ANCIENT CATACLYSMIC FLOODS IN THE PACIFIC NORTHWEST; ANCESTORS TO THE MISSOULA FLOODS

The Missoula Floods left erosional and depositional features behind in eastern Washington, now termed the scablands. At least 89 floods occurred when Glacial Lake Missoula’s ice dam repeatedly ruptured and re-formed between 15 to 18 thousands years ago, with 40 of the floods reaching Wallula Gap (Allen et al., 2009). Much evidence of older flood deposits has been wiped out by the Missoula Floods. By searching geological literature, locations with preserved evidence of much older flood deposits can be found. Geologists have sited magnetic reversals in flood sediments that date some deposits to at least 780,000 years before present. Further evidence can be found in the stages of caliche development in paleosols, which takes longer than 15,000 years to form. Field sites referenced by other authors as well as newly discovered sites have been visited, providing a basis for correlation of the flood deposits across eastern Washington and Oregon. At one site, Reese Coulee, as many as eight ancient flood deposits and paleosols are present (Bjornstad, 2006).