Just to add to what’s been said, I heard that Jaques Cousteau, the french adventurer and scuba diver went to the bottom of the meeting of the waters and the water was still divided all the way to the bottom (a very long way down).

On the reason they are divided, I think I heard that it is pressure, and not just temperature, that makes them not mix, although I guess these two are related (inversely) anyway.

I remember seeing those pink/brown river dolphins there, it was very odd seeing dolphins in a river.

The amazon river was so amazing to me coming from Australia where we call a small trickle a river, or even something that only has water in it once every 8 years or so. It was so big that in parts you can’t see the other side (although not where I went to).

The comment on the southern cross rung a bell for me. I remember it struck me as odd seeing the southern cross in south america because I had always thought of it as a uniquely Australian thing (when of course it is a southern hemisphere thing) and it reminded me of home when I was almost as far away as you can get.