The Amazon River of South America is the largest river in the world with a total river flow greater than the next ten largest rivers combined. The Amazon, which has the largest drainage basin in the world, accounts for approximately one-fifth of the world's total river flow.

In its upper stretches the Amazon river is called Apurímac (in Peru) and Solimões (in Brazil).

During the wet season, parts of the Amazon exceed 190 kilometres (120 mi) in width. Because of its vast dimensions, it is sometimes called The River Sea. At no point is the Amazon crossed by bridges. This is not because of its huge dimensions; in fact, for most of its length, the Amazon's width is well within the capability of modern engineers to bridge. However, the bulk of the river flows through tropical rainforest, where there are few roads and even fewer cities, so there is no need for crossings.

While the Amazon is the largest river in the world by most measures, the current consensus within the geographic community holds that the Amazon is the second longest river, just slightly shorter than the Nile.

Drainage area

The Amazon Basin, the largest drainage basin in the world, covers about 40 percent of South America, an area of approximately 7,050,000 square kilometres (2,720,000 sq mi). It gathers its waters from 5 degrees north latitude to 20 degrees south latitude. Its most remote sources are found on the inter-Andean plateau, just a short distance from the Pacific Ocean.

The Amazon River and its tributaries more than triples over the course of a year. In an average dry season, 110,000 square kilometres (42,000 sq mi) of land are water-covered, while in the wet season, the flooded area of the Amazon Basin rises to 350,000 square kilometres (140,000 sq mi).

The quantity of water released by the Amazon to the Atlantic Ocean is enormous: up to 300,000 cubic metres (11,000,000 cu ft) per second in the rainy season, with an average of 209 000 m³/s from 1973 to 1990. The Amazon is responsible for about 20% of the total volume of freshwater entering the ocean. Potable water can be drawn from the ocean while still out of sight of the coastline, and the salinity of the ocean is notably lower 500 kilometres (310 mi) out to sea.

There is a natural water union between the Amazon and the Orinoco basins, the so-called Casiquiare canal. Actually the Casiquiare is a river distributary of the upper Orinoco, which flows southward into the Rio Negro, which in turn flows into the Amazon. The Casiquiare is the largest river on the planet that links two major river systems, a so-called bifurcation.

Origins

The Amazon river has a series of major river systems in Peru and Ecuador, some of which flow into the Marañón and Ucayali, others directly into the Amazon proper. Among others, these include the following rivers: Morona, Pastaza, Nucuray, Urituyacu, Chambira, Tigre, Nanay, Napo, and Huallaga.

The most distant source of the Amazon was firmly established in 1996, 2001, and 2007 as a glacial stream on a snowcapped 5,597 m (18,363 ft) peak called Nevado Mismi in the Peruvian Andes, roughly 160 km (99 mi) west of Lake Titicaca and 700 km (430 mi) southeast of Lima. The waters from Nevado Mismi flow into the Quebradas Carhuasanta and Apacheta, which flow into the Río Apurímac which is a tributary of the Ucayali which later joins the Marañón to form the Amazon proper. (While this is the point at which most geographers place the beginning of the Amazon proper, in Brazil the river is known at this point as the Solimões das Águas.) Soon thereafter the darkly colored waters of the Rio Negro meet the sandy colored Rio Solimões, and for over 6 km (4 mi) these waters run side by side without mixing.

After the confluence of Río Apurímac and Ucayali, the river leaves Andean terrain and is instead surrounded by floodplain. From this point to`the Marañón, some 1,600 km (990 mi) , the forested banks are just out of water, and are inundated long before the river attains its maximum flood-line. The low river banks are interrupted by only a few hills, and the river enters the enormous Amazon Rainforest.

The river systems and flood plains in Brazil, Peru, Ecuador, Colombia and Venezuela whose waters drain into the Solimões and its tributaries are called the "Upper Amazon". The Amazon River proper runs mostly through Brazil and Peru, and it has tributaries reaching into Venezuela, Colombia, Ecuador, and Bolivia.

Flooding

Not all of the Amazon's tributaries flood at the same time of the year. Many branches begin flooding in November, and may continue to rise until June. The rise of the Rio Negro starts in February or March, and it also begins to recede in June. The Madeira rises and falls two months earlier than most of the rest of the Amazon.

The average depth of the river in the height of the rainy season is 40 metres (130 ft) and the average width can be nearly 40 km (25 mi).

The main river (which is between approximately one and six miles (10 km) wide) is navigable for large ocean steamers to Manaus, 1,500 kilometres (930 mi) upriver from the mouth. Smaller ocean vessels of 3,000 tons or 9,000 tons and 5.5 metres (18 ft) draft can reach as far as Iquitos, Peru, 3,600 kilometres (2,200 mi) from the sea. Smaller riverboats can reach 780 kilometres (480 mi) higher as far as Achual Point. Beyond that, small boats frequently ascend to the Pongo de Manseriche, just above Achual Point.

Geography

At some points, for long distances, the river divides into two main streams with inland and lateral channels, all connected by a complicated system of natural canals, cutting the low, flat igapó lands, which are never more than 5 metres (16 ft) above low river, into many islands.

From the town of Canaria at the great bend of the Amazon to the Negro, only very low land is found, resembling that at the mouth of the river. Vast areas of land in this region are submerged at high water, above which only the upper part of the trees of the sombre forests appear. Near the mouth of the Rio Negro to Serpa, nearly opposite the river Madeira, the banks of the Amazon are low, until approaching Manaus, they rise to become rolling hills. At Óbidos, a bluff 17 m (56 ft) above the river is backed by low hills. The lower Amazon seems to have once been a gulf of the Atlantic Ocean, the waters of which washed the cliffs near Óbidos.

