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In the summer of 1922, and for the next seven years, Bretz conducted field research of the Columbia River Plateau.
He had been interested in unusual erosion features in the area since 1910 after seeing a newly published topographic map of the Potholes Cataract.
Glacial deposits overlaid with centuries of windblown sediments (loess) have scattered steep, southerly-sloping dunes throughout the Columbia Valley, ideal conditions for orchard and vineyard development at higher latitudes.
After analysis and controversy, geologists now believe that there were 40 or more separate floods, although the exact source of the water is still being debated.
During the last deglaciation that followed the end of the Last Glacial Maximum, geologists estimate that a cycle of flooding and reformation of the lake lasted an average of 55 years and that the floods occurred several times over the 2,000-year period between 15,000 and 13,000 years ago. Within the Columbia River drainage basin, detailed investigation of the Missoula floods' glaciofluvial deposits, informally known as the Hanford formation, has documented the presence of Middle and Early Pleistocene Missoula flood deposits within the Othello Channels, Columbia River Gorge, Channeled Scabland, Quincy Basin, Pasco Basin, and the Walla Walla Valley.The peak flow of the floods is estimated to be 40 to 60 cubic kilometers per hour (9.5 to 15 cubic miles per hour).Waitt's proposal was based mainly on analysis from glacial lake bottom deposits in Ninemile Creek and the flood deposits in Burlingame Canyon.Based on the presence of multiple interglacial calcretes interbedded with flood deposits, magnetostratigraphy, optically stimulated luminescence dating, and unconformity truncated clastic dikes, it has been estimated that the oldest of the Pleistocene Missoula floods happened before 1.5 million years ago.Because of the fragmentary nature of older glaciofluvial deposits, which have been largely removed by subsequent Missoula floods, within the Hanford formation, the exact number of older Missoula floods, which are known as Ancient Cataclysmic Floods, that occurred during the Pleistocene cannot be estimated with any confidence.