Aros.

The river came from two wells in the Southeastern hills and mountains of Dorthonion. There was one ford of the river; Arosslach. It served as border for 80 miles between Himlad and Dor-Dinen before it meet the Northern end of Neldoreth, Region and Doriath and as border between Doriath and Himlad for 125 miles South until it met river Celon and turned West, folloving the edge of Region 150 miles to the junktion with Sirion and the Fens of Sirion; Aelin-Uial.