During the basalt eruptions, the southern and western Columbia Plateau, which included the current location of the Wallula Gap, began to fold.

[3][4] 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 (190 cu mi) of water per day entering could be discharged as it passed into Lake Condon downstream.

[6] 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.

[7] Twin Sisters is a basalt pillar that inspired the mythology of the Wallula Gap near the Columbia River just upstream of Port Kelley, Washington.

According to the Cayuse Indian tribe, who lived on the Columbia Plateau, the following legend recounts the origin of the rocks:[8][9] Coyote, the Trickster spirit hero of many native stories, fell in love with three sisters who were building a salmon trap on the river near here.

[10]This site is a popular hiking destination for avid hikers who want to view the geology of the Wallula Gap and the effects of the ancient Missoula Floods.