Amazon River Basin

The Amazon River basin is the largest in the world, covering an area of about 2,720,000 square miles (7,044,768 square kilometers). It is home to more plant and animal species than any other region and contains thousands of species that have yet to be discovered or named.

Geography

The Amazon River in South America is the world’s largest river, although, at around 4,000 miles (6,437 kilometers), it is not quite the longest; Africa’s Nile River, at around 4,132 miles (6,650 kilometers), is slightly longer. However, the amount of water flowing down the Amazon is about sixty times greater than that of the Nile. The Amazon carries more water than the Nile, the Mississippi, and the Yangtze Rivers combined, accounting for more than one-fifth of all the river water in the world. Its basin—the region drained by a river and its tributaries—includes parts of Brazil, Bolivia, Peru, Ecuador, and Colombia. It covers more than 2.7 million square miles, or 7 million square kilometers, an area about three-fourths the size of the continental United States. More than a thousand tributaries flow into the Amazon, and the basin contains about 14,092 miles (22,680 kilometers) of navigable water.

There are two different types of tributaries, dubbed black-water and white-water rivers. The black-water rivers, such as the Urubu, the Río Negro, and the Uatumã, come from very old geologic areas that are poor in salts, very acidic, and filled with sediments. Anglers avoid them because they have small fish populations. The water is actually tea colored because of heavy concentrations of tannic acid. The black-water rivers have few nutrients and carry much-decaying vegetation from riverbanks. The decaying matter, chiefly leaves and branches, consumes much of the oxygen in the water. Because of low oxygen levels, these rivers support few forms of life; they are so muddy that little light can penetrate, and few plants can live in them, further reducing the food supply for fish. Indigenous people living along these rivers call them “starvation rivers” because little food can be obtained. On the other hand, they also support fewer insects and mosquitoes, making the surrounding areas more livable for human beings. The white-water, or clear-water, tributaries come from areas of little erosion. The waters from these streams, such as the Xingu and the Tapajos, are clear enough that the bottom is visible in many places.

The Amazon begins high in the Andes Mountains in Peru, less than 160 kilometers from the Pacific Ocean, at about 5,200 meters. For its first 970 kilometers, the Amazon headstream, known as the Apurímac River, drops through the mountains at a rate of about 5 meters per kilometer. It passes through grasslands used for grazing sheep and alpaca and by Cuzco, the old capital of the Inca Empire and the region’s largest city. After passing through the Montana, an area of dense forests and steep valleys, the Apurímac flows into the Urubamba, forming Ucayali River. It then heads north for about 325 kilometers, where the stream is joined by several smaller tributaries coming from Peru and Ecuador.

Where the Ucayali joins the Maranon, about 160 kilometers above the city of Iquitos, the middle course of the Amazon begins. The elevation is about 105 meters, and the river is more than 3 kilometers wide. It then travels through 405 kilometers of unpopulated territory before it enters Brazil. For the next 1,296 kilometers, the Amazon becomes a slow-moving, meandering stream, forming many huge lakes. It is along this stretch that the Río Negro, named for its very dark waters, enters from the north. The Río Negro is 6.5 kilometers wide and carries water from Colombia and Venezuela. A few hundred miles downstream, the Madeira, the Amazon’s longest tributary, enters from the south. Its waters come from as far away as La Páz, the capital of Bolivia, to the Paraguay River in far southern Brazil. Below the Madeira, the Amazon narrows to little more than half a kilometer wide, but it is about 130 meters deep. It still has about 800 kilometers to go before it reaches the ocean.

The Xingu is the last major river to enter the Amazon, and its junction is considered the place where the delta—the triangular part of a river formed by soil deposits just before it enters the sea—begins. The Xingu is about 1,620 kilometers long and rises in the Brazilian highlands. The Amazon delta stretches along the Atlantic coast for about 325 kilometers to the north and inland for about 485 kilometers. The delta contains the island of Marajó. The city of Belem is in the delta and is the official port of entry for the Amazon basin.

Climate and Flora

Most of the Amazon basin has a rainy tropical climate. Annual rainfall in the delta averages about 215 centimeters per year, with 180 centimeters in the middle region and 300 to 400 centimeters per year in the upper course. The middle course and the delta have a rainy season from December through May, during which as much as 45 centimeters of rain can fall every month. During this time, the floodplain, called the várzea, is soaked and becomes a giant lake that can be anywhere from 50 to 200 kilometers wide. Above the mouth of the Madeira, the Amazon flows through a different type of floodplain called the terra firme, where the soil is generally very poor in nutrients and unsuitable for agriculture. Even in the várzea, however, less than 2 percent of the floodplain can be used for farming. Mostly, there is a tropical rainforest of light woods, brush, wild cane, and grasses. The soil is poorly drained and often clayey. Cattle are grazed on Marajó Island, and farmers have brought in water buffalo, but little else can be grown or raised in the region.

There are more than thirty thousand species of flowering plants in the basin. That number represents about one-third of all the flowering plants found in South America and three times the number found in Europe. The amount of vegetation in various areas of the river is influenced by the rise and fall of the water. Some plants are drowned during floods, while others thrive. During periods of low water, the floodplain has little vegetation save for some grasses and wild rice. The grasses become floating islands during flood season, sometimes reaching sizes of over 0.6 kilometers in length and several hundred meters wide. The islands are home to passion fruit, morning glories, and giant water lilies more than 0.6 meters in diameter. Ants, spiders, and grasshoppers also live on the islands. The root zone becomes a home for insects and an important food source for fish. Many fish also eat the fruit of the plants. The islands are trapped in bushes and trees at low water, and the plants begin to rot, turning the water a deep, murky brown.

In the várzea, forest trees and bushes must adapt to survive during the flood season, which is at its peak from May until July. During this period, plants become waterlogged, and the forests, or igapo, are constantly under one to two meters of water. Sometimes, the shoreline is swamped all year, and many plants and trees will die. Young plants are often underwater for seven to ten months per year. A lack of light also limits their growth, as bigger trees block out the light. In many cases, it takes seedlings ten to twenty years to reach a size that will enable them to survive. There are other problems; for example, kapok, or silk-cotton trees, have been among the most successful plants in the várzea. This species has been almost entirely eliminated by loggers cutting down the trees to be made into boxes.

Fauna

Approximately thirteen hundred species of birds live in the Amazon, or over 10 percent of all bird species on the planet. Some 50 percent of these species are found in this region and nowhere else. The bird population includes hummingbirds, macaws, owls, parakeets, parrots, and toucans. Most species favor a particular level of rainforest, either on the ground or in the lower, middle, or upper tree level. Only a few kinds of birds move between levels. The Amazon is the richest bird region in the world. There is also an enormous population of other flying animals: more than fifty different species of bats are represented, making them the region's most diverse group of mammals.

There are no crocodiles in the Amazon, but there is the crocodile's smaller relative, the caiman. The black caiman can reach a length of 5.5 meters. Unfortunately, it is endangered due to overhunting. Its shiny black skin, used for handbags and expensive shoes, is a sought-after commodity for hunters.

More than 3,000 species of fish are believed to be in the Amazon River. In comparison, the Congo River in Africa has an estimated 560 fish species, and the Mississippi River has fewer than 300. There are so many species in the Amazon because there were no mass extinctions in the river caused by glaciers or ice ages. The huge size of the basin also provides for numerous kinds of different environments, each of which can be uniquely exploited by other species. There are rapids, waterfalls, lakes, and streams, each with varying types of water and vegetation. Both freshwater and saltwater species are found. Lungfish, which have survived from the Paleozoic era 150 million years ago, are common.

About 43 percent of the fish are characins, including piranhas and neons, which are familiar to many aquarium owners. Piranhas are a diverse group of around sixty species, twenty of which are found in the Amazon and can be very dangerous. They hunt in schools of up to one hundred fish and are constantly searching for food. Normally, they are found in the floodplain lakes of the várzea. Various kinds of catfish and stingrays are found in deeper water. One species, the piraíba catfish, is the largest fish in the river, growing as long as 3.6 meters and weighing as much as 200 kilograms. The Amazon River is also home to many freshwater dolphins, which can reach lengths of 2.4 meters.

The phylum Arthropoda is well represented in the Amazon region, with more than one million species in the area, many of which are still unclassified. They are, without a doubt, the most successful animals in the rainforest. About 90 percent are insects (jointed invertebrates, or animals without backbones), 9 percent are spiders, and 1 percent are other, more obscure creatures. Insects include flies, beetles (the most diverse group), ants, scorpions, millipedes, centipedes, and symphylans. The latter are centipede-like and live on the floodplain floor. The floor-dwelling species generally migrate upward into the trees when the rainy season begins. In the middle and upper tree levels are spiders, moths, butterflies, and various other winged species. The most abundant species at all levels are the ants, many of which have adapted to live on one particular kind of tree or plant at one specific level. Millions more live on the ground, where huge numbers of mollusks, segmented worms, and flatworms also thrive.

Several types of lizards live in the basin, iguanas being the best known. Iguanas are typically green but can quickly change color to blend in with their background. They have long toes and sharp nails that enable them to climb trees, as most live at the tops of trees. Iguanas are vegetarians and can reach a size of more than 0.3 meters in length. Another kind of lizard, the teiid, can reach lengths of 1.5 meters. Only a few frog species are in the region, primarily because they have difficulty competing with fish for food and are easy prey for piranhas and other flesh-eating fish. There are more toads than frogs in the Amazon, as toads can more easily hide in the leaf litter that covers the ground.

The Amazon is home to the world’s largest living rodent, the capybara, which looks like a large guinea pig and lives in groups of a dozen or more along the riverbanks, feeding on grasses and other plants. However, the capybara is being rapidly reduced in numbers because it is intensively hunted for food by locals. Other rodents include the toro, a spiny rat that is a common night hunter; tree-dwelling porcupines; anteaters; leaf-eating sloths; five species of opossums; and several species of armadillos. Most of these groups migrated into the basin from the north when Central America became connected to South America via Panama around three to four million years ago.

There are some hoofed mammals in the basin, but most species of this type have become extinct. The tapir is the oldest hoofed mammal in the basin and the only one that enters the river, usually for bathing and to eat fish. Monkeys are abundant in the region, with more than forty species found. There are two main groups: callitrichids, members of the family Callitrichidae, which include marmosets and tamarins, and cebids, members of the family Cebidae, which include capuchins and squirrel monkeys. Callitrichids live in small troupes in the trees and are most active during the day; cebids are much larger, have relatively large brains, and resemble African apes.

There are nearly three hundred species of snakes in the Amazon, most living on the floodplain or the riverbank. Only the coral snakes and vipers are poisonous. The area’s pit vipers are among the most dangerous snakes in the world. They hunt at night, reach lengths of a meter, and live on a diet of small animals. The anaconda is the largest snake in the basin, reaching a length of six meters or more. It is a constrictor and, therefore, not poisonous; instead, it kills by squeezing its victims to prevent them from breathing, causing death by suffocation. The anaconda is most active at night, grabbing birds, rodents, turtles, or caimans and coiling around them to kill them and then break their bones to be easier to swallow and digest. Boa constrictors use the same method of capturing their food, which is common in the water and on the shore.

Manatees are the largest animals in the Amazon, weighing 450 kilograms or more. These water-dwelling mammals eat grasses and plants, congregate in large numbers, and can stay underwater for more than an hour before coming to the surface to breathe. Other animals include coatis, which resemble raccoons; several kinds of deer; and peccaries, or wild hogs. Human beings were the last species to enter the basin, perhaps about ten thousand years ago.

Studying the Amazon River and its surrounding areas remains vital for ecological and economic reasons. The river is essential to the global environment, especially in increasing understanding of climate change. The Amazon remains the center of Earth's biodiversity and interconnectivity; protecting its contents protects Earth's valuable natural resources. Because the Amazon River and its basin contain vast amounts of carbon, protecting the area is vital to mitigating the negative effects of global climate change. Threats, such as pollution, deforestation, and human-induced hydrologic changes, threaten the Amazon. 

Principal Terms

basin: the region drained by a river system, including all of its tributaries

delta: the area at the mouth of a river that is built up by deposits of soil and silt

fauna: the animal population of a region

flora: the plant species of a region

tributaries: rivers that flow into larger rivers

várzea: the part of the rainforest that is flooded for up to six months per year

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