Aelin-uial, or the Meres of Twilight, was a region of pools and marshes where the river Aros flowed into Sirion in the south-west of Doriath.
Location
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.
Description
The Aelin-uial was under the control of Doriath. Thingol's marchwardens kept hidden ferries among the reeds, which they could use to cross in between Sirion's eastern and western banks unseen by anyone.
It was here that Turgon and Finrod slept and Ulmo put into their dreams the idea of building their hidden strongholds located at Gondolin and Nargothrond.
Near north of here, Morwen stopped in her travel to Nargothrond, and Mablung and his men showed themselves to her, as they had been following her in secret by order of Thingol. She refused the offer of coming back to Doriath and accepted them as escort in her travel. Then Mablung lead her to the Pools and took the hidden rafts used by the messengers between Doriath and Nargothrond, and they crossed the Aelin-uial in the white mists of the night.
It was also in the meres that the Elves who guarded the western marches of Doriath found Húrin, whom they then brought before King Thingol.