The Columbia River system is one of the great river systems of North America, draining much of the Pacific Northwest, as well as parts of the western United States and British Columbia. The river system has had a long and complex history, slowly evolving over the past 17 m.y. The Columbia River and its tributaries have been shaped by flood basalt volcanism, Cascade volcanism, regional tectonism, and finally outburst floods from Glacial Lake Missoula. The most complex part of river development has been in the northern part, the Columbia Basin, where the Columbia River and its tributaries were controlled by a subsiding Columbia Basin with subtle anticlinal ridges and synclinal valleys superimposed on a flood basalt landscape. After negotiating this landscape, the course to the Pacific Ocean led through the Cascade Range via the Columbia Trans-Arc Lowland, an ancient crustal weakness zone that separates Washington and Oregon. The peak of flood basalt volcanism obliterated the river paths, but as flood basalt volcanism waned, the rivers were able to establish courses within the growing fold belt. As the folds grew larger, the major pathways of the rivers moved toward the center of the Columbia Basin where subsidence was greatest. The finishing touches to the river system, however, were added during the Pleistocene by the Missoula floods, which caused local repositioning of river channels.

Abstract Like nowhere else on Earth, repeated cataclysmic floods—first of molten lava, then of water from Ice Age floods—decimated southeastern Washington during the late Cenozoic. Beginning ca. 17 Ma, successive outpourings of Columbia River basalt spread for hundreds of kilometers from volcanic vents located in the southern and eastern Columbia Plateau. Up to 300 separate basalt flows have been identified, reaching cumulative thicknesses of 5 km in the Pasco Basin. With the close of basalt volcanism ca. 6 Ma, only a few million years elapsed before the Pacific Northwest succumbed to a new era of flooding. Outburst floods are associated with regular glacial cycles that have occurred periodically over the past 1–2 m.y. from one or more Pleistocene, ice-marginal lakes. During the last glacial cycle (15,000–20,000 calendar yr) alone, as many as 100 separate flood events, mostly from glacial Lake Missoula, are postulated. In the Channeled Scabland, after removing a blanket of loess, differential erosion through hundreds of meters of layered basalt with widely contrasting variations in fracture patterns and structure resulted in a unique assemblage of erosional landforms including multi-tiered cataract canyons, buttes, mesas, and rock basins. A number of depositional features, including huge flood bars blanketed with giant current ripples, as well as ice-rafted erratics and bergmounds, are also prevalent.

Neogene sedimentation on and adjacent to the Columbia Plateau in Oregon, Washington, and Idaho was related to volcanism and tectonism. During emplacement of the largest volume of middle Miocene flood basalts (Grande Ronde, Picture Gorge, and Wanapum Basalts), local drainage disruption and gradient diminishment caused deposition in lakes and by sluggish mixed-load streams at or near the flow margins (e.g., Latah, lower Ellensburg, and Simtustus Formations). The Pasco basin was the principal subsiding feature at this time, but because of its central position on the basalt plateau, it received only minor accumulations of detrital and organic-rich sediments. The Mascall and Payette Formations (and equivalents) were deposited in subsiding basins along the southern and southeastern plateau margins. As basalt eruptive frequency and volume diminished in late Miocene time (Saddle Mountains Basalt), deposition occurred primarily in response to intrabasin tectonism and Cascade volcanism. A well-integrated through-flowing river system transported detritus from the surrounding highlands across the plateau. Late Miocene sedimentation along the western plateau margin was strongly influenced by large volcaniclastic sediment loads from the Cascade Range (upper Ellensburg, Dalles, and Deschutes Formations). Elsewhere, fluvial and lacustrine deposition occurred in response to basin subsidence (e.g., Ringold and Idaho Formations) or influx of coarse clastics into shallow basins (e.g., Alkali Canyon and McKay Formations, Thorp Gravel). Widespread unconformities and provenances indicative of drainage reversals in the Blue Mountains region may reflect a transition from primarily compressional to extensional deformation along the southern margin of the plateau between 12 and 10 Ma.