MAPPING THE MISSOULA FLOODS, EASTERN WASHINGTON

During the last glacial maximum, massive floods from glacial Lake Missoula swept across eastern Washington. These floods eroded hundreds of square kilometers of eastern Washington, carving the Channeled Scabland. Geomorphic mapping of erosional and depositional features delineates flood pathways, maximum flood stages, and area and extent of flood deposits. We’ve conducted this mapping chiefly with lidar, supplemented by 10-m DEMs, aerial photography, existing mapping, and field observations. The current effort focuses on a 9,000-km² area of the Spokane and Columbia valleys between Spokane and Grand Coulee, and a 1,000-km² area of the northern Cheney-Palouse scabland tract. Large erosional features include stripped and scoured basalt surfaces, in places eroded into basin-and-butte topography of high vertical relief; particularly common in the Cheney-Palouse tract. The stripped bedrock carved into loess-covered upland shows the extent and magnitude of flood channeling. Large coarse-grained gravel bars identify areas where currents slowed. Visible trim lines and divide crossings enable estimates of maximum flood stages that can be fit by surfaces representing approximate maximum flood inundation. The highest post-flood terraces along the Spokane and Columbia Rivers, as much as 300 m above the valley bottom, grade to different levels of glacial Lakes Columbia and Spokane (respectively impounded by the Okanogan and Columbia ice lobes). Younger Holocene alluvium flanks most modern channels and waterways. This mapping is part of an ongoing endeavor to compile a 1:250,000 scale geomorphic map and geodatabase for ice-age floods in the U.S. Pacific Northwest.