Glacial Lake Missoula and the Floods That Remade the West

Roughly fifteen thousand years ago, a finger of the Cordilleran ice sheet crept south into the Idaho panhandle and plugged the Clark Fork valley with a wall of glacial ice nearly two thousand feet high. Behind that dam, meltwater pooled into a vast inland sea geologists now call Glacial Lake Missoula. At its greatest, the lake held roughly five hundred cubic miles of water, roughly half the volume of present-day Lake Michigan, and spread across more than two thousand nine hundred square miles of western Montana, submerging the future site of the Missoula valley under hundreds of feet of water.

The dam, however, was made of ice rather than rock. Periodically, the rising water pressure floated the ice dam off its bed, and the entire lake drained in a catastrophic torrent. Geologists estimate the outburst floods reached speeds approaching sixty-five miles per hour and a discharge roughly ten times the combined flow of all the world's rivers today. The water raced west through Idaho and across eastern Washington, gouging the strange scarred terrain now called the Channeled Scablands, a stripped, tilted landscape of coulees, dry waterfalls, and giant ripple marks up to fifty feet high and three hundred feet long.

This catastrophe did not happen once. The ice dam reformed, the lake refilled, and the cycle repeated dozens of times over roughly two thousand years. Geologists count at least forty separate megaflood events, perhaps as many as one hundred.

The man who first understood what had happened was a University of Chicago geologist named J. Harlen Bretz. In the nineteen twenties, Bretz proposed that the Scablands had been carved by a single inconceivable flood. His peers were appalled. Catastrophism had been out of fashion since the Enlightenment; slow, gradual processes were the respectable explanation for landscape change. When Bretz presented his findings to the Geological Society of Washington in nineteen twenty-seven, the room was packed with eminent geologists prepared to denounce him. His theory was called preposterous.

Decades passed. In nineteen forty, a Montana geologist named Joseph Pardee presented evidence of the giant ripple bars left on the floor of the old lake basin, proof that a massive body of water had drained with violent speed. Gradually the geological community came around. In nineteen seventy-nine, J. Harlen Bretz received the Penrose Medal, the highest honor of the Geological Society of America, at the age of ninety-six. He reportedly told his son that all his enemies were dead, so he had no one left to gloat over.