The area was first mined by the Iberians and Tartessians in the third millennium B.C., and it remains a viable source of copper even today. Over time, it acquired its characteristic red color from heavy metals that crept in as a side effect of mining. The color's not all that trickled in, either. The river is actually even more lethal than its uninviting appearance suggests. The pH of the water hovers around 2, placing it on par with stomach acid. Yes, Rio Tinto could literally digest you.
Being the stubborn bastard that it is, life has still somehow found a way to persevere in the hellish environment of Rio Tinto. Anaerobic extremophile bacteria have been found to thrive in its waters, the conditions of which are so out of this world that NASA is collecting samples of these bacteria to simulate searching for life on other planets. Meanwhile, we now know what it would look like if the Earth was to get into a knife fight.