Only about 10% of the water discharged by the Amazon enters the mighty stream downstream of Óbidos, very little of which is from the northern slope of the valley. The drainage area of the Amazon Basin above Óbidos city is about 5 million square kilometres (2,000,000 sq mi), and, below, only about 1 million square kilometres (400,000 sq mi or around 20%), exclusive of the 1.4 million square kilometres (540,000 sq mi) of the Tocantins basin.

In the lower reaches of the river, the north bank consists of a series of steep, table-topped hills extending for about 240 kilometres (150 mi) from opposite the mouth of the Xingu as far as Monte Alegre. These hills are cut down to a kind of terrace which lies between them and the river.

On the south bank, above the Xingu, an almost-unbroken line of low bluffs bordering the flood-plain extends nearly to Santarém, in a series of gentle curves before they bend to the south-west, and, abutting upon the lower Tapajós, merge into the bluffs which form the terrace margin of the Tapajós river valley.

Mouth

The definition of what exactly and how wide is the mouth of the Amazon is a matter of dispute, because of the area's peculiar geography. Most particularly, sometimes the Pará River is included, whereas sometimes it is just considered the independent lower reach of the Tocantins River. The Pará river estuary alone is 60 km (37 mi) wide. The Pará and the Amazon are connected by a series of river channels called furos near the town of Breves; between them lies Marajó, an island almost the size of Switzerland that is the world's largest combined river/sea island.

If the Pará river and the Marajó island ocean frontage are included, the Amazon estuary is some 330 kilometres (210 mi) wide. In this case, the width of the mouth of the river is usually measured from Cabo Norte, in the Brazilian state of Amapá, to Ponta da Tijoca near the town of Curuçá, in the state of Pará. By this criterion, the Amazon is wider at its mouth than the entire length of the River Thames in England.

A more conservative measurement excluding the Pará river estuary, from the mouth of the Araguari River to Ponta do Navio on the northern coast of Marajó, would still give the mouth of the Amazon a width of over 180 kilometres (110 mi). If only the river's main channel is considered, between the islands of Curuá (state of Amapá) and Jurupari (state of Pará), the width falls to just about 15 kilometres (9.3 mi) - but that is still impressive for any river. It also has a 13% dumpage rate into the basin.

Tidal bore (pororoca)

The tension between the river's strong push and the Atlantic tides causes a phenomenon called a tidal bore, a powerful tidal wave that flows rapidly inland from the sea up the Amazon mouth and nearby coastal rivers several times a year at high tide. Tidal bores also occur in other river mouths around the world, but the Amazon's are among the world's highest and fastest, probably second only to those of Qiantang River in China. In the Amazon, the phenomenon is locally known as the pororoca.

The pororoca occurs especially where depths do not exceed 7 metres (23 ft). It starts with a very loud roar, constantly increasing, and advances at the rate of 15–25 km/h (9–16 mph), with a breaking wall of water 1.5–4.0-metres (5–13 ft) high that may travel violently several kilometres up the Amazon and other rivers close to its mouth. It is particularly intense in the rivers of the coast of the state of Amapá north of the mouth of the Amazon, such as the Araguari River, but can be observed in Pará rivers as well.

The bore is the reason the Amazon does not have a protruding delta; the ocean rapidly carries away the vast volume of silt carried by the Amazon, making it impossible for a delta to grow past the shoreline. The region also has very high tides, sometimes reaching 6 metres (20 ft) and has become a popular spot for river surfing.

Wildlife

More than one-third of all species in the world live in the Amazon Rainforest, a giant tropical forest and river basin with an area that stretches more than 5.4 million square kilometres (2.1 million sq mi), and is the richest tropical forest in the world. The Amazon River has over 3,000 recognized species of fish and that number is still growing.

Along with the Orinoco, the river is one of the main habitats of the boto, also known as the Amazon River Dolphin (Inia geoffrensis). It is the largest species of river dolphin, and it can grow to lengths of up to 2.6 metres (8.5 ft). The boto is the subject of a very famous legend in Brazil about a dolphin that turns into a man and seduces maidens by the riverside. The tucuxi (Sotalia fluviatilis), also a dolphin species, is found both in the rivers of the Amazon Basin and in the coastal waters of South America.

The Amazonian Manatee is another conspicuous mammal living in the waters of the Amazonian basin.

Also present in large numbers is the notorious piranha, a carnivorous fish which congregates in large schools, and may attack livestock and even humans. However, only a few of its species are known to attack humans, most notably Pygocentrus nattereri, the Red-bellied Piranha.

The bull shark (Carcharhinus leucas) has been reported 4,000 kilometres (2,500 mi) up the Amazon River at Iquitos in Peru. The arapaima, known in Brazil as the pirarucu (Arapaima gigas), is a South American tropical freshwater fish. It is one of the largest freshwater fish in the world, reportedly with a maximum length in excess of 4.5 metres (15 ft) and weight up to 200 kilograms (440 lb). Another Amazonian freshwater fish is the arowana (or aruanã in Portuguese), such as the Silver arowana (Osteoglossum bicirrhosum), which is also a predator and very similar to the arapaima, but only reaches a length of maximum 120 centimetres (47 in). The candirú are a number of genera of parasitic, freshwater catfish in the family Trichomycteridae; all are native to the Amazon River. They sometimes attack humans and have been known to enter the urethras of bathers. The electric eel (Electrophorus electricus) is also found in the river.

The anaconda snake is found in shallow waters in the Amazon Basin. One of the world's largest species of snake, the anaconda spends most of its time in the water, with just its nostrils above the surface. In addition to the thousands of species of fish, the river supports crabs, algae, and turtles.

credited to wikipedia and flickr: balsa