Clark Fork River
Imagine a glacier filling this valley so that only a few mountaintops poked
above the ice. The glacier, a southern tongue of the great Cordilleran ice
sheet that covered western Canada in the last ice age, dammed up the Clark Fork
River, creating Glacial Lake Missoula. When the ice dam (which stood in the far
right of this image) burst under the lake's enormous pressure, up to 500 cubic
miles of water barreled down this valley in just 48 hours.