The Nemiscau River flows over 100 kilometres (62 mi) northeast to southwest in the Nord-du-Québec (administrative region) passing south of the hamlet of Nemaska.

Neighboring hydrographic watersheds are: At the end of its course, the river flows into the northern edge of Lake Nemiscau, which matches the shape of a U open towards the northeast.

In 1982, the Commission de toponymie du Québec (English: Quebec Names Board) formalized the movement toponymique pointing this river "Rivière Nemiscau".

Subsequently, the maps of Claude-Charles Bacqueville de La Potherie use the spelling "R. Rupert" and those of Father Laure in 1731–1732 "R.

The place name Nemiscau was recovered, but moving to the northeast to identify the branch of the river remained unnamed until the 1940s.