DURATION AND TIMING OF GLACIAL OUTBURST FLOODS FROM PLEISTOCENE LAKE MISSOULA

Effects of catastrophic drainage of Pleistocene glacial Lake Missoula were simulated using a sophisticated model for flooding and inundation of rugged, three-dimensional terrain. We used a three-dimensional domain of square cells 250m on a side, covering much of the Pacific Northwest of the United States. We simulated the Columbia River blocked with ice near the Okanogan valley to force flow through Grand Coulee and simulated a dam near the northernmost extent of the Clark’s Fork of the Columbia River to create a model of glacial Lake Missoula with a stage of 1250m. Sudden removal of the dam holding Lake Missoula simulated a dambreak flood, and created high model flood stages that match nearly all field indicators of high stage attributed to the Missoula Floods. Catastrophic rupture of the ice dam produced rapid flooding of eastern Washington, rapid filling of Pasco, Yakima, and Umatilla Basins, and then slow drainage of these basins through the Columbia Gorge. Details of this flood scenario are significant for interpreting field exposures. Flow occurs across the Cheney-Palouse Scablands and empties into the Snake River and then Pasco Basin before floodwaters racing along the Columbia River enter Pasco Basin from the north. Maximum rate of filling of Pasco, Yakima, and Umatilla Basins occurs when the rate of influx from the Cheney Palouse Scablands and Snake River is comparable to the influx through the Columbia River system. This initial scenario is unsustainable for more than a few days, however, as dropping stage in the Rathdrum-Spokane valley starves the flow into the Cheney-Palouse. By the third day most flow is through the Columbia River drainage. After three days, Lake Missoula has almost completely drained and the lake volume has been transferred to Pasco, Yakima, and Umatilla basins, with a minor amount draining through the Columbia Gorge downstream. Drainage of these basins through the Gorge requires another 25 days.