 Until about 10 million years ago, the Salmon-Clearwater River flowed through the Wallula Gap alone.As the flood plain in the Columbia Basin continued to tilt, the Columbia River was forced eastward and joined the Salmon-Clearwater River about 6 million years ago.About 2.5 million years ago, the Snake River was forced northward by the Blue Mountain uplift and was captured by the Salmon-Clearwater River near the Idaho-Oregon border.