What causes Brazil's 'Meeting of the Waters'?
Image: Danocoo1/Reddit.com

Almost 10 kilometres from the inland city of Manaus in northern Brazil, ‘the Meeting of the Waters’ is the point where two of Amazon River’s largest tributaries - a smaller river that flows into a bigger ‘parent’ river - converge but never mix.

The Solimões River forms the lighter half, its ‘cafe au lait’ colouring owed to the rich sediment that runs down from the Andes Mountains, including sand, mud and silt. Known as a ‘white water river’, the Solimões River stretches over a 1600 km distance. 

The darker side is the Rio Negro, and it gets its ‘black tea’ hue from leaf and plant matter that has decayed and dissolved in the water. It might look dark and murky, but the Rio Negro carries little or no sediment, and according to the European Space Agency website, is considered one of the cleanest natural waters in the world. On really clear days, water visibility in this black water river can exceed nine metres. 

If this really was milky coffee and black tea, the Meeting of the Waters would require the worst’s most humungous cup. Robert Meade, who spent decades studying these rivers for the U.S. Geological Survey, told Nasa’s Space Observatory website, “Put in terms of the sheer quantities of water, what we are seeing here is a volume of water at least a dozen times greater than the total of the water falling over the Niagara, Iguassu, and Victoria Falls combined.”

The Solimões River and Rio Negro flow side-by-side over distance of six kilometres. The reason they never mix is because of the stark differences in temperature, speed and water density between the two. The Solimões is faster, cooler and denser, its waters flowing up to 6 km/h at 22 degrees Celsius, and the warmer, slower waters of the Rio Negro flow at a more leisurely 2 km/h, and maintain a temperature of around 28 degrees Celsius.

After several kilometres, the two rivers eventually converge thanks to a blast of fast-moving whitewater, and become part of the Lower Amazon River.

Here's a video of the Meeting of the Waters: