The Lorentz River is located in the Indonesian province of South Papua in Western New Guinea, about 3,500 km northeast of the Indonesian capital Jakarta. 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. After it became part of Indonesia, it was renamed to Unir, as in the language of the local Asmat people, while the Lorentz name is still in use.

The Asmat are an ethnic group of New Guinea, residing in the province of South Papua, Indonesia. The Asmat inhabit a region on the island's southwestern coast bordering the Arafura Sea, with lands totaling approximately 18,000 km2 (7,336 mi2) and consisting of mangrove, tidal swamp, freshwater swamp, and lowland rainforest.