Why is the Amazon River important? The river in numbers

The Amazon starts in the Andes of Peru, in high mountains far from any city or road a visitor would normally use. In NASA's telling, the river is born in the Peruvian Andes as small mountain streams that merge into one of the mightiest rivers anywhere.³ The treaty organization agrees that the journey begins at a source in Peru.¹ Which stream counts as the source, though, is a real question.

In 2007 a team of Brazilian researchers challenged the older belief that the river begins in northern Peru.¹² The team claimed the river originated instead in southern Peru, somewhere on snowcapped Mount Mismi, also called Nevado Mismi.¹³ The distinction matters because the choice of source changes the river's length, which brings back the old debate with the Nile, and shows how a question that sounds simple can depend on a patient field expedition.

For a traveler the debate is mostly a curiosity, since no visitor from Manaus goes near the source. It still helps to know that any length you hear on a boat is an estimate with a story behind it. A guide who tells you the length is exact is rounding, and the honest version is that length depends on identifying the source, a point that never fails to impress a quiet table of listeners.

The Amazon is not a single river of a single color. It changes name, color and character as it flows, and Manaus is where that change is easiest to see. At Manaus the Solimões, the upper Amazon, is joined by the Negro, and NASA treats that junction as the birthplace of the Amazon.¹⁰ Upstream of that meeting the main stem is called the Solimões in Brazil, which is why Manaus guides treat the Solimões and the Negro as separate rivers until the confluence.¹⁸

Its water changes character along the way, which is the first thing a visitor on the Manaus waterfront notices. The Solimões gets its tan color from Andean sediment and is the shallower of the pair, having silted up its own valley.^19,20 The Negro, by contrast, is stained black-tea brown by organic matter from the forest floor.²¹ NASA's confluence image shows the Negro almost 20 km wide in one corner, wider than most visitors expect of a river.²²

It swells and shrinks with the seasons, and the change is large enough to redraw the shoreline. The flood peaks from May to July, when the water climbs several meters, according to NASA.¹⁶ The gauge at the Manaus port has logged anything from 13.63 meters to 29.77 meters over the years.^23,24 A river with that range changes the landscape twice a year, and it is one reason the same tour looks different on different dates.

The river reaches the Atlantic Ocean in Brazil, as the treaty organization puts it, after crossing the whole width of the continent.¹ NASA reports millions of cubic feet of water entering the Atlantic each second, with the outflow carried a very long way from the shore.¹¹ The mouth is so wide and the flow so strong that river water reaches far out to sea, which is how a river can leave its mark on the ocean it joins.

The mouth is also where the river's reputation ends and its scale becomes obvious. The same NASA page grants that the Nile is longer, but the Amazon moves the most fresh water of any river, nearly 20 percent of Earth's discharge into the oceans.² The comparison is simple: a longer river does not mean a bigger river, and the Amazon is the clearest example, with the Nile the usual counterpoint.

A short boat trip from Manaus will not take you to the mouth, which lies far downstream, but a ferry of 36 hours reaches Santarém while Belém is a 5-day journey by local boat, both listed from Manaus as transportation rather than sightseeing. For most visitors, the mouth stays an idea, and the Meeting of the Waters at Manaus is the place where the river becomes real, close enough to touch and easy to reach in a single day.