The Okanogan lobe of the Cordilleran Glacier moved down the Okanogan River valley and blocked the ancient route of the Columbia River, backing up water to create Lake Spokane. As the Okanogan lobe grew the Columbia was rerouted into the Grand Coulee. Flowing across the current Grand Coulee-Dry Falls region, the ice-age Columbia then entered the Quincy Basin near Quincy, Washington & joined Crab Creek at Moses Lake, following Crab Creek's course southward past the Frenchman Hills and turning west to run along the north face of the Saddle Mountains, there to rejoin the previous and modern course of the Columbia River just above the main water gap in the Saddle Mountains, Sentinel Gap.