From Portland, head west on Hwy 84. The waterfall is visible from the highway, about 30 miles west of downtown Portland.
The Columbia River Gorge is amazingly inspiring for landscape photography, and has multiple large waterfalls along it's waterfront, most visible and/or alongside Highway 84. It's easily the most beautiful drive in the continental United States.
Multnomah Falls, shot from afar along the Columbia River Gorge just outside of Portland, Oregon in Mount Hood National Forest