Early theories suggested that glaciers diverted the Columbia River into what became the Grand Coulee and that normal flows caused the erosion observed . In 1910 Joseph T. Pardee described a great Ice Age lake , " Glacial Lake Missoula " , a glacier dammed lake with water up to deep , in northwest Montana and in 1940 he reported his discovery that giant dunes high and feet apart had formed the lake bed . In the 1920s J Harlen Bretz looked deeper into the landscape and put forth his theory of the dam breaches and massive glacial floods from Lake Missoula
massive glacial floods
Missoula Floods
Grand Coulee