#63278: The Upper Grand Coulee

The Grand Coulee, a National Natural Landmark, was carved by the massive Ice Age Floods. An ice dam blocked the Clark Fork River on the Idaho/Montana border, creating glacial Lake Missoula, which contained more water than Lakes Erie and Ontario combined. When that ice dam gave way, the massive waters rushed forward in to Washington, carving coulees and depositing huge boulders, gravel bars and sand dunes along the Coulee Corridor.