Wallula Gap is a large water gap of the Columbia River through basalt anticlines in the Columbia River Basin in the U.S. state of Washington, just south of the confluence of the Walla Walla and Columbia rivers. It has been recognized as a National Natural Landmark by the National Park Service as a site that provides an important illustration of the geological history of the United States.
Contents |
In southeastern Washington, eastern Oregon, and southern Idaho, flood basalt flows of extremely fluid basaltic lava spread in all directions from long fissures, building broad fields of gently sloping lava that spread widely over great distances. Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows have been extensively exposed by erosion (with extensive displays of columnar basalt), and measure almost 2 km in total thickness.[1]
The basalts flows which are visible in the Wallula Gap were laid down during the Miocene Epoch, about 17 million years ago. They are part of the massive fissure flood basalts of the Columbia River Basalt Group. During the basalt eruptions, the southern & western Columbia Plateau, which included the current location of the Wallula Gap, began to fold. The bending (or shear) of the ancient lava flows is clearly visible in the folded layers of basalt exposed in the steep walls of the gap.[2][3]
Early in the history of folding, the ridge crest at Wallula Gap must have been the lowest along the ridge. The ancient Salmon-Clearwater River began to flow across the ridge over this low point, cutting a channel. As the ridge continued to fold upward, erosion cut through and the water gap was formed. 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.[2][3]
The Wallula Gap was widened about the 14th millennium BC, by the historic flow of the Salmon, Snake, and Columbia rivers combined with the glacial waters that poured across the Channeled Scablands during the Missoula Floods. The Wallula Gap constrained the flow such that less than 1/5 of the 800 km³ of water per day entering could be discharged as it passed into Lake Condon downstream. As a result, the floods filled the Pasco Basin and formed, for a short period, Lake Lewis. The large volumes of flood water passing through the gap contributed substantially to the erosion of the gap, as is evidenced by the shear walls and by scab-features such as "Twin Sisters".[4]
During the Missoula Floods water backed up in the Pasco Basin because of the narrow opening of the Wallula Gap. The floods were so large in volume that they could not rapidly discharge through the gap, even though it is 2 km (1.2 mi) wide. Maximum flood waters reached elevations of about 380 meters. The hydraulic head required to drive the flood waters through the gap, combined with the elevated levels downstream of the constriction resulted in a head of about 250 meters. The peak flow is estimated to have been about 10 million cubic meters per second.
The Lewis and Clark Expedition (a.k.a. Corps of Discovery) first saw the Wallula Gap on October 18, 1805, as they headed downstream from their camp on the confluence of the Columbia and Snake rivers. The Corps camped near Spring Gulch Creek that night, and proceeded through Wallula Gap the next day.[5]
In the 1840s pioneers headed west on the Oregon Trail found Wallula a logical stopping place to convert their wagons to boats and then continue the trip via the Columbia River."[6]
|
||||||||||||||||||||||