§ Of the Coming of the Elves and the Captivity of Melkor The Silmarillion

In the changes of the world the shapes of lands and of seas have been broken and remade; rivers have not kept their courses, neither have mountains remained steadfast; and to Cuiviénen there is no returning. But it is said among the Elves that it lay far off in the east of Middle-earth, and northward, and it was a bay in the Inland Sea of Helcar; and that sea stood where aforetime the roots of the mountain of Illuin had been before Melkor overthrew it Many waters flowed down thither from heights in the east, and the first sound that was heard by the Elves was the sound of water flowing, and the sound of water falling over stone.