NASA satellite image of Lake Diefenbaker showing the Qu'Appelle River Dam at the southeast end and the Gardiner Dam at the northeast end
During the time of glaciation on North America, the retreating glacier would block the flow north and would force the water flow down the Qu'Appelle River. When the glaciers retreated further, water would then flow north. Before the Gardiner Dam was built, spring flows were high enough to allow water down the Qu'Appelle but would dry up later in the fall. Now as the Qu'Appelle Dam always retains the water of Lake Diefenbaker, water is released into the Qu'Appelle to maintain flows all year long. This serves the farmers along the Qu'Appelle who use it for irrigation and watering their livestock.