Why are Columbia River flows lower in autumn and winter?

The Columbia River experiences low flows in autumn and winter primarily because its peak discharge is driven by the melting of snow in its mountainous watershed, which occurs during late spring and early summer.1

Because the river's natural hydrological cycle is tied to this seasonal snowmelt, the volume of water decreases significantly during the colder months when precipitation is stored as snow in the mountains rather than flowing directly into the river system. These low autumn and winter flows historically created water shortages for the river's hydroelectric power plants. To mitigate this, dams such as the Grand Coulee Dam are used to store water and augment these low flows, particularly when electricity demand is at its highest.12