From Tolkien Gateway

The Mindeb was a river of Beleriand, a northern tributary to the river Sirion.

The Mindeb began in the Ered Gorgoroth near the Pass of Anach, and was met by a tributary coming from the Crissaegrim. Upon its western bank was the land of Dimbar while on the east were the valley of Nan Dungortheb and the Forest of Neldoreth.[1] In its lower reaches it bordered Doriath and thus was part of the Girdle of Melian.[2]