MAPPING THE MISSOULA FLOODS: THE COLUMBIA RIVER GORGE

Late Pleistocene floods from Glacial Lake Missoula surged though the Columbia River Gorge, inundated the Willamette Valley, and flowed out to the Pacific Ocean dozens of times over the course of a few thousand years at the end of the Last Glacial Maximum. Geomorphic mapping of the Missoula Flood province for a 230-km-long stretch of the Columbia Valley between Biggs Junction, OR, and Longview, WA, from lidar, 10 m DEMs, aerial photography, and existing geologic maps shows a wide variety of erosional and depositional features as far downstream as the Cowlitz River Valley in western Washington. Erosional features such as stripped and plucked bedrock are prevalent where flood waters were confined, such as in the narrow Columbia Gorge. Massive coarse-grained flood bars are found in areas where the immense flows slackened due to widening flow paths or to eddying in Columbia River tributary mouths. Expansive deposits of slackwater sand and silt rhythmites blanket extensive backwater areas such as the Willamette Valley. Mapping of Missoula Flood features can help constrain the timing, magnitude, and geomorphic consequences of the cataclysmic floods. This map is part of a planned ~1:250,000-scale map and associated database of the Missoula Flood province that extends from Clark Fork, Idaho to Astoria, Oregon.