The Missoula and Bonneville floods—A review of ice-age megafloods in the Columbia River basin

• Published in: Earth-Science Reviews
• Main Authors: O’connor, Jim E., Baker, Victor R., Waitt, Richard B., Smith, Larry N., Cannon, Charles M., George, David L., Denlinger, Roger P.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier BV 2020
• Subjects: Megafloods; Columbia River basin; western North America; Missoula floods; Bonneville flood; Channeled Scabland; Crab Creek; Glacial Lake; Moses; Ice Sheet
• Online Access: https://archimer.ifremer.fr/doc/00624/73634/73074.pdf; https://doi.org/10.1016/j.earscirev.2020.103181; https://archimer.ifremer.fr/doc/00624/73634/

The Channeled Scabland of eastern Washington State, USA, brought megafloods to the scientific forefront. A 30,000-km2 landscape of coulees and cataracts carved into the region’s loess-covered basalt attests to overwhelming volumes of energetic water. The scarred landscape, garnished by huge boulder bars and far-travelled ice-rafted erratics, spurred J Harlen Bretz’s vigorously disputed flood hypothesis in the 1920s. First known as the Spokane flood, it was rebranded the Missoula flood once understood that the water came from glacial Lake Missoula, formed when the Purcell Trench lobe of the last-glacial Cordilleran ice sheet dammed the Clark Fork valley in northwestern Idaho with ice a kilometer thick. Bretz’s flood evidence in the once-remote Channeled Scabland, widely seen and elaborated by the 1950s, eventually swayed consensus for cataclysmic flooding. Missoula flood questions then turned to some that continue today: how many? when? how big? what routes? what processes? The Missoula floods passed through eastern Washington by a multitude of valleys, coulees and scabland tracts, some contemporaneously, some sequentially. Routings and their timing depended on the positions of various lobes of the multi-pronged Cordilleran ice sheet and the erosional development of the channels themselves. The first floods mostly followed the big bend of Columbia valley looping through north-central Washington. But the south-advancing Okanogan ice lobe soon blocked that path, forming long-lasting glacial Lake Columbia in the impounded Columbia valley. Missoula floods into this lake were diverted south out of the Columbia valley and into eastern Washington coulees and scabland tracts. At least four floods entered Moses Coulee, but then as the Okanogan lobe advanced over and blocked the head of that coulee, more eastern paths took the water, including Grand Coulee and the Telford-Crab-Creek and Cheney-Palouse scabland tracts. Flood routing also depended on the erosion of the coulees. At some point, headward erosion of upper Grand ...