Foster Coulee Cordilleran Glacier moved down the Okanogan River valley and blocked the ancient route of the Columbia River, backing up water to create Glacial Lake Columbia. Initially, floodwater discharged from Glacial Lake Columbia by running up through the head of Grand Coulee and down through Foster Coulee to rejoin the Columbia River. After the floods, meltwater from the glacier reversed course and flowed east through the coulee, forming a hanging valley above the west rim of Grand Coulee and a debris fan on the Grand Coulee floor that can be seen today. As the glacier moved further south, Foster Coulee was
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