Lake Lewis was a large transient lake in the Pacific Northwest region of North America, formed by periodic Missoula Floods along the Columbia River paleocourse between 21,000 and 16,000 years ago. Lake Lewis was formed when the restricted flow of waters from periodic cataclysmic floods from Glacial Lake Missoula, pluvial Lake Bonneville, and perhaps fro…Lake Lewis was a large transient lake in the Pacific Northwest region of North America, formed by periodic Missoula Floods along the Columbia River paleocourse between 21,000 and 16,000 years ago. Lake Lewis was formed when the restricted flow of waters from periodic cataclysmic floods from Glacial Lake Missoula, pluvial Lake Bonneville, and perhaps from subglacial outbursts, backed up through the constriction formed by the Wallula Gap in the Horse Heaven Hills. Water also backed up further downstream on the Columbia River between Washington and Oregon, delaying the drainage of Lake Lewis. The water remained for a period of weeks before the flood waters drained through Wallula Gap, just southeast of the Tri-Cities area. Lake Lewis reached an elevation of about 1,200 feet above sea level before subsiding.
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