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- Serni
- The fourth of the Five Rivers of Lebennin in Gondor. It began in the plains of Lebennin and met the Bay of Belfalas north of the Mouths of Anduin at the city of Linhir.
- The name is usually written Serni, although in the Preface to The Adventures of Tom Bombadil Tolkien spelt it Sernui.
- Shirebourn
- A river of the Shire with the source in the uplands of the Green Hill Country of the Eastfarthing. Thence it ran south and then southeast, meeting the Thistle Brook at Willowbottom and then flowing eastwards to Baranduin. There it formed the boggy region known as the Overbourn Marshes.
- Silverlode
- A river of Lothlórien rising in the eastern Misty Mountains near the East Gate of Moria. It then ran through Lothlórien where it met Nimrodel and emptied into Anduin. The Company of the Ring followed this river when they travelled from Moria to Lothlórien.
- Silverlode was called Celebrant in Sindarin and Kibil-nâla in Dwarvish. It also formed the northern boundary of the Field of Celebrant.
- Sirannon
- The Gate Stream of Khazad-dûm. Beginning at the Silvertine, it flowed past the gates of Moria towards the river Glanduin. Sirannon encountered the Stair Falls under the gates of Moria, and its sound could have been heard from miles around.
- During the War of the Ring, the Company of the Ring found that the Gate Stream had been dammed by someone or something, and before the gates of Moria there was a foul lake where the Watcher in the Water dwelt.
- Sirion
- A river of Middle-earth in the First Age, the principal river of Beleriand. During most of its course it was the border between East and West Beleriand. Karen Wynn Fonstad estimates its length as 390 miles in her Atlas of Middle-earth. Sirion flowed southwards along the border of the Ered Wethrin, passing through the Fens of Serech before entering the valley between the Ered Wethrin and the Pass of Sirion. It then continued south into Beleriand, with the Forest of Brethil to the west, and Dimbar and then Doriath to the east. After leaving Doriath it ran though Fens of Sirion before falling below ground in the Falls of Sirion at Andram (the Long Wall), where the ground fell steeply. Three leagues southwards the Sirion exited the underground caves at the Gates of Sirion. It then flowed southwards through Nan-Tathren until it reached the Bay of Balar, part of Belegaer, at the Ethir Sirion, Mouths of Sirion. Barad Eithel, at the source of the Sirion in the Ered Wethrin, was a chief fortress of Fingolfin and his son Fingon. Further south, in the Pass of Sirion, lay Tol Sirion in the centre of the river.
- Sirith
- The third of the Five Rivers of Lebennin in Gondor. Celon was its tributary, while Sirith met Anduin at Pelargir.
- Snowbourn
- A river of Rohan arising in the White Mountains under the mountain Starkhorn. It flowed through Harrowdale where Edoras was built, and then flowed east until it met the river Entwash. It separated Rohan's Eastfold from the West Emnet.
Read more about this topic: List Of Middle-earth Rivers