The main river of Númenor, and indeed one of only two rivers on the island of any significance. Siril rose in Noirinan, the Valley of the Tombs on the southern side of Númenor's tall central peak of the Meneltarma. From there it flowed southwards, passing through the western fringes of Arandor, the Kingsland, before running on across the inland plains of the Mittalmar. As it passed through the southern parts of the Mittalmar, the stream of Siril began to slow, and its course became more winding.

After crossing the Mittalmar's southern border, the river's wandering course formed the boundary between the two great southern promontories of the isle of Númenor, the Hyarnustar and the Hyarrostar. As it approached the Sea on Númenor's southern coast, Siril began to break into numerous separate streams, creating an extensive marshland around its mouths as it formed a complex delta. Through the marshes of Siril's mouths lay the scattered villages of a people who lived by fishing. The largest of these was a shoreland township named Nindamos, on the coasts to the east of Siril's delta some hundred and fifty miles southwards from the river's springs under the Meneltarma.