The Columbia Plateau... known as the Columbia Basalt Plain, is the prominent geographic feature of the interior Columbia River Basin. The plateau formed between 6 million and 16 million years ago as the result of successive flows of basalt. Its northern border is marked by the Columbia River and the mouth of the Okanagon River. On the south, the plain includes the drainage of the Deschutes River in Oregon. On the east the plateau includes the Camas Prairie of central Idaho, and on the west it runs to the foothills of the Cascade Mountains. The Columbia River makes a looping S-curve through the central Washington part of the Columbia Plateau.