When the dam breaks. (prehistoric flooding)(Four Disasters That Shaped the World)(Cover Story)

The deluges that flooded northwestern North America during the Ice Age began about 15,000 years ago, but they didn't hit geologist Richard Waitt until 1977. Standing in a gorge in the Walla Walla Valley of eastern Washington State in that year, Waitt found himself staring at two layers of ash that, by all accounts, shouldn't have been there at all. The ash, spewed out by volcanic Mount St. Helens, was sitting among a 40-layer stack of sediments thought to have been deposited during a single catastrophic flood -- a flood hundreds of times greater than any seen on the Mississippi River.

The trouble was that the ash layers were almost at the bottom of the stack. That's why Waitt was staring. The water would have had to remain high to deposit the rest of the stack. But volcanic ash doesn't just drift down nicely through hundreds of feet of turbulent water in neat layers. It does settle on dry ground that way, however. "I thought about that all winter," Waitt recalls. "How can I get that ash …