Aelin-uial

The twilit meres

South of the forests of Doriath, where the Aros came in from the east to meet Sirion, the great river slowed and spread out into a country of marsh, pools and reed-beds that the Sindar called Aelin-uial, the Meres of Twilight. The waters were still and the air was thick with mist, so that the place lay always in a half-light, and the Elves of Doriath held that the power of Melian reached out over the meres and made them a further guard of the Hidden Kingdom. Nothing lived there but birds, and no one crossed it without knowing the ways. Below the meres Sirion gathered itself again and rushed down over the Falls of Sirion, and then, at the Gates of Sirion, plunged into tunnels beneath the Andram and ran underground for a great distance before emerging on the far side of the Long Wall.

The edge of the Guarded Realm

The meres were the southern boundary of Thingol's protected land and the reason that Doriath could not be approached from the south by any army. Beyond them, across Sirion to the west, lay the country under the eyes of Nargothrond, and Nivrim, the strip of Doriath west of Sirion, ran down to the northern shore of the fens. The Narog country, Nan-tathren and the coast lay south past the Andram, reached by the road that ran down the western bank of Sirion. When Melian left Middle-earth after Thingol's death, her enchantment faded from the meres as from the forests, and the twilight fens became simply a hard country to cross. They went under the Sea with the rest of Beleriand at the end of the First Age.