The Vale(s) of Sirion was the wide valley of the River Sirion, that separated West Beleriand from East Beleriand. The term seems to apply especially to that part of the river's course around the Pass of Sirion that lay to the north of Doriath.
These waters were protected by Ulmo until some point prior to F.A. 495; at that time, two messengers from Círdan bore tidings to Orodreth that the Evil of the North had defiled the springs of Sirion, and Ulmo's power had withdrawn.