Moses Coulee the southern portion of the plateau. These streams had combined into a canyon that drained to the Columbia near the current mouth of Moses Coulee. These side drainages are still visible today along the coulee walls as truncated streambeds, interspersed with gable-like highlands. The Okanogan lobe of the Cordilleran Glacier moved down the Okanogan River valley, covering 500 mi² of the Waterville Plateau and blocked the ancient route of the Columbia River, backing up water to create Glacial Lake Columbia and Lake Spokane. Initially water discharged from Lake Columbia by running up through the head of Grand Coulee and down
Grand Coulee Dam of electricity. Each generator is supplied with water by an individual penstock. The largest of these feed the Third Powerplant and are in diameter and can supply up to . The dam's power facilities originally had an installed capacity of 1,974 MW but expansions and upgrades have increased generation to 6,809 MW installed, 7,079 MW maximum. Grand Coulee Dam generates 21 TWh of electricity annually. This means the dam generates about 2,397 MW of power on average, which results in a total plant factor efficiency of 35%. In 2014, 20.24 TWh of electricity was generated. Grand Coulee Dam's spillway is