Northwest Discovery Water Trail Traveler's Forum

Wallula Gap, located on the Columbia, just north of the Oregon-Washington border, is one of my favorite geographic features on the Columbia. 

Here is some info on the formation of the Gap, from Geologist Bruce Bjornstad of the Pacific Northwest National Laboratory, as presented to the Geological Society of America's annual meeting in Salt Lake City on October 17, 2005:

    "... The story of Wallula Gap begins back in the Miocene Epoch, about 17 million years ago. At that time, lava flows of basalt ran out of giant fissures in the Earth's crust east of here, near the Idaho-Oregon-Washington border. During basalt volcanism, the southern and western portions of the Columbia Plateau, including Wallula Gap, began to warp and fold under stresses deep in the Earth's crust. The bending of the ancient lava flows is clearly visible in the folded layers of basalt exposed in the steep walls of the gap.

    Early in the history of folding, the ridgecrest here was slightly lower than elsewhere along the ridge. This caused first the ancient Salmon-Clearwater River (precursor to the Snake River), and then the Columbia River to flow across the ridge over this low point. As the ridge continued to bend upward, river erosion kept pace, and a water gap developed. Until about 10 million years ago only the ancestral Salmon-Clearwater River flowed through Wallula Gap. It wasn't until about 6 million years ago that the Columbia River joined in, where it has continued to flow ever since. Somewhere between 2 and 3 million years ago, the ancestral Salmon-Clearwater River captured the Snake River in the vicinity of Hell's Canyon, along the Idaho-Oregon border; this added significantly to the amount of water draining through the gap. ..."

(Posted on http://englishriverwebsite.com/LewisCla … a_gap.html)