Owyhee Plateau
The Yellowstone hot spot first appeared about 17 million years ago with rhyolite eruptions on the Owyhee Plateau of southeast Oregon and northern Nevada. At about the same time, the Columbia River Basalts, containing over 150,000 cubic miles of lava, were erupting in eastern Oregon and Washington from north-northwest-trending fissures. Some of the more extensive lava flows moved westward down the ancestral Columbia River to the Pacific Ocean. The continental divide was likely located north of Boise in the Salmon River Mountains where the Idaho Batholith formed a resistant topographic high area. The Albion Range was uplifted near the Nevada-Utah border and drainage in eastern Idaho was eastward across a broad alluvial surface, above the eroded Idaho-Wyoming fold-thrust belt.
Owyhee Plateau Cross-section of the plateau.

Images courtesy of the U.S. Forest Service, Department of Agriculture.