One of the world’s waterier wonders is this two-toned confluence of two massive Amazon tributaries.

About 10 kilometers outside of the Brazilian city of Manaus is the site known as the Meeting of Waters (Encontro das Águas). Here, the dark waters of the aptly-named Rio Negro encounter the sandy upper Amazon, known as the Rio Solimões. Although the rivers converge, they do not mix. Instead, they snake stubbornly alongside each other for nearly four miles, forming a polychrome boundary that is visible from space.

Geologist Robert Meade described the scale of the Meeting of Waters as “Six Mississippis worth of café-au-lait colored water […] converging here with two Mississippis worth of black-tea-colored water.” The coffee-like coloring of the Solimões is due to the various sediments (sand, mud, silt) it accrues running down from the Andes. In contrast, the Negro carries very little sediment – its dark, tea-like color is primarily caused by dissolved plant matter. It runs slower and is about 6 degrees Celsius warmer than the Solimões, whose swifter, cooler, denser waters rebuff the smaller river’s invitation, making it difficult for their flows to merge. 

When the two rivers eventually sync up and come together to form the impressive Lower Amazon, there's a good ways left to go. The River Sea, as it is aptly called, continues across the continent. And although it will encounter several more tributaries along its route to the Atlantic Ocean, the Meeting of Waters remains the encounter to see.