The parallel lines etched on the side of Mount Jumbo are testimony to a series of dramatic geologic events, the Lake Missoula Floods. Occurring as recently as 15,000 years ago, the floods resulted when the Clark Fork River, which had been dammed by glaciers near the entrance to Lake Pend d'Oreille near Sandpoint, Idaho, broke through its constraints. As the melting glacier dam failed, an estimated 200 to 400 cubic miles of escaping water created a river that contained ten times the water of all the rivers in the world. The torrent rocketed downstream with a force that tore through 3,000 square miles of Columbia Plateau, carving magnificent geologic formations in its wake.
The dramatic event repeated itself over the years as glaciers again moved into position to block the entrance to Lake Pend d'Oreille, refilling Lake Missoula to the combined volume of Lake Ontario and Lake Erie. As the glaciers again melted, the violent flooding reoccurred, creating the "scablands," some 2,000 miles of raw, peeled ground that stretches from Spokane west to the Cascades and south to the Snake River. Lake Missoula's tremendous flooding represents one of the most significant geological events in the history of the world.