Don't confuse the town of Coulee City on the south shore of Banks Lake along a major US highway for Grand Coulee. The town with the dam is further north and in a more remote part of the state.
From the west: US Hwy 2 east to WA State Hwy 155 north. In the Seattle Metro area, there are short cuts to US Hwy 2 from Bothell on WA State Hwy 522, and Duvall (east of Redmond) on WA State Hwy 203. Otherwise, take the I-5 Fwy exit #194 in Everett where US Hwy 2 begins. Distance from Seattle: 230 miles
From the east: US Hwy 2 west to WA State Hwy 174 west (but runs northwest). Distance from Spokane: 88 miles
The small segment of the Columbia River at Grand Coulee flows north. This is the opposite of the river's normal flow from Canada to the Pacific Ocean. Photography can be a challenge due to the massive size of the dam, and the inability to get far enough away before passing the relatively short horizon. If possible, bring a wide-angle lens, and avoid the hours of the day when the sun is just barely above the dam face.