Map of the Wallula Gap Area
McNary Master Plan
Supporting Data
Item 11
Wallula Gap - National Natural Landmark
Letter from U.S. Department of the Interior
1. Site: Wallula Gap, Benton and Walla Walla Counties, Washington.
2. Description: This 1780 ha (4,400 acres) site is located in southern Washington, 25 km (16 miles) south of Pasco. The Horse Heaven Hills are the southern border of the Central Basin area of the Columbia Basin, and Wallula Gap is opening through which all of the Columbia and Snake River Waters escape the basin. Wallula Gap is the largest, the most spectacular, and the most significant of the several large water gaps through basalt anticlines in the Columbia Basin. During the Pliocene and early Pleistocene, the thick basalt flows of what is now southern Washington were wrinkled and uplifted into a long northeast-southwest trending anticlinal ridge which we call the Horse Heaven Hills. The uplift of this structure was more rapid than the ability of the ancestral Columbia River to downcut, thus a great lake, Lake Lewis, was impounded in the Pasco Basin area. The remains of the lake beds formed in this lake are the Ringold Formation. Once Lake Lewis was filled to overflowing, it established an outlet over the lowest saddle in the ridge at the location of Wallula Gap. The Columbia then poured down the steep south flank of the Horse Heaven Hills anticline in cascades and waterfalls which eroded slowly downward and headward until the ridge was completely breached. Wallula Gap was formed, and the waters of Lake Lewis, along with considerable quantities of the Ringold sediments, drained down the Columbia Gorge into the sea. During the late Pleistocene, the greatly increased meltwaters from the ice sheets and Cascade glaciers caused floods, perhaps annually, which encountered the constriction of Wallula Gap. Also, the gap was possibly dammed by jammed icebergs and ice cakes, causing the temporary and intermittent rebirth of Lake Lewis. This ephemeral lake left bottom deposits called the Touchet Beds. Superimposed on this period of great drainage were at least two gigantic floods, Bonneville and Spokane, the likes of which have never been documented elsewhere on Earth. These giant floods filled the basin with a treat temporary lake, which rapidly drained out through Wallula Gap. At the height of the Spokane Flood, the water flow through Wallula Gap has been estimated at 164 cubic kilometers per day (39.5 cubic miles per day), a rate nearly twice the combined flow of all the present rivers on Earth. A rare onion (Allium robinsonii) is reported to occur within the site. Prairie falcons (Falco mexicanus) have also been reported to occur within, and adjacent of, the site.
3. Owner: U.S. Government, administered by the U.S. Army Corps of Engineers, a public hospital district; and private.
4. Proposed by: W. Frank Scott, Columbia Plateau, Geologic Theme Study, 1978.
5. Significance: All of the floodwaters that cut the channeled scablands of southeastern Washington from the Bonneville and Spokane floods were channeled through the water gap called Wallula Gap. More than 833 cubic km of water per day (200 cubic miles per day) were delivered to a gap that could discharge less than 166 cubic km per day (40 cubic miles per day), causing the remaining water to pond in the Pasco Basin and the Yakima and Touchet Valleys, to form an intermittent lake - Lake Lewis. This outlet of Wallula Gap, constricted in a relative sense only [since it is more than 1.6 km (1 mile) from rim to rim], controlled the discharge from the Pasco Basin, and is dramatic and illustrative evidence of the magnitude of these floodwaters.
6. Land Use: Wallula Gap is the principal route of commerce into south-central Washington. Through it, barge traffic travels up the Columbia River to Pasco and beyond - up the Snake River. Two railroads and a highway also pass through the gap. The waters of Lake Wallula are used for recreation. The upland areas are used for grazing. The old river floodplain is now part of the reservoir storage behind McNary Dam.
7. Dangers to Integrity: None.
8. Special Conditions: None.
9. Evaluated by: William Copeland, Oregon Natural Heritage Program; Rexford Daubenmire; W. Frank Scott, Washington University; Edwin Tisdale; and Donald E. Trimble, U.S. Geological Survey.
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