From Tolkien Gateway

Nivrim was a narrow woodland composed of mostly oaks, that lay north of the Aelin-uial and extending to Taeglin. It was the only part of the kingdom of Doriath west of the River Sirion and was encompassed within the Girdle of Melian.[1] A guarded bridge crossed Sirion just south of the inflow of Esgalduin, connecting the land with the rest of Doriath. Mablung and three companions were leading Nienor to this bridge but were assailed by Orcs when they rested, and Nienor ran away.[2]


The name means "west-march" from Doriathrin nivon and rim[3]