From Tolkien Gateway

Thalos was the second of the six rivers that rushed down from the Blue Mountains through Ossiriand to meet the great River Gelion, between the rivers Ascar and the Legolin.[1] It was in the high valley of its upper course that Bëor and his followers, the first Men to cross into Beleriand, made their camp, and where they were discovered by Elf King Finrod Felagund.[2]

The name comes from the Ilkorin word thalos ("torrent") used as a name.[3]


Seven Rivers of Ossiriand
Gelion · Ascar · Thalos · Legolin · Brilthor · Duilwen · Adurant