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

Abstract

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 Coulee lowered the divide saddle between the west-running Columbia valley and the deep and wide Grand Coulee heading southwest. Still uncertain is when this happened and the consequences with respect to the stage and extent of glacial Lake Columbia and to flood access to the other, higher, flood routes. Downstream, all flood routes converged into Pasco Basin, flowed through Wallula Gap and the Columbia River Gorge into the Pacific Ocean, following submarine canyons and depositing sediment layers on abyssal plains.Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.and perhaps older ones affected the Snake River Plain, some likely sourced in lakes dammed by alpine glaciers in central Idaho.

Characterization of a glacial paleo-outburst flood using high-resolution 3-D seismic data: Bjørnelva River Valley, SW Barents Sea

“…Therefore, the deep Bjørnelva River Valley could have been created following the same high-energy processes involved in the formation of the valleys in the English Channel (Gupta and others, 2007; Collier and others, 2015), the Münsterland Embayment (Meinsen and others, 2011), or the Coronation – Spondin Scabland in the Canadian Prairies (Sjogren and Rains, 1995). The Bjørnelva River Valley could have been formed by multiple separate Jøkulhlaups with wide ranges of magnitudes, as also suggested for megafloods in North America and Greenland (Baker and Bunker, 1985; Keisling and others, 2020; O'Connor and others, 2020). Glacier-outburst generated floods can last minutes to years (e.g.…”

“…While the deposits from the Bjørnelva River outburst flood differ from the largest identified events on Earth, the erosional power of the Bjørnelva River is comparable to other paleofloods (Sjogren and Rains, 1995; Meinsen and others, 2011; O'Connor and others, 2020).…”

“…Cataclysmic glacial outburst floods erode large channels and canyon networks (Smith, 1993; Larsen and Lamb, 2016), and abrupt cessation of these flows can leave coarse deposits (Baker, 1973; Sjogren and Rains, 1995; O'Connor and others, 2020). The landscape of the Coronation – Spondin Scabland in the Canadian Prairies formed in front of the retreating Laurentide Ice Sheet and outburst floods left discontinuous gravel beds, large boulders and bedrock clasts (Sjogren and Rains, 1995).…”

“…The large amounts of fresh water released from today's retreating ice sheets contribute to the global rise in sea level and affect ocean circulations and climate patterns (Richardson and Reynolds, 2000; Teller and others, 2002; Lewis and others, 2006). Locally, meltwater can be stored in dammed proglacial lakes that can be suddenly released forming catastrophic floods with effects to be felt several hundreds of kilometers downstream (Carrivick and Tweed, 2013; Harrison and others, 2018; O'Connor and others, 2020). These outbursts are so violent that they can reshape landscapes (Alho and others, 2005) by carving several tens of meters into the bedrock (Lewis and others, 2006; Gupta and others, 2007; Meinsen and others, 2011; Høgaas and Longva, 2016), and redepositing large amounts of eroded sediments in low-lying areas (Sjogren and Rains, 1995; Murton and others, 2010).…”

“…These outbursts are so violent that they can reshape landscapes (Alho and others, 2005) by carving several tens of meters into the bedrock (Lewis and others, 2006; Gupta and others, 2007; Meinsen and others, 2011; Høgaas and Longva, 2016), and redepositing large amounts of eroded sediments in low-lying areas (Sjogren and Rains, 1995; Murton and others, 2010). Such catastrophic events occurred throughout the Pleistocene deglaciations when huge fluxes of water from the wasting of continental ice sheets created floods of immense magnitude and relatively short duration (Collier and others, 2015; O'Connor and others, 2020). Prehistoric floods related to glacial lake outbursts discharged 100–500 times more water than the largest modern river (Baker, 2002).…”