Amazon Rainforest Tours and Travel | The Tributaries of the Amazon
The Amazon is a river of many rivers stretching almost coast to coast across the northern regions of South America. The seventeen longest of the Amazon's 1,100 tributaries flow more than a thousand miles. Two of its tributaries, the Negro and the Madeira, are the fifth and sixth largest rivers in the world by volume.
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The Amazon and its tributaries are divided into two segments. The Upper Amazon is comprised of the tributaries starting in Colombia, Venezuela, Ecuador, Peru, Bolivia, and Brazil that flow into the Solimoes River. The Lower Amazon includes the tributaries east of Manaus and the main channel the rest of the way to the Atlantic.
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Just east of Iquitos, Peru, the north-flowing Maranon and Ucayali rivers merge. It is here at their confluence that the Amazon technically begins and takes the name. Where the borders of Peru, Colombia and Brazil meet, the river's name then switches to Solimoes until its confluence with the Negro River, after which it regains the name Amazon.
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The river basin has two fresh water systems, whitewater (or varzea) and blackwater (or igapo), so named for the color of their water which varies according to geology and chemical composition.
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People see photos of the murky Amazon and think pollution when what they are actually seeing is sediment. The Amazon's whitewater tributaries originate in the Andes where nutrient-rich sediments wash away. The churning of sediment makes these tributaries appear white, though most carry so much of this material that the rivers are actually milky brown in color. The Amazon whitewater tributaries are neutral to slightly acidic and periodically flood. These include the Madre de Dios, Beni, Jutai, Madeira, and Napo rivers. The Solimoes and the Amazon itself are whitewater rivers.
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The blackwater tributaries are slow-moving, dark brown, very clear and very acidic, making them inhospitable to parasites, bacteria and insect larva. This is a plus for visitors since it means that mosquitoes in and around these tributaries are less of a nuisance. On the other hand, the lower diversity of trees results in a lower diversity of rainforest wildlife. This is not true of fish species. Having adapted to the conditions, they are quite diverse in these rivers and easy to see because of the water's clarity. The rainforest areas surrounding blackwater rivers are regularly inundated during flood season.
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Blackwater rivers have a low pH and nutrient content because the river beds are composed of ancient rock that no longer decomposes and because tannin is leached from decaying floodplain vegetation due to almost constant flooding. Though found elsewhere, most of the world's blackwater rivers are Amazon tributaries. Among these blackwater tributaries are the Negro and Urubu as well as the Yarapo River near Iquitos, Peru. A few Amazon tributaries are clearwater rivers, with an even lower pH than blackwater rivers. These include the Tapajos River in Brazil, the Trombetas River flowing from Venezuela and the Xingu River from the south, all east of Manaus.
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The black and whitewater systems meet near Manaus, Brazil, and flow side by side for several miles described locally as "the meeting of the waters." This phenomenon makes an interesting day-trip for visitors on Amazon tours in Manaus.
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