The Lorentz River (also Unir or Undir) is located in Western New Guinea, Indonesia. It originates in the central east-west mountain range of New Guinea, and flows southwards into the Arafura Sea at Flamingo Bay. During the first two Dutch expeditions to Southern New Guinea (1907-10) it was called Northern River. In 1910 it was renamed after the Dutch explorer Hendrikus Albertus Lorentz.