Mystery Of The Megaflood

One of the Earth's strangest geological riddles is the evidence for a huge catastrophe that struck eastern Washington State thousands of years ago. It took scientists decades to figure out that a colossal flood had carved out bizarre landscape features strewn across thousands of square miles. On "Mystery of the Megaflood," NOVA gets to the bottom of what created this compelling detective story.

"Mystery of the Megaflood" features a dogged geologist sticking to his bold theory for decades despite virtual professional banishment. Eventually, other geologists joined his cause and filled in the intricate details, which NOVA recreates in stunning computer animation to show what may be one of the most spectacular series of events ever to occur on our planet.

The so-called "scablands" are a vast region of weird terrain 200 miles east of Seattle, including gorges hundreds of feet deep, enormous pits, huge boulders scattered as if dropped by giants, undulating hills that look like huge ripples, strange layers of silt and ash, and a "waterfall" five times wider than Niagara, but without any water (see Explore the Scablands). The name "scablands" perfectly suits the scarred and wounded landscape, which baffled most geologists throughout the 19th century and much of the 20th. To them, no plausible explanation fit all the facts.

For example, there is no large river cutting through the scablands that could have carved the features over millions of years—as the Colorado River did in the Grand Canyon. Nor is there evidence that the area was buried beneath glaciers that produced extensive erosion—as occurred in large sections of the American and Canadian Rockies.

But during the 1920s a geologist named J Harlen Bretz outlined a startling hypothesis. His fieldwork convinced him that the scablands were not the result of slow geological weathering, but of an enormous catastrophe that had taken place almost overnight when a titanic flood engulfed the region. Many of his colleagues ridiculed the idea, especially because it smacked of "catastrophism," a discredited view that Earth had been shaped by sudden cataclysms rather than by slow evolutionary change.

Bretz was unable to say where all the water had come from, but a colleague named Joseph Thomas Pardee was certain that the answer lay in the region around Missoula, Montana, where the surrounding mountains held evidence that an enormous lake had once filled the basin (see Ice Age Lake). This lake formed when a glacier plugged the valley below Missoula during the last ice age, creating a natural dam and eventually a body of water that was half the size of Lake Michigan. If that dam suddenly burst, the path of the released water would rush directly over the scablands, scouring exactly the kinds of features that are observed there today.

All that was needed was a natural mechanism to breach the dam and release Lake Missoula—no easy feat since the glacier was probably thousands of feet thick. Such a mechanism was finally discovered after a much smaller glacial dam burst in Iceland in 1996, causing incredible devastation in the valley below.

NOVA takes viewers on a virtual tour inside a glacier to see how tremendous pressure creates tunnels of supercooled water that, