Amazon River, the greatest river of South America and the largest drainage system in the world in terms of the volume of its flow and the area of its basin. The total length of the river—as measured from the headwaters of the Ucayali-Apurímac river system in southern Peru—is at least 4,000 miles (6,400 km), which makes it slightly shorter than the Nile River but still the equivalent of the distance from New York City to Rome. Its westernmost source is high in the Andes Mountains, within 100 miles (160 km) of the Pacific Ocean, and its mouth is in the Atlantic Ocean, on the northeastern coast of Brazil. However, both the length of the Amazon and its ultimate source have been subjects of debate since the mid-20th century, and there are those who claim that the Amazon is actually longer than the Nile.

The vast Amazon basin (Amazonia), the largest lowland in Latin America, has an area of about 2.7 million square miles (7 million square km) and is nearly twice as large as that of the Congo River, the Earth’s other great equatorial drainage system. Stretching some 1,725 miles (2,780 km) from north to south at its widest point, the basin includes the greater part of Brazil and Peru, significant parts of Colombia, Ecuador, and Bolivia, and a small area of Venezuela; roughly two-thirds of the Amazon’s main stream and by far the largest portion of its basin are within Brazil. The Tocantins-Araguaia catchment area in Pará state covers another 300,000 square miles (777,000 square km). Although considered a part of Amazonia by the Brazilian government and in popular usage, it is technically a separate system. It is estimated that about one-fifth of all the water that runs off Earth’s surface is carried by the Amazon. The flood-stage discharge at the river’s mouth is four times that of the Congo and more than 10 times the amount carried by the Mississippi River. This immense volume of fresh water dilutes the ocean’s saltiness for more than 100 miles (160 km) from shore.

The extensive lowland areas bordering the main river and its tributaries, called várzeas (“floodplains”), are subject to annual flooding, with consequent soil enrichment; however, most of the vast basin consists of upland, well above the inundations and known as terra firme. More than two-thirds of the basin is covered by an immense rainforest, which grades into dry forest and savanna on the higher northern and southern margins and into montane forest in the Andes to the west. The Amazon Rainforest, which represents about half of the Earth’s remaining rainforest, also constitutes its single largest reserve of biological resources.
Since the later decades of the 20th century, the Amazon basin has attracted international attention because human activities have increasingly threatened the equilibrium of the forest’s highly complex ecology. Deforestation has accelerated, especially south of the Amazon River and on the piedmont outwash of the Andes, as new highways and air transport facilities have opened the basin to a tidal wave of settlers, corporations, and researchers. Significant mineral discoveries have brought further influxes of population. The ecological consequences of such developments, potentially reaching well beyond the basin and even gaining worldwide importance, have attracted considerable scientific attention.

The first European to explore the Amazon, in 1541, was the Spanish soldier Francisco de Orellana, who gave the river its name after reporting pitched battles with tribes of female warriors, whom he likened to the Amazons of Greek mythology. Although the name Amazon is conventionally employed for the entire river, in Peruvian and Brazilian nomenclature it properly is applied only to sections of it. In Peru the upper main stream (fed by numerous tributaries flowing from sources in the Andes) down to the confluence with the Ucayali River is called Marañón, and from there to the Brazilian border it is called Amazonas. In Brazil the name of the river that flows from Peru to its confluence with the Negro River is Solimões; from the Negro out to the Atlantic the river is called Amazonas.

Hydrology
Most of the estimated 1.3 million tons of sediment that the Amazon pours daily into the sea is transported northward by coastal currents to be deposited along the coasts of northern Brazil and French Guiana. As a consequence, the river is not building a delta. Normally, the effect of the tide is felt as far upstream as Óbidos, Brazil, 600 miles (970 km) from the river’s mouth. A tidal bore called the pororoca occurs at times in the estuary, prior to spring tides. With an increasing roar, it advances upstream at speeds of 10 to 15 miles (16 to 24 km) per hour, forming a breaking wall of water from 5 to 12 feet (1.5 to 4 metres) high.

Soils
The vast Amazonian forest vegetation appears extremely lush, leading to the erroneous conclusion that the underlying soil must be extremely fertile. In fact, the nutrients in the system are locked up in the vegetation, including roots and surface litter, and are continuously recycled through leaf fall and decay. Generally, the soils above flood level are well-drained, porous, and of variable structure. Often they are sandy and of low natural fertility because of their lack of phosphate, nitrogen, and potash and their high acidity. Small areas are underlain with basaltic and diabasic rocks, with reddish soils (terra roxa) of considerable natural fertility. The terra preta dos Indios (“black earth of the Indians”) is another localized and superior soil type, created by past settlement activity.

Plant life
The overwhelmingly dominant feature of the Amazon basin is the tropical rainforest, or selva, which has a bewildering complexity and prodigious variety of trees. Indeed, as many as 100 arboreal species have been counted on a single acre of forest, with few of them occurring more than once. The Amazon forest has a strikingly layered structure. The sun-loving giants of the uppermost reaches, the canopy, soar as high as 120 feet (40 metres) above the ground; occasional individual trees, known as emergents, rise beyond the canopy, frequently attaining heights of 200 feet (60 metres). Their straight, whitish trunks are splotched with lichens and fungi. A characteristic of these giant trees is their buttresses, the basal enlargements of their trunks, which help stabilize the top-heavy trees during infrequent heavy winds. Further characteristics of the canopy trees are their narrow, downward-pointing “drip-tip” leaves, which easily shed water, and their cauliflory (the production of flowers directly from the trunks rather than from the branches). Flowers are inconspicuous. Among the prominent members of the canopy species, which capture most of the sunlight and conduct most of the photosynthesis, are rubber trees (Hevea brasiliensis), silk-cotton trees (Ceiba pentandra), Brazil nut trees (Bertholletia excelsa), sapucaia trees (Lecythis), and sucupira trees (Bowdichia). Below the canopy are two or three levels of shade-tolerant trees, including certain species of palms—of the genera Mauritia, Orbignya, and Euterpe. Myrtles, laurels, bignonias, figs, Spanish cedars, mahogany, and rosewoods are also common. They support a myriad of epiphytes (plants living on other plants)—such as orchids, bromeliads, and cacti—as well as ferns and mosses. The entire system is laced together by a bewildering network of woody ropelike vines known as lianas.

Landforms and drainage patterns
The Amazon basin is a great structural depression, a subsidence trough that has been filling with immense quantities of sediment of Cenozoic age (i.e., dating from about the past 65 million years). This depression, which flares out to its greatest dimension in the Amazon’s upper reaches, lies between two old and relatively low crystalline plateaus, the rugged Guiana Highlands to the north and the lower Brazilian Highlands (lying somewhat farther from the main river) to the south. The Amazon basin was occupied by a great freshwater sea during the Pliocene Epoch (5.3 to 2.6 million years ago). Sometime during the Pleistocene Epoch (about 2,600,000 to 11,700 years ago) an outlet to the Atlantic was established, and the great river and its tributaries became deeply entrenched in the former Pliocene seafloor.
The modern Amazon and its tributaries occupy a vast system of drowned valleys that have been filled with alluvium. With the rise in sea level that followed the melting of the Pleistocene glaciers, the steep-sided canyons that had been eroded into the Pliocene surface during the period of lower sea levels were gradually flooded. In the upper part of the basin—in eastern Colombia, Ecuador, Peru, and Bolivia—more-recent outwash from the Andes has covered many of the older surfaces.

Climate
The climate of Amazonia is warm, rainy, and humid. The lengths of day and night are equal on the Equator (which runs only slightly north of the river), and the usually clear nights favour relatively rapid radiation of the heat received from the sun during the 12-hour day. There is a greater difference between daytime and midnight temperatures than between the warmest and coolest months. Hence, night can be considered the winter of the Amazon. At Manaus the average daily temperature is in the upper 80s F (about 32 °C) in September and the mid-70s F (about 24 °C) in April, but the humidity is consistently high and often oppressive. During the winter months of the Southern Hemisphere, a powerful south-polar air mass occasionally drifts northward into the Amazon region, causing a sharp drop in temperature, known locally as a friagem, when the mercury may register into the 50s F (about 14 °C). At any time of the year, several days of heavy rain can be succeeded by clear, sunny days and fresh, cool nights with relatively low humidity. In the lower reaches of the river basin, cooling trade winds blow most of the year.

Animal life
To give a succinct overview of the complete fauna of Amazonia is as impossible as it is to adequately describe the great diversity of its flora; in part this is because many of the region’s species have yet to be identified. The rivers and streams of the basin teem with life, and the forest canopy resonates with the cries of birds and monkeys and the whine of insects. There is a notable paucity of large terrestrial mammal species; indeed, many of the mammals are arboreal.
More than 8,000 species of insects alone have been collected and classified. Myriads of mosquitoes may transmit diseases including malaria and yellow fever. Leaf-cutter ants (of the genera Atta and Acromyrmex) are prevalent, as are the ubiquitous small black flies known as piums in Brazil. Fireflies, stinging bees, hornets, wasps, beetles, cockroaches, cicadas, centipedes, scorpions, ticks, red bugs, and giant spiders are abundant. Most spectacular, however, are the hundreds of species of brilliantly coloured butterflies; sometimes thousands of butterflies gather in the afternoon on riverside sandbanks.