NEW INSIGHTS INTO THE TIMESCALES AND ROUTING HISTORY OF THE LATE PLEISTOCENE MEGAFLOODS IN THE PACIFIC NORTHWEST (Invited Presentation)

The Missoula floods are the largest fresh-water floods known from Earth’s geologic record. These megafloods were caused by the rapid draining of Glacial Lake Missoula when the ice dam holding it back failed. The point of release was northeast of Spokane, Washington from where floodwaters flowed into the Columbia River and ultimately into the Pacific Ocean. Numerous questions remain about both the routes and timing of flood events. Presented here are new cosmogenic ¹⁰Be surface exposure ages that provide key constraints on the timescales of flooding and associated glacial retreat. The flood routes were principally dependent on the southern extent of the Cordilleran Ice Sheet. Our ages reveal multiple flooding events of varying magnitude, with the largest flood occurring ~18.2 ka. The retreat of the Okanogan and Purcell Trench lobes began ~15.5 ka and resulted in the Upper Grand Coulee opening as a flood route. The youngest floods (~14 ka) were routed down the Northern Columbia River valley and likely were derived from Glacial Lake Columbia.