The Peace River (French: rivière de la Paix) is a 1,923-kilometre-long (1,195 mi) river in Canada that originates in the Rocky Mountains of northern British Columbia and flows to the northeast through northern Alberta. The Peace River joins the Athabasca River in the Peace-Athabasca Delta to form the Slave River, a tributary of the Mackenzie River. The Finlay River, the main headwater of the Peace River, is regarded as the ultimate source of the Mackenzie River. The combined Finlay–Peace–Slave–Mackenzie river system is the 13th longest river system in the world.