The word ‘Amazon’ evokes images of a lush rainforest and a large river in South America. Well, at least for some us. Recent public surveys in the U.S. show that in the last decade, and mainly among younger people, the word ‘Amazon’ evokes the giant internet retailer (amazon.com). At any rate, the word ‘Amazon’ implies superlatives; a region where some of its components are the world’s largest or biggest. The superlatives also apply on the other direction. The Amazon region is the least known and the least understood in many aspects of its natural history. In the following paragraphs I will expose some interesting facts about the Amazon River.
Having grown up in the city of Lima Peru, and seeing what the east and west sides of the Andes look like, it came to me as a big surprise learning that the Amazon River (at some point of its geological history) flowed into the Pacific rather than the Atlantic Ocean. Interestingly, the western side of the Andes is technically a desert. The west slope receives only seasonal rains that support Andean scrub vegetation and pockets of dry Andean forest. The east slope of the Andes could not be more contrasting from its western counterpart. The east slope is a lush moss, bromeliad, orchid-laden, and extremely wet forest. The lowlands constitute an extension of the east slope, but not as wet as the latter.
The Andes divide most of South America into two contrasting life zones.
But this is not the way it used to be 65 million years ago. The Amazon basin has not always been an area of lush tropical rainforest. At several times during its history, the basin has been the location of huge lakes and shallow seas. Salt deposits up to 600 meters thick (nearly 2000 feet!) have been found in some locations, indicating that the lowlands east of the Andes, at one time, may have been desert-like, drying up the shallow seas and creating the salt deposits. As for the switch from flowing into the Pacific Ocean, a series of geological events cut the Amazon River’s flow to the west from flowing into the Pacific, and forced the Amazon River to flow eastward. These changes took place when the westward-moving South American tectonic plate crashed headlong into the eastern-moving Nazca Plate. The Nazca plate was forced beneath the South American plate, lifting up the Andes Mountains in a process that continues to this day, as evidenced by the many earthquakes and high volcanic activity of the Andes region.
The Andes of today bisect South America into east and west watersheds with the west slope flowing westward to the Pacific and the east flowing eastward to the Atlantic Ocean. Most of the east side of the Andes constitutes the Amazon basin or watershed of the Amazon river. This is by far the world’s biggest watershed. The Amazon basin is 7,050,000 square km in area (or about 2,500,000 square miles), and covers about 40% of South America. Of this area, approximately 5,000,000 square km is covered by high tropical rainforest, with the remainder covered by savannah (“campo”) or scrubby woodland (“cerrado”). The Amazon basin covers significant portions of the countries of Brazil, Peru, Colombia, Ecuador, Venezuela and Bolivia, though the major part of the watershed lies within Brazil. The next largest tropical watershed, the Congo River, at 3,690,000 square km, is only half the size of the Amazon basin. This means that the Amazon collects water from a huge area which is discharged into the Atlantic Ocean. The average discharge of water into the Atlantic Ocean by the Amazon is approximately 1/5th and 1/6th of the total discharge into the oceans of all of the world’s rivers! This discharge is 4-5 times that of the Congo River (the second largest in ocean discharge), and 10 times that of the Mississippi. The Rio Negro, a tributary of the Amazon, is the second largest river in the world in terms of water flow, and is 100 meters (over 300 feet) deep and 14 kilometers (~9 miles) wide near its mouth at Manaus, Brazil. About 1,100 other sizeable tributaries empty into the Amazon River.
The length of the Amazon River is a superlative that is in dispute. Its total length from its source springs in the Andes (taking the Ucayali River as the continuation of the main river into the Andes), is estimated at 6518 km ( ~4075 miles) (not including all river bends, and measuring the short distance around Marajó Island in the mouth of the Amazon). This is exceeded only by the Nile River (including the Kagera River) of Africa with a total length of 6671 km (4170 miles). If you measure the long-way around Marajo Island (the world’s largest freshwater island), however, the Amazon is slightly longer than the Nile itself The Amazon headwaters are located high in the Andes at an elevation of about 5,200 meters (17,000 feet), and only 190 kilometers (120 miles) from the Pacific Ocean. Moreover, two of the tributaries of the Amazon, the Juruá and the Madeira Rivers, are both over 3,300 km (2,060 miles) long.
