The Columbia Plateau, Oregon Plateau, Snake River Plain, and Northern Nevada Rift compose a single magmatic system containing all the essential characteristics ascribed to a mantle plume genesis. A mobile mantle is delineated by volcanic migrations, divisible into two types: (1) Rapid, radial migrations (~10--100 cm/yr) are associated with impingement and spreading of the Yellowstone plume head along the Chief Joseph, Steens Mountain--Picture Gorge, and Northern Nevada Rift magmatic trends from ~16.6 to 15.0 Ma. (2) Subsequent (post-15.0 Ma), slower migrations (1--5 cm/yr) are associated with shearing off of the plume head, generating the Snake River Plain hot spot track above the plume tail, and with westward asthenospheric drag of the plume head beneath the Oregon Plateau. The plume head provided a melt component to Imnaha and Grande Ronde Basalts. Depleted mantle lithosphere lying above the plume head provided a melt component to Steens Basalt and Picture Gorge Basalt and to younger eruptions of high-alumina olivine tholeiite. The plume head currently resides beneath a broad lithospheric swell, marked by young volcanism, high heat flow, and slow P wave travel times. The periphery of the plume head is delineated by the cratonic margin to the east, a gravity discontinuity and a set of wrinkle ridges to the north, and a prominent belt of young high-alumina olivine tholeiites and active volcanoes adjacent to the Cascade volcanic arc to the west.