Why is surplus Columbia River electricity sent to the American Southwest during summer?

Surplus hydroelectric power from the Columbia River is sent to the American Southwest during the summer because of the complementary nature of regional energy demands. While the Pacific Northwest experiences its peak power demand during the winter, the American Southwest has a high demand for electricity during the summer, largely due to cooling needs.

To balance these seasonal fluctuations, the power network in the Pacific Northwest is linked with the grid in the American Southwest. This allows the regions to trade energy: the Northwest exports its surplus hydroelectric power to the Southwest during the summer, while the Southwest sends its steam-generated power to the Northwest during the winter when demand in the north is at its highest.1