The width of the Amazon at Iquitos, Peru (3,600 km/2,250 miles from the ocean) is about 2 km. Ocean-going ships can easily access the Port of Iquitos at high water, as the mean depth of the current-canal of the Amazon is between 40 and 50 m (or up to 150+ feet deep), and in places, over 100 m (over 300 feet) deep. Even hundreds of miles away from the ocean, sections of the bottom of the river channel actually lie below
The width of the Amazon River in Iquitos-Peru is about 2 km. The river increases in width eastward
sea level! Interestingly, the Amazon River bed has its own geography. The river bed below the mouth of the Rio Negro show giant sand dunes as long as 600 meters (2000 feet) and up to 12 meters (39 feet) in height. These dunes are gradually moved downstream in the same manner that wind moves sand-dunes in deserts.
The mouth of the Amazon is over 320 km wide (approximately 200 miles), and contains the world’s largest freshwater island, Marajó Island, with an area of 48,000 square km (about the size of Switzerland). In the Atlantic Ocean beyond the mouth of the Amazon, and resting on the continental shelf, the Amazon sediment cone has a length of about 680 km and a width of 250 km. These fine grained deposits (mostly clay/mud particles) on the ocean floor are over 11,000 meters (36,000 feet) thick. This is mostly sediment that has been carried downriver from the Andes Mountains, the Guianan Shield (to the North) and the Brazilian Shield (to the south), by the river current, and which settled out of the water column once the river current dissipated into the waters of the Atlantic Ocean.
Average rainfall across the whole Amazon basin is approximately 2,300 mm (or ~7.5′) annually. In some areas of the northwest portion of the Amazon basin, yearly rainfall can exceed 6,000 mm (almost 20′)!
The mouth of the Amazon splits into multiple chanels expanding an area of approximately 300 km.
That much water and endless aquatic habitat has resulted in many species of fish often specialized in specific water types and watersheds. The Amazon basin is home to over 2,500 species of fish, more species than are recorded for the entire Atlantic Ocean, and some experts estimate that there may be as many as 6,000 species! These range from giant 3-meter air-breathing fish (Arapaima gigas) and river catfish weighing up to 600-700 lbs, to tiny tetras, electric eels, sting-rays, needlefish, fresh-water flying-fish, and knife-fish. As it is the case with the Amazon, the fish fauna of many river systems is poorly known, and new species are discovered yearly, even in the “better-known” areas. The fishes, insects and plants of the Amazon are among the least known aspects of this tropical paradise.
There is indeed a lot to learn about the Amazon not only about animals, but also the local people who
interact and depend on the rainforest for their daily lives. Our team of naturalists first focused on the animals and their interaction with the rainforest. The focus has been refined to include, perhaps the most important element, the humans that live in it. If the animals and the rainforest were complicated and difficult to understand, including the human component makes it even more complicated…but the challenge is exciting and the goals rewarding.
Alfredo Begazo is a member of the team of naturalists with Amazon Voyages.
The Amazon. Limnology and Landscape Ecology of a Mighty Tropical River and its Basin. (1984) H. Sioli, editor. Dr. W. Junk Publishers, Dordrecht (ISBN 90-6193-108-8).
The Palms of the Amazon. (1995) A. Henderson. Oxford University Press, Oxford, 362 pages. (ISBN 0-19-508311-3).
Field Guide to the Palms of the Americas (1995; A. Henderson, G. Galeano and R. Bernal; Princeton University Press, Princeton, New Jersey; ISBN 0-691-08537-4)
A Neotropical Companion. 2nd Ed. (1997) J. C. Kricher. Princeton University Press, Princeton, New Jersey, 451 pages. (ISBN 0-691-04433-3