Afros and Gruir were two rivers in Tol Eressëa, joining at the bridge of Tavrobel, according to the early version of the legendarium in The Book of Lost Tales. The Tower of Tavrobel was located near them.
The Battle of the Heath of the Sky-roof, which took place during the Faring Forth, and ended in complete disaster for the Elves of Tol Eressëa, took place near Tavrobel.
Following the battle, the remaining Elves, and in one outline, Eriol, fled across the rivers Afros and Gruir in order to escape the invading Men.