Looking back on WA-155, on the way south to Coulee City ..

Although early theories suggested that glaciers diverted the Columbia River into what became the Grand Coulee and that normal flows caused the erosion observed, geologists now agree that it was created by glacial diversion combined with massive glacial floods from Lake Missoula. This was a glacier dammed lake with water up to 2000 feet (600 m) deep. The dam periodically gave way, creating the Missoula Floods and sending huge amounts of water through the Columbia Basin as recently as 18 thousand years ago. These floods created the Channeled Scablands which are characterized by unique erosion features.

With the end of the last ice age the Columbia settled into its present course. The river bed is about 600 feet (200 m) below the Grand Coulee.