A river in southern Doriath (S/121). The meaning of this name is unclear.

Conceptual Development: This name dates back to the earliest Lost Tales, where the name G. Aros appeared (LT2/236). In The Etymologies from the 1930s, Tolkien designated this name as Ilkorin Aros, related to the adjective Ilk. arn “red” due to the colour of its waters (Ety/YAR). Since Tolkien abandoned the Ilkorin language, it is unclear what the meaning of this name would have been in his later writings.

A river in Doriath described as “a river with reddish water” (Ety/YAR). This name seems to include a variant of arn “red” or perhaps the root ᴹ√YAR, combined with the suffix -os “land” also seen in thalos.

See S. Aros for further discussion.