Between 18,000 and 13,000 years ago, glaciers advanced and retreated over eastern and central Washington. This ice formed a fragile dam on the Clark Fork River, which slowly filled with water, forming glacial Lake Missoula. The lake spanned hundreds of miles over present-day Montana. But the ice dam that held the water in place was fragile. Periodically, the dam burst open, sending the entire lake hurtling down the western slope of the continent. It is estimated that these floods carried as much as fifty cubic miles of dirt and rock with them. Over time, the ice dams reformed, creating a millennia-long flood cycle that might have taken place 60 or even 100 times.
These hills are typical of landscapes created by the slow movement of glaciers. Rippling features in Montana prairies, the “channeled scablands," and the mounds of loess in the Washington Palouse also suggest the flow of water. As far back as the beginning of the twentieth century, scientists have theorized on what could have caused these features. In 1923, the geologist J. Harlen Bretz published the first paper on the theory that massive floods created these features. More modern techniques have provided even stronger evidence, matching local rock and soil to materials spread across the West and the shore of the Pacific. The broad strokes of this geologic history are widely agreed on today, but the floods still presents mysteries to scientists.
The most important legacy of these ice-age floods is the ground beneath our feet. These floods moved soil nutrients and broke through thick layers of volcanic rock, priming the area for farming. By gouging canyons across the state, they even influenced the course of the Columbia basin, setting the stage for the human history that would follow. When we stand on the banks of the Columbia today, we are standing in the history of the local Native American cultures and of later settlers. But we are also standing in the long history of Earth itself.