Acheron, river in Thesprotía in Epirus, Greece, that was thought in ancient times to go to Hades because it flowed through dark gorges and went underground in several places; an oracle of the dead was located on its bank. In Greek mythology it is a river in Hades, and the name sometimes refers to the lower world generally. Several other rivers in Greece are also called Acheron, which traditionally means River of Woe.