An outrageous hypothesis
In the 1920s, geologist J Harlen Bretz proposed a theory that was dismissed by the scientific community for decades. He suggested that the bizarre, eroded landscape of eastern Washington—a region he named the Channeled Scablands—was formed not by slow, gradual processes, but by a catastrophic flood of unimaginable scale. For forty years, his "outrageous hypothesis" was rejected in favor of uniformitarianism, the principle that the same slow geological processes seen today have always been at work.
The source of the flood was a mystery until another geologist identified Glacial Lake Missoula. Near the end of the last ice age, a lobe of the Cordilleran Ice Sheet crept south and blocked the Clark Fork River in Idaho, creating an ice dam up to 2,000 feet high. Behind it, water collected, forming a vast lake that held over 500 cubic miles of water—Lake Erie and Lake Ontario combined.
When the immense pressure of the water lifted the ice dam, it failed catastrophically. The lake drained in as little as 48 hours. The discharge rate reached an estimated 10 cubic kilometers per hour, a flow ten times greater than all the rivers of the world combined, rushing across the landscape at speeds up to 65 miles per hour. This cycle of dam formation and failure repeated dozens of times over about 2,500 years, with the last major floods occurring between 14,000 and 18,200 years ago.
A landscape scoured to bedrock
The torrents of water and ice were powerful enough to strip away hundreds of feet of soil and loess, exposing the hard volcanic basalt beneath. The floodwaters carved a 16,000-square-mile network of interconnected, dry channels, canyons (coulees), and basins. This created dramatic geological features in North America.
One feature is Dry Falls. This remnant of a waterfall that was once 3.5 miles wide and 400 feet high—four times the size of Niagara Falls. The flood also carved the Grand Coulee, a canyon that is in places up to three miles wide with walls a thousand feet high.
As the floodwaters slowed, they created other enormous features. Giant current ripples, some as high as 50 feet with several hundred feet between crests, formed on the landscape. The floods also transported and deposited countless glacial erratics, boulders of granite and other rock types not native to the area. Some of these erratics are the size of small houses. Bretz's theory was finally accepted in the 1970s, vindicated by satellite imagery that showed the channels exactly as he had mapped them decades earlier.