The river Lethe was associated with Lethe a goddess of forgetfulness. Making her a daughter of Eris or Oceanus.
Hades
Lethe
The Lethe River ran through the plain of Lethe in Hades. Also known as the stream of oblivion, the river flowed around the cave of Hypnos where the murmuring waters caused drowsiness. The shades of the dead were required to drink the water in order to forget their earthly life. Lethe is frequently used as a metaphor for the Underworld in general.
Hades
the Acheron (river of woe), the Cocytus (river of lamentation), the Phlegethon (river of fire), the Styc (river of unbreakable oath by which the gods took vows), and the Lethe (river of forgetfulness).
In Greek Mythology, Lethe is the Underworld river of oblivion/forgetfulness.
It is called Lethe.
Hades is a term that can be applied to the Underworld (the abode of the dead) or as a name of the God of the Underworld. There are five rivers in the Underworld, but the most well known is the river Styx. The other four are Acheron, Cocytus, Lethe, and Phlegethon.
The River Lethe is the river of forgetfulness and is one of the five rivers of Hades. It was believed that the newly dead who drank from the River Lethe would lose all memory of their past existence. The other four rivers of the underworld are Acheron, Cocytus, Phlegethon, and Styx. Acheron, which means "river of woe", is the river used by the old ferryman Charon would transport the dead from the land of the living to the realm of Hades. Cocytus means "river of lamentation". The people who were not properly buried were doomed to wander the banks of this river for most of their afterlife. Phlegethon was the river of fire. Although the fire burned, it did not consume anything with its flames. Styx means "hate". This was the river that separate the realm of the living from that of the dead. Many accounts say it wraps around Hades nine times.
Lethargy is an extreme drowsiness or lassitude. The word comes from the Greek lethe, "forgetfulness." In literature, Lethe is the name of a River in Hades, the Waters of Oblivion, which may account for the word's modern sense of a deathlike inertia.
The Lethe, from which we get the word "lethargic". Anyone who was blessed with eternity in the Elysian Fields would drink from the Lethe to forget their earthly woes.
The word "lethargic" comes from Lethe, a river in the Greek Underworld, Tartarus (or Hades), whose water, when drunk, caused forgetfulness of the past.