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Amazon River

15:13 | 26/05/2019

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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 Central and Northern Andes and the Amazon River basin and drainage network.Encyclopædia Britannica, Inc.

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.

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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 Riveris Solimões; from the Negro out to the Atlantic the river is called Amazonas.

 
The debate over the location of the true source of the Amazon and over the river’s precise length sharpened during the second half of the 20th century, as technological advances made it possible to explore deeper into the extremely remote locations of the Amazon’s headstreams and to more accurately measure stream lengths. Beginning in the 1950s, explorers of the region cited various mountains in Peru as possible sources, but they did so without taking precise measurements or applying hydrological research. An expedition in 1971, sponsored by the National Geographic Society, pinpointed Carruhasanta Creek, which runs off the north slope of Mount Mismi in southern Peru, as the source of the river. This location became widely accepted in the scientific community and remained so until the mid-1990s—although a Polish expedition in 1983 contended that the source of the river was actually another stream, nearby Apacheta Creek. (The Carruhasanta and Apacheta streams form the Lloqueta River, an extension of the Apurímac.)

With the introduction of Global Positioning System (GPS) technology in the 1990s, researchers again attempted to navigate the entire length of the Amazon. The American geographer Andrew Johnston of the Smithsonian Institution’s Air and Space Museum in Washington, D.C., employed GPS gear to explore the various Andean rivers that flow into the Amazon. Using the definition of the river’s source as being the farthest point from which water could flow into the ocean and where that water flows year-round (thereby eliminating those rivers that freeze in winter), he concluded that the source was Carruhasanta Creek on Mount Mismi.

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By the early 21st century, advanced satellite-imagery technology was allowing researchers to match the river’s dimensions even more precisely. In 2007 an expedition that included members of Brazil’s National Institute for Space Research and other organizations traveled to the region of Carruhasanta and Apacheta creeks in an attempt to determine which of the two was the “true” source of the Amazon. Their data revealed that Apacheta was 6 miles (10 km) longer than Carruhasanta and carries water year-round, and they concluded that Apacheta Creek was indeed the source of the Amazon River. The team then proceeded to measure the river’s length. As part of this process, they had to determine from which of the Amazon’s three main outlets to the sea to begin the measurement—the Northern or Southern channels, which flow north of Marajó Island, or Breves Channel, which flows southward around the western edge of the island to join the Pará River estuary along the southern coast of the island. They chose to use the southern channel and estuary, since that constituted the longest distance from the source of the river to the ocean (at Marajó Bay); according to their calculations, the southern outlet lengthened the river by 219 miles (353 km). Their final measurement for the length of the Amazon—from Apacheta Creek to the mouth of Marajó Bay—was about 4,345 miles (6,992 km).

This team of researchers, using the same technology and methodology, then measured the length of the Nile River, which they determined to be about 4,258 miles (6,853 km); that value was some 125 miles (200 km) longer than previous calculations for the Nile but nearly 90 miles (145 km) shorter than the length the group gave for the Amazon. These measurements infer that the Amazon may be recognized as the world’s longest river, supplanting the Nile. However, a river like the Amazon has a highly complex and variable streambed—made more so by seasonal climatic factors—which complicates the process of obtaining an accurate measurement. Thus, the final length of the river remains open to interpretation and continued debate.

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.

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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.

The Amazon River’s main outlets are the two channels north of Marajó Island, a lowland somewhat larger in size than Denmark, through a cluster of half-submerged islets and shallow sandbanks. There the mouth of the river is 40 miles (64 km) wide. The port city of Belém, Brazil, is on the deep water of the Pará River estuary south of Marajó. The Pará is fed chiefly by the Tocantins River, which enters the Pará southwest of Belém. The port city’s link with the main Amazon channel is either north along the ocean frontage of Marajó or following the deep but narrow furos (channels) of Breves that bound the island on the west and southwest and link the Pará River with the Amazon. There are more than 1,000 tributaries of the Amazon that flow into it from the Guiana Highlands, the Brazilian Highlands, and the Andes. Six of these tributaries—the Japurá (Caquetá in Colombia), Juruá, Madeira, Negro, Purus, and Xingu rivers—are each more than 1,000 miles (1,600 km) long; the Madeira River exceeds 2,000 miles (3,200 km) from source to mouth. The largest oceangoing ships can ascend the river 1,000 miles to the city of Manaus, Brazil, while lesser freight and passenger vessels can reach Iquitos, Peru, 1,300 miles (2,090 km) farther upstream, at any time of year.

How Long Is the Amazon River?
According to some, it’s longer than the Nile.

The sedimentary axis of the Amazon basin comprises two distinct groups of landforms: the várzea, or floodplain of alluvium of Holocene age (i.e., up to about 11,700 years old), and the terra firme, or upland surfaces of Pliocene and Pleistocene materials (those from 11,700 to 5,300,000 years old) that lie well above the highest flood level. The floodplain of the main river is characteristically 12 to 30 miles (19 to 50 km) wide. It is bounded irregularly by low bluffs 20 to 100 feet (6 to 30 metres) high, beyond which the older, undulating upland extends both north and south to the horizon. Occasionally these bluffs are undercut by the river as it swings to and fro across the alluvium, producing the terra caída, or “fallen land,” so often described by Amazon travelers. At the city of Óbidos, Brazil, where the river width is some 1.25 miles (2 km), a low range of relatively hard rock narrows the otherwise broad floodplain.

