Grand Coulee Columbia Rivers pathway and onto the southern plateau creating an ice dam. This dam backed up the waters of the Columbia into Glacial Lake Columbia and later during the Missoula Floods forced those waters into eastern Washington, creating the Scablands. The river at Grand Coulee found no existing valley and thus forged its own pathway across the divide, creating the Upper Coulee. The plateau is not level, but is marked with wrinkles and upfolds of the basalt. The diverted waters of the Columbia, encountered the monoclinal flexure, a steep warping up of toward the northwest. Lake Columbia topped the ridge
Foster Coulee Foster Coulee Foster Coulee is a coulee in Douglas County, Washington. Like the larger Moses Coulee nearby, it was formed during the Missoula Floods at the end of the last ice age, some 14,000 years ago. Washington State Route 17 between Bridgeport, Washington and Coulee City, Washington follows East Foster Creek along much of the coulee's valley floor. East Foster Creek now flows east to west through Foster Coulee to Grand Coulee. The United States Geological Survey quadrangle map "Foster Coulee" is named for this prominent feature. Foster Coulee was formed during the Missoula Floods. The Okanogan lobe of the