ROUTES TO WENATCHEE OF COLUMBIA VALLEY MEGAFLOODS FROM GLACIAL LAKE MISSOULA AND OTHER SOURCES

Immense late Wisconsin debâcles from glacial Lake Missoula in Montana flooded the Wenatchee reach of Washington's Columbia valley at different times by different routes. The earliest, about 15,500 yr BP, 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 and backflooded through Wenatchee. The Okanogan lobe then also blocked Moses Coulee and the westmost floodway was Grand Coulee that empties into Quincy basin. From Quincy basin's western outfalls to Columbia valley, dozens of huge and then smaller Missoula floods backflowed long distance to Wenatchee. These great Missoula floods dwindled and then ceased before 13,000 BP. The Okanogan lobe continued to dam glacial Lake Columbia for another few centuries until it failed catastrophically about 12,700 BP. Glacier Peak ash fell about 11,600 BP. Some time later two smaller floods swept down Columbia valley from a source in British Columbia. Field evidence and timing argue against these late floods coming from lower Pend Oreille valley, the mainstem Columbia above Castlegar, or Rocky Mountain Trench. Some evidence—bouldery bars, denuded valley sides, local scabland—suggests these late megafloods escaped glacial Lake Kootenay.