§ Of Beleriand and Its Realms The Silmarillion

Now the great and fair country of Beleriand lay on either side of the mighty river Sirion, renowned in song, which rose at Eithel Sirion and skirted the edge of Ard-galen ere he plunged through the pass, becoming ever fuller with the streams of the mountains. Thence he flowed south for one hundred and thirty leagues, gathering the waters of many tributaries, until with a mighty flood he reached his many mouths and sandy delta in the Bay of Balar. And following Sirion from north to south there lay upon the right hand in West Beleriand the Forest of Brethil between Sirion and Teiglin, and then the realm of Nargothrond, between Teiglin and Narog. And the River Narog rose in the falls of Ivrin in the southern face of Dor-lómin, and flowed some eighty leagues ere he joined Sirion in Nan-tathren, the Land of Willows. South of Nan-tathren was a region of meads filled with many flowers, where few folk dwelt; and beyond lay the marshes and isles of reed about the mouths of Sirion, and the sands of his delta empty of all living things save birds of the sea.