Wallula Gap

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Coordinates: 46°2′40″N, 118°56′48″W

Looking to the north from about the midway point within the Walulla Gap.
Looking to the north from about the midway point within the Walulla Gap.

Wallula Gap is a large water gap 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

[edit] Geology

[edit] Flood basalts

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]

[edit] Creation of the Wallula Gap

View of Wallula Gap from Wallula, Washington Main Street
View of Wallula Gap from Wallula, Washington Main Street

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]

[edit] Missoula Floods

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 1/5th of the 800 km³ of water per day entering could be discharged. 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 "The 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 mile) 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.

[edit] History

[edit] Lewis & Clark

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]

[edit] Images

Picture taken from location slightly above the location of Fort Nez Percés as seen in 2006, looking to the south toward the Walulla Gap.

Looking from the northern end toward the south within the Walulla Gap.

Looking to the west across the Walulla Gap.

The Sisters: a scabland residual of the Missoula Floods on the east bank of the Columbia River in the Wallula Gap.

[edit] References

  1. ^ USGS Material on shield-volcano eruptions
  2. ^ a b Channeled Scabland of Eastern Washington:The Geologic Story of the Spokane Flood; U.S. Government Printing Office; 1982.
  3. ^ a b Carson, Robert J. and Pogue, Kevin R. (1996). Flood Basalts and Glacier Floods:Roadside Geology of Parts of Walla Walla, Franklin, and Columbia Counties, Washington. Washington State Department of Natural Resources (Washington Division of Geology and Earth Resources Information Circular 90). ISBN none. 
  4. ^ Wallula Gap - National Natural Landmark
  5. ^ Moulton, Gary E. (1991). The Journals of the Lewis & Clark Expedition: March 23—June 9, 1806. University of Nebraska Press, Lincoln, Nebraska. ISBN 0-8032-2898-8. 

[edit] External links

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Ice Age Floods in the Pacific Northwest