Koksoak, Rivière, 874 km long (to head of Caniapiscau River), final leg of a river system that drains a vast area (133 000 km2) of northern Québec. The main branch is formed by the Caniapiscau River, which rises in Lac Caniapiscau, and its main tributary, the Rivière Swampy Bay, and flows generally north-northwest. The more westerly branch is formed by Rivière aux Mélèzes and its tributaries Rivière du Gué and Rivière Delay.
The Caniapiscau and Mélèzes join to form the Koksoak, which flows north 145 km past Kuujuak to Ungava Bay. The name is likely a translation of an Inuit word meaning "big river," and the Koksoak is over 1600 m wide near its mouth. A dam on the Caniapiscau River has tripled the size of Lac Caniapiscau, and water is being diverted to the James Bay hydroelectric development via La Grande Rivière.