Why is the final card called the "river"?

anyone having knowledge on it

It's named after the river of Hades

It seem like this is historical.

In the 19. century it was a normal punishment for caught cheaters at the last card dealt to be thrown into the Mississippi (when played on the boat).
There may be other explanations.
This one is the funniest.

The Evolution of the Term Rive.
Some historians believe the name originated from the old English term rive, which meant to split or to break apart. Over time, rive may have evolved phonetically into river. However, there is less historical evidence supporting this theory.