Two Amazon rivers run side by side for 6km without mixing

At Manaus, Brazil, the dark, warm, slow-moving Rio Negro meets the pale, cooler, faster Rio Solimões, and for about 6km the two waters flow next to each other in the same channel with a visible boundary between them. Differences in temperature, speed and sediment keep them apart until roughly 60km downstream, where they finally blend into what's then called the Amazon River.

— Wikipedia contributors, Meeting of Waters — Rio Negro / Rio Solimões confluence, Manaus