The river originates at the confluence of the Bow and Oldman rivers north of Grassy Lake, Alberta at the incorporated community of Bow Island, not to be confused with the town of Bow Island, Alberta. The waters of these two rivers, in turn, originate from winter snowpack and rainfall in the Rocky Mountains near the British Columbia and Montana border. Glacier and snowmelt from the Rocky Mountains and foothills contributes nearly 90% of the annual flow, with most of that contribution during July and August. The Red Deer River is a major tributary of the South Saskatchewan merging east of the Alberta-Saskatchewan border. The Lake Diefenbaker reservoir was created with the construction of the Gardiner and Qu'Appelle River dams in Saskatchewan. Water from the South Saskatchewan flowing through the dams provides approximately 19 percent of the hydro-electricity generated by SaskPower.