Oelinuial

The Aelin-uial was the maze of lakes and fens formed where the river Aros met the Sirion, before it fell beneath the Andram. These were lands heavy with enchantment from the Girdle of Melian guarding Doriath to the north of the Aelin-uial, and also from the power of Ulmo the Vala, which was very strong in this area.

At the southernmost end of the Aelin-uial, the waters of Sirion met once more at the Falls of Sirion.

Aelin-uial is a Sindarin name, composed of aelin "meres", plural of ael + uial "twilight".

A variant name used by Tolkien was Umboth Muilin (also spelt Umboth-muilin and Umboth-Muilin), which translates to "Pools of Twilight". The Noldorin name Oelinuial ("Pools of Twilight"; variant forms Elinuial and Eilinuial), of the Etymologies, became Aelin-uial in the published Silmarillion.

From Tolkien Gateway.