Bottom Sediments Provide Additional Clues to Glacial Lake Missoula's History

During the latter stages of the last ice age, 21,000 to 14,000 years ago, an immense lake was created in  western Montana basins when  glacial ice, greater than 640-meters (2,100 feet) high, dammed the Clark Fork River at the site now occupied by Lake Pend Oreille, in northern Idaho.  This lake, known as glacial Lake Missoula, held as much water as the combined volumes of Lakes Erie and Ontario. Once the water level reached a critical depth, the ice dam began to float allowing water to find its way under the ice, when suddenly the ice dam collapsed releasing an enormous discharge of water that had only one way to go. In only a few days between 2100 to 2500 cubic kilometers (500 to 600 cu. miles) of torrential floodwater swept across northern Idaho, eastern Washington, and Oregon, down the Columbia River gorge finding its way to the Pacific Ocean at Portland, Oregon. In its wake it left giant flood bars, trains of huge current ripples, and a ravaged landscape known as the “Channeled Scabland” in eastern Washington. The Lake Missoula floods, often referred to as "the ice age flood story," probably involved the largest freshwater discharges in the geologic record; about 20 times greater than the average worldwide runoff, with computer modeled peak discharges between 2.7 to 14 million cu. meters/sec or 2.3 to 12 cubic miles/hour (Clark, et al., 1984).

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For more than 100 years, numerous investigators have endeavored to decipher the relationship between glacial events, the Lake Missoula floods, and the scabland complex of eastern Washington. Publications and scientific presentations, beginning in 1910 until today, have produced some of the most acrimonious scientific debates ever recorded in the geological literature (Pardee, 1910, 1942; Bretz, 1930, 1969; Baker, 1973; Baker and Bunker, 1985). In his comprehensive review of the evidence for repeated catastrophic outbursts from glacial Lake Missoula, Bretz (1969) remarked that very little was known about the lake bottom sediments, except that they were varved and contained randomly distributed ice rafted debris. He believed that an investigation of these deposits would provide valuable clues to the question of how many times the lake drained and refilled. Bretz also suggested that each lake sequence should be separated by an unconformity, with bogs and forests occupying the drained lake floor only to become buried when a new lake formed.  Some studies suggest dozens and even as many as 90 or more lake drainings and floods (Alt and Chambers, 1970; Chambers, 1971, 1984, 1989; Waitt, 1980, 1984, 1985; Alt, 2001; Hanson, et al., 2012).