Lake Diefenbaker, reservoir in Saskatchewan, Canada

Lake Diefenbaker is the largest body of water in southern Saskatchewan and stretches across a wide area with a shoreline that extends for many kilometers. The water is bordered by several provincial parks that offer beaches, campgrounds, and hiking trails, while boat launches and marinas provide easy access to the water.

Lake Diefenbaker was formed by two dams: the Gardiner Dam on the South Saskatchewan River and the Qu'Appelle River Dam on the Qu'Appelle River. Construction began in 1959 and the reservoir filled in 1967. Before the dams, high water on the South Saskatchewan caused dangerous ice downstream at Saskatoon, while the Qu'Appelle often dried up in summer. The lake is named after John G. Diefenbaker, a former Prime Minister of Canada.

The lake takes its name from John G. Diefenbaker, Canada's 13th prime minister. Before the dams were built, a 400-ton glacial erratic called Mistasiniy in the Qu'Appelle Valley served Plains Cree and other Indigenous peoples as a gathering place for millennia; it was destroyed with dynamite on 1 December 1966, and fragments were set in a cairn at Elbow Harbour and at Chief Poundmaker's grave.

Highways 4 and 42 cross the lake: Highway 4 uses a causeway and bridge at the western end, while Highway 42 relies on the cable-operated Riverhurst Ferry between Riverhurst on the south bank and Lucky Lake on the north bank. Highway 44 crosses the Gardiner Dam, and Highways 19 and 628 also reach the shore. Campgrounds, beaches and boat launches sit in Danielson, Douglas and Saskatchewan Landing provincial parks and in Palliser Regional Park, about 8 km west of Riverhurst.

Both the current world-record rainbow trout and the world-record burbot were caught in Lake Diefenbaker, and it is one of only two Saskatchewan waters where rainbow trout reproduce. Before the reservoir filled, the 400-ton glacial erratic Mistaseni stood in the Qu'Appelle Valley as a gathering place for Plains Cree; it was dynamited on 1 December 1966, and fragments went to a cairn at Elbow Harbour and to Chief Poundmaker's grave.