A LARGE LATE PLEISTOCENE MEGAFLOOD AT 17.2 KA; DOWN THE COLUMBIA RIVER PRIOR TO BLOCKAGE BY THE OKANOGAN LOBE OF THE CORDILLERAN ICE SHEET

The awe-inspiring landforms of eastern Washington resulted from catastrophic outburst floods triggered by the intermittent failure of an ice dam impounding Glacial Lake Missoula. However, the magnitudes, routings, and timing for these megafloods remain largely unconstrained. Here we use surface exposure dating of high-elevation ice-rafted boulders to define the age and magnitude of one of the largest floods. Cosmogenic nuclide ¹⁰Be ages from Wallula Gap and upstream in the Columbia Valley near Wenatchee document the occurrence of a very large megaflood at ~17.2 ka. The high elevations of these boulders indicate a peak discharge of ~ 10 x 10⁶ m³/s at Wallula Gap, equivalent to the largest known event in the sequence of last-glacial Missoula floods. The timing of this large flood is consistent with salinity records from the eastern Pacific that indicate a peak in freshwater influx at that time. The high ice-rafted boulders in the Columbia Valley near Wenatchee confirm that at least one very large megaflood traveled through an open upper Columbia River route. This must have occurred prior to blockage of that route by the Okanogan lobe during growth to its maximum extent when it adv­­anced across the Columbia River and southward another 70 km onto the Waterville Plateau.