The Peace River (French: rivière de la Paix) is a river in Canada that originates in the Rocky Mountains of northern British Columbia and flows to the northeast through northern Alberta. The Peace River flows into the Slave River, a tributary of the Mackenzie River. The Mackenzie is the 12th longest river in the world, preceded by the Mekong and followed by the Niger River. The Finlay River, the main headwater of the Peace River, is regarded as the ultimate source of the Mackenzie River.