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
Grand Coulee wide as Niagara Falls and three times as high. Steamboat Rock high and a in area, now stands as an isolated rise, but for a time it created two cataract. When the falls passed north of Steamboat Rock, it found a granite base beneath the basal flows. Granite lacks the close vertical joints of basalt, was resisted the erosion from the cataract's plunge. It remains as hills on the broad floor of the Coulee. Some gravel-bar deposits are visible along the Route 155. They provide evidence of eddies in the lee of rock shoulders. Dry Falls is at the head