Which underworld river is called the river of woe?

The underworld river called the river of woe is Acheron.

Acheron is one of the five rivers associated with the Greek underworld. Its name is traditionally connected with sorrow or woe, matching its role as a gloomy boundary between the world of the living and the realm of the dead. In many classical accounts, newly dead souls crossed Acheron with Charon as their ferryman.

The river’s mythology varies across authors. Homer describes Acheron as a river of Hades into which Cocytus and Phlegethon flow. Pausanias and several later Greek writers also place Charon on Acheron. Roman poets sometimes assign Charon’s crossing to the Styx instead, which explains why the two rivers are frequently confused in modern retellings.

Acheron was not purely imaginary to ancient audiences: a real river in Epirus, northwestern Greece, was identified with the mythic waterway. That river is about 52 kilometres long and reaches the Ionian Sea. Cocytus means the river of lamentation, Lethe is linked with forgetfulness, and Styx is associated with an oath-binding sacred river.