Which of these was the principal river of the underworld in Greek mythology?

When you cross the river styx, crossing from the world to the underworld, be sure to have payment for the ferryman Charon. This is where the practice of leaving coins on the eyes or in the hands of the deceased began.

Sassy, I saw Styx in concert twice back in the 70's/80's.....awesome!!

Just like the rock band styx