Saskatchewan, river, c.340 mi (550 km) long, formed by the confluence of the North Saskatchewan (c.760 mi/1,220 km long) and the South Saskatchewan (c.550 mi/890 km long) rivers near Prince Albert, central Sask., Canada; the system drains most of the Canadian prairie provinces. It flows generally east past Nipawin, across the Manitoba line, then past The Pas and through Cedar Lake to Lake Winnipeg. The North Saskatchewan River rises in the Columbia ice field at the foot of Mt. Saskatchewan, SW Alta., and flows generally east past Edmonton, into Saskatchewan prov., and then past North Battleford to Prince Albert. Its chief tributaries are the Clearwater, Brazeau, Vermillion, and Battle rivers. The South Saskatchewan River is formed in S Alberta by the junction of the Bow and Oldman rivers. It flows east past Medicine Hat, then northeast into Saskatchewan prov., past Saskatoon, to Prince Albert; it receives the Red Deer River. The Bow–South Saskatchewan–Saskatchewan system is c.1,200 mi (1,930 km) long. Completion (1967) of the Gardiner and Qu'Appelle Valley dams, major elements of the South Sasketchewan River Project, impound Lake Diefenbaker, a huge reservoir. The dams and reservoir provide hydroelectric power and irrigation for a large region south of Saskatoon. The Saskatchewan River and its branches were once important thoroughfares for explorers and trappers.