River of the underworld in Greek mythology (5)
I believe the answer is:
lethe
(Other definitions for lethe that I've seen before include "Mythological river in Hades" , "In Greek myth, the river of forgetfulness flowing through Hades" , "where elephants never drink?" , "Classical river of forgetfulness" , "River whose waters caused the souls of the dead to forget completely their life on Earth" .)