Missoula floods

Near the end of the last ice age, a lobe of the Cordilleran ice sheet blocked the Clark Fork valley in present-day Montana and Idaho, impounding Glacial Lake Missoula, which held about 2,000 cubic kilometres of water, about as much as Lake Erie and Lake Ontario together. When the ice dam failed, the lake drained within days across Idaho and Washington, over the Columbia River basin and out to the Pacific. The dam re-formed and the process repeated many times, mostly between about 15,000 and 13,000 years ago, though some estimates start earlier.¹

J Harlen Bretz, a geologist at the University of Chicago, first proposed in the 1920s that the odd landscape of eastern Washington, the Channeled Scablands, with its dry waterfalls, huge gravel bars and house-sized boulders, was cut by a vast flood. Most geologists, favouring slow change, rejected the idea for decades. In the 1940s Joseph Pardee pointed to Lake Missoula as a source, and in the 1960s aerial photos and studies showed giant ripples and channels that could only be flood features. Bretz received the Penrose Medal in 1979, aged 96. Peak flows may have reached about 10 cubic miles (roughly 40 cubic kilometres) an hour.¹