The streams that rise in the ancient crystalline highlands are classified as either blackwater (Jari, Negro, and Tocantins-Araguaia) or clearwater (Trombetas, Xingu, and Tapajós). The blackwater tributaries have higher levels of humic acids (which cause their dark colour) and originate in nutrient-poor, often sandy uplands, so they carry little or no silt or dissolved solids. Clearwater tributaries have a higher mineral content and lower levels of humic acids. Some rivers flow as clearwater during the rainy season and blackwater during the dry season. Where such blackwater tributaries enter the main river, they are sometimes blocked off to form funnel-shaped freshwater lakes or estuaries, as at the mouth of the Tapajós.

The mother river, the Marañón above Iquitos, rises in the central Peruvian Andes at an elevation of 15,870 feet (4,840 metres) in a small lake in the Cordillera Huayhuash above Cerro de Pasco. The Huallaga and Ucayali, major right-bank affluents of the Marañón, originate considerably farther south. The headwaters of the deeply entrenched Apurímac and Urubamba, tributaries at the confluence of the Ucayali, reach to within 100 miles (160 km) of Lake Titicaca, the farthest of any stream in the system from the great river’s mouth.

The Madeira River, the second largest affluent of the Amazon, has a discharge of perhaps two-thirds that of the Negro. Silt from its turbid waters has choked its lower valley with sediments; where it joins the Amazon below Manaus, it has contributed to the formation of the 200-mile- (320-km-) long island of Tupinambarana. Beyond its first cataract, 600 miles (970 km) up the river, its three major affluents—the Madre de Dios, the Beni, and the Mamoré—provide access to the rubber-rich forests of the Bolivian Oriente; the meandering Purus and Juruá rivers that flank the Madeira on the west are also important tributaries that lead into those forests. Mamoré’s tributary, the Guaporé, opens up to the Mato Grosso Plateau.

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.

BRITANNICA DEMYSTIFIED
How Long Is the Amazon River?
According to some, it’s longer than the Nile.

At the Óbidos narrows, the flow of the river has been measured at 7,628,000 cubic feet (216,000 cubic metres) per second; its width is constricted to little more than a mile. Here the average depth of the channel below the mean watermark is more than 200 feet (60 metres), well below sea level; in most of the Brazilian part of the river its depth exceeds 150 feet (45 metres). Its gradient is extraordinarily slight. At the Peruvian border, some 2,000 miles (3,200 km) from the Atlantic, the elevation above sea level is less than 300 feet (90 metres). The maximum free width (without islands) of the river’s permanent bed is 8.5 miles (14 km), upstream from the mouth of the Xingu. During great floods, however, when the river completely fills the floodplain, it spreads out in a band some 35 miles (55 km) wide or more. The average velocity of the Amazon is about 1.5 miles per hour, a speed that increases considerably at flood time.

The rise and fall of the water is controlled by events external to the floodplain. The floods of the Amazon are not disasters but rather distinctive, anticipated events. Their marked regularity and the gradualness of the change in water level are due to the enormous size of the basin, the gentle gradient, and the great temporary storage capacity of both the floodplain and the estuaries of the river’s tributaries. The upper course of the Amazon has two annual floods, and the river is subject to the alternate influence of the tributaries that descend from the Peruvian Andes (where rains fall from October to January) and from the Ecuadoran Andes (where rains fall from March to July). This pattern of alternation disappears farther downstream, as the two seasons of high flow gradually merge into a single one. Thus, the rise of the river progresses slowly downstream in a gigantic wave from November to June, and then the waters recede until the end of October. The flood levels can reach from 40 to 50 feet (12 to 15 metres) above low river.

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.

The main influx of atmospheric water vapour into the basin comes from the east. About half of the precipitation that falls originates from the Atlantic Ocean; the other half comes from evapotranspiration from the tropical forest and associated convectional storms. Rainfall in the lowlands typically ranges from 60 to 120 inches (1,500 to 3,000 mm) annually in the central Amazon basin (e.g., Manaus). On the eastern and northwestern margins of the basin, rainfall occurs year-round, whereas in the central part there is a definite drier period, usually from June to November. Manaus has experienced as many as 60 consecutive days without rain. Moreover, in 2005 the Manaus region experienced a devastating drought, which caused parts of the river to dry up; this made transportation difficult, depleted drinking supplies, and left millions of rotting fish in the riverbed. Such extreme periods of drought are uncommon to the Manaus region, but fluctuations in the river’s level—thought to be related to climatic events and continued deforestation in the area—have continued to be of concern. The dry season is not sufficiently intense to arrest plant growth, but it may facilitate the onset and spread of fires, whether arsonous or natural. To the west the Andes form a natural barrier that prevents most of the water vapour from leaving the basin. The influence of mountains on rainfall is indicated by the high levels of precipitation in the upper piedmont and by the cloud-steeped Andean flanks, which feed the rivers that form a large part of the Amazon system. The highest amounts of precipitation, up to 140 inches (3,500 mm), are recorded in the upper Putumayo along the Colombian border.

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