In the Amazon in Brazil there are two great tributaries, the Rio Negro and the Rio Solimoes. The Rio Negro is a dark brown, because of all the dead vegetation that seeps into it from the rain forest. The Rio Solimoes is a milky tan color, because of all the sediment it has collected in the Andes mountains. Amazingly, these two bodies of water don't mix for 6 miles, because of their differing density, temperature, velocity and PH.
Notice how the pale and black water doesn't mix? Mother nature is a racist, segregation is bad.
(Even though I am clearly joking, I guarantee there will be people that get offended by this.)