Eunoe is a feature of Dante's Commedia created by Dante as the fifth river of the dead (taking into consideration that Cocytus was described as a lake rather than a river). Penitents reaching the Garden of Eden at the top of Mount Purgatory are first washed in the waters of the river Lethe in order to forget the memories of their mortal sins. They then pass through Eunoe to have the memories of their good deeds in life strengthened.
Upon completing his or her sentence in Purgatory, a soul is washed in the rivers Lethe and Eunoe (in that order) by Matelda. It is unclear who Matelda was in real life, but, nonetheless, her function is to cause the penitent to forget his or her sins (now that these sins have been purgated) and then sip from the waters of Eunoe so that the soul may enter heaven full of the strength of his or her life's good deeds.
Dante makes particular reference to the dolce ber or "sweet drink/draught" of Eunoe at the end of Purgatorio when he explains that he wished he had greater space to write of the water that "never would have sated me." (trans. C.S. Singleton)
Other mythological rivers borrowed by Dante from Greek lore:
Eunoë is also the name of a nymph reported in Greek mythology; Eunoë