Artist's painting (Byron Pickering) of Glacial Lake Missoula

According to geologists, Glacial Lake Missoula was formed about 15,000 years ago during the last ice age when the Clark Fork River was dammed by ice near Sandpoint, Idaho. The water was backed up almost to the Continental Divide on the east, through the Bitterroot Valley to the south and as far north as Polson, which was the terminus of another tongue of the British Columbia glacier. The glacial ice went from over 5,000 feet thick in British Columbia to approximately a quarter of a mile deep at Polson. Periodically, the dam broke at Sandpoint creating tremendous floods that scoured the scab lands in the Columbia Basin and created the Columbia Gorge.