Many geologic features of the region of the northwestern United States known to geologists as the Channeled Scabland were once thought to be the work of gradual erosion. But early in the twentieth century, geologist J.Harlen Bretz theorized that the features were caused by a single catastrophic flood approximately 15,500 to 13,000 years ago, during the last ice age: floodwaters deposited gravel bars hundreds of feet high and carved out huge potholes and canyons. A series of high-water marks on the sides of hills in the vicinity of the then-extant Lake Missoula suggested to geologists that this lake (which no longer exists today) had been the source of the floodwaters.

However, in 1977 geologist Richard Waitt observed evidence apparently contradicting the notion of a single catastrophic flood. Two layers of ash that had been spewed out in past eruptions of the Mount St. Helens volcano made up part of a 40-laver stack of sediments thought to have been deposited by the flood. Despite the fact that the rest of the stack of sediments could not have been deposited had the water not remained high, the ash avers were almost at the bottom of the stack. Volcanic ash could not have drifted down through hundreds of feet of turbulent water to settle in neat layers; it does, however, settle in that manner on dry land. By hypothesizing that multiple catastrophic floods, rather than a single flood, had

occurred, Waitt was able to suggest that each layer of ash had settled on dry land after one flood and then was buried under sediments deposited by a subsequent flood.

Waitt’s theory posits that a protruding lobe of the vast Laurentide lce Sheet had crept southward, damming up a river, and that water pooled up behind the dam, as high as 1,800 feet, to create Lake Missoula. By observing smaller-scale floods from ice-dammed lakes in places like iceland, geologists have suggested how Lake Missoula could have been repeatedly unleashed. These outburst floods, called jokulhlaups, occur when the level of the water reaches about nine-tenths of the height of the dam. The ice dam becomes slightly buoyant and rises, allowing water to leak underneath and eventually create a tunnel through which the water rushes. At glacial Lake Missoula Waitt thinks that the tunnel enlarged so much that the ice dam collapsed and the lake drained out; soon afterward the southward-creeping ice lobe stemmed the flow and the process started over again.

The example of the Channeled Scabland has enabled geologists to propose jokulhlaups as the cause of other landforms previously attributed to erosion or to grinding by glacial ice, not only in North America, but also in Asia where jokulhlaups nearly as great as those that burst from Lake Missoula may have carved the landscape in the Altai Mountain region of south-central Siberia.