Ok, I found myself.
Originally the flop was called the turn and the turn was simply known as 4th street. Then the term flop came from the sound the dealer makes when he “flops” the first three cards. As this became colloquial, 4th street became known as the “turn”. While the river came from the Mississippi gambling riverboats on which poker was played in the 1800's. Sometimes cheaters would deal themselves an extra card after the hand was supposed to be complete. That would improve their own hand. If caught, they would often be thrown overboard, into the river.
Another version of river is one of the games on riverboats was called "up the river down the river". Final card became known as the river.
Or it came from "short stud". The seventh card was dealt facedown gave rise to its nickname, Down-the-River. Down-the-River was shortened to "the river".
If someone is interested... But it's all speculations.
Oh, did you know that poker started out as a game called poque (French) and that Southern US'ers on the riverboats were pronouncing it poh kuh? Another speculation...