A river in Gondor appearing on the maps of The Lord of the Rings (LotR/1186), it is simply the gerund sirith “flowing” used as a name (SA/sîr, VT42/11).
Conceptual Development: In Lord of the Rings drafts from the 1940s, N. Sirith appeared the list of rivers of Gondor (TI/312).
sirilla participle *"flowing", "Qenya" participle of siri- "flow" (Narqelion, cf. QL:xiv)