As the combined flood waters from Glacial Lakes Missoula and Columbia surged westward on the path of the Columbia River the rampaging floods were deflected south by the eastern margin of the Okanogan Lobe of the Cordilleran Ice Sheet.

As the concentrated flood waters spilled over a monoclinal fold at the southern end of the present day Upper Grand Coulee, they tore away at the underlying basalt bedrock creating a recessional cataract that eventually extended 20 miles northward to the older path of the Columbia River.

As the flood waters continued streaming south they hugged the south-trending edge of the monocline and began creating another recessional cataract at the present day location of Soap Lake. The resulting Lower Grand Coulee eventually extended 20 miles northward until the cataract recession ended at the site of Dry Falls when the last of the major floods ended.

These spectacular coulees and their boulder strewn expansion bars mutely but exquisitely testify to the awesome power of the Ice Age Floods.