Lake Diefenbaker is a large multi-purpose reservoir on the South Saskatchewan River in southern Saskatchewan. The reservoir was developed as a joint undertaking by the Canadian and Saskatchewan governments to provide for a wide range of uses including power production, flood control, irrigation, industrial water supply, recreation and augmentation of flows in the Qu'Applelle River by diversion through the Qu'Appelle Dam. The structures were designed and constructed by the Prairie Farm Rehabilitation Administration. Construction began in 1958 and the reservoir was filled in 1967.

Inflow to Lake Diefenbaker is primarily from the South Saskatchewan River which originates as the Oldman, Bow and Red Deer Rivers in the mountains of southwestern Alberta. These rivers drain much of southern Alberta which, except for the mountain regions, is mainly treeless prairie used for grain and livestock production. Approximately 75% of the drainage to Lake Diefenbaker is in Alberta. The quality of water entering Lake Diefenbaker is generally very good and the lake represents a major source of high quality water in an area where water quality on other water bodies is generally poor, usually with high levels of minerals and nutrients. As a result a number of projects to draw drinking water for communities in southern Saskatchewan, including Regina and Moose Jaw, have been and are being examined.

The South Saskatchewan River traversed the sandy soils of the Missouri Coteau where Lake Diefenbaker is now situated and the valley is a deep, broad, strongly eroded meltwater channel cut 60 to 150 meters into the glacial deposits and underlying bedrock. In the upper reaches of the lake the deep valley is still evident. The recreational potential of the lake is significant, providing a healthy sport fishery, large sandy beaches and excellent boating. There are a number of cottage developments along its shores and three provincial parks (Q).