VARIED ROUTINGS OF LAKE MISSOULA MEGAFLOODS DOWN THE CHANNELED SCABLAND AND COLUMBIA VALLEY

Immense late Wisconsin debâcles from glacial Lake Missoula in Montana flooded the Channeled Scabland and Columbia valley at different times by different discharges and by different routes. The earliest, about 15,500 ¹⁴C yr BP (uncalibrated), raged 335 m deep down the Columbia and built high features like huge Pangborn bar at East Wenatchee. When the advancing Okanogan lobe of Cordilleran ice blocked the northwest ‘great bend’ of Columbia valley, several great Missoula floods descended Moses Coulee. The Okanogan lobe then also overrode Moses Coulee and the westmost floodway became Grand Coulee. All these immense Missoula floods also flooded overland down the eastern Cheney-Palouse scabland tract. Dozens of huge and then smaller Missoula floods descended Grand Coulee, Quincy basin, and lower Crab Creek. The great Missoula floods dwindled and ceased before 13,000 BP.

The high limit of Missoula floods down the Columbia mainstem descends from 485 m altitude at Wenatchee to 375 m in northern Pasco basin to 365 m at Wallula Gap. The high limit of megaflood down lower Snake valley (from Cheney-Palouse scabland) descends from 400 m altitude at the Palouse confluence to 330 m at Snake River Junction—graded downcurrent at Wallula Gap to a flood level of about 325 m. At Wallula Gap, peak flood via the Snake is some 40 m below the peak flood via the mainstem Columbia and Moses Coulee. Peak flow down the Cheney-Palouse tract in the east apparently came long after peak flow down the Columbia and Moses Coulee in the west.

After the Missoula floods ceased, a smaller megaflood descended mainstem Columbia valley about 12,500 BP from glacial Lake Columbia in northern Washington and yet smaller ones about 11,200 BP from glacial Lake Kootenay in southern British Columbia.