Possible Clues:

• River in Dante's 'Inferno'
• Underworld river in Greek mythology
• Mythical river of forgetfulness?
• River of forgetfulness flowing through Hades
• River of forgetfulness, in myth
• Mythical Hades River of Forgetfulness
• Offered by Ruth Rendell to reveal secrets of a long-running soap?
• In Greek mythology, a river in Hades that caused forgetfulness in those who drank its waters
• Mythological river
• Infernal river