisingly
modest beginnings. The river forms high in the Peruvian Andes in the
Ucayali valley, a mere 100 miles (161 km) from the Pacific Ocean, at an
elevation of 18,673 feet. Not far from its headwaters and at an altitude of
9,850 feet (3,000 m) lies the Sacred valley of the Inca, the site of ancient
Indian ruins. Machu Picchu sits only a few kilometers away, evidence that
this upland area once supported farming and a sophisticated culture, both
of which disappeared hundreds of years ago and are represented today
only by artifacts.
Explorers have long known that streams descending from the snowcovered
Andes feed the Amazon. The two largest branches of the nascent
Amazon are the Ucayali, which descends from the headwaters, and the
Marañón, which originates further north in the Andes. Each river flows
north, though the Marañón takes a sharp turn to the east before meeting
the Ucayali in northern Peru, about 50 miles south of Iquitos. Soon thereafter,
they are joined by the Napo River, which descends from Ecuador.
From that point on, the rivers form the main trunk of the Amazon.
The Amazon wends its way east, hugging the equator as it goes, never
further than two degrees latitude from the Earth’s middle. After dropping
nearly 14,000 feet (4,267 m) in its first 600 miles (966 km), the river
now flattens. From Iquitos to its point of discharge into the Atlantic—a
distance of 2,300 miles (3,700 km)—the river descends a mere 300 feet
(91 m), dropping less than one inch per mile on its ocean-bound journey.
Without the momentum downhill provides, the water moves languidly,
in no hurry to reach the sea. That the Amazon flows at all in this stretch
is because the river is pushed by runoff from the Andes.
Moving slowly, the Amazon heads east, joined by tributaries that
descend from Peru, Colombia, and Ecuador to the west; Venezuela and
Guyana to the north; and Bolivia to the south. One such tributary is depicted
in the color insert on page C-2 (top). Here the Rio Pastaza flows
through eastern Ecuador, eroding sediments and carving a channel deep
into the Andean foothills, a pattern repeated throughout the region. As a
result of continued tectonic activity, the Andes are still rising, and their
relative instability, manifested in earthquakes and volcanic activity, contributes
to the erosion of their sediments, which are deposited throughout
the Amazonian floodplain.
The Amazon unites with its best-known tributary, the Río Negro, at
the bustling port city of Manaus, home to 1 million people. The channel
broadens into a giant expanse of water called a ria just downstream from
Manaus. Here the riverbanks stand several miles apart and separate still
further during flood season. So broad is the channel, in fact, that early
Portuguese explorers dubbed the river Rio Mar, or Ocean River.
28 G Rivers
Moving seaward, the Amazon continues to grow in girth and eventually
divides in two, flowing around an enormous island called Marajó.
The river’s northern branch, though a major shipping route, becomes
a maze of islands and channels. The southern branch, called the Pará
River, provides a port for oncoming boats and gives rise to the city of
Belem. With a population of 1.4 million, Belem is the largest city in
eastern Brazil.
The Amazon passes through a large estuary, which stretches some
200 miles (322 km) across the coastal landscape before finally discharging
into the ocean. The Atlantic, unlike the Mediterranean (site of the
Nile’s delta) is a high-energy coastal environment, and so the Amazon
does not build a true delta (see the figure in chapter 4) but instead discharges
into the roiling sea, which sweeps the river’s sediments far from
shore. Billowing sediment plumes (muddy swirls of silt-laden water) are
visible some 150 miles (241 km) out to sea.