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Amazon, Brazil

NileGuide Expert tip:

Seeing this from a boat is awesome enough, but from the air it is truly spectacular – helicopter and floatplane flights are available. Though pricey, they are worth it. On a killjoy note, on the Amazon winds and currents can be deceptive and storms and very large waves can spring up very quickly. It would be extremely unwise to think about piloting a boat yourself without a local boatman

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At least three of these natural phenomena occur in the Amazon basin. The other two are smaller and occur where the Rio Branco debouches into the Rio Negro (above Manaus) and at Santarém, where the Tapajos's clear waters enters the muddy ones of the Amazon. But the one just downriver from Manaus is the best-known, the largest and the most spectacular.  This is the meeting of two massive river systems, the Solimões and the Negro. It is where the Amazon river officially begins.

 

The Solimões has traveled from the Andes to reach this point, and its waters are heavy with volcanically-derived silt, eroded from these comparatively new mountains (many parts less than 5 million years old, and some freshly emerging). The Negro hasn't traveled as far, but it has flowed over the Roraima Formation, whose grantites, gneises and rosy pink sandstones are some of the oldest rocks on the planet. They've been around for nearly 3 billion years and any nutrients they had are long gone. What rocks remain are super-hard, and there is so little left top erode that the Negro carries only 3% of sediment load that the Solimões does. Plants growing along the banks of the Negro and its tributaries have to scavenge for life's essentials – and deter leaf-eating insects with chemical weapons - tannins. Once fallen, such leaves give up their tannins to the river water, coloring it a deep dark red. A cupful tastes like tea (because, minus the caffeine and plus a little bit of dolphin poo, that it pretty much what it is).

 

The upshot of all interaction between biology and geology is that the two rivers have waters that are very different temperatures (the near-black Rio Negro is warmer, of course), and densities (the Solimões is more soup-like) and general water chemistry. So great is the difference, that it takes over 40 km for the two to mix completely. Dive in from a paused boat and you can feel the difference in temperature and texture. For fish the difference is so great that they can be temporarily stunned in they cross from one water block to another – something the local dolphins know very well, and they congregate at this huge hydrological junction for an easy meal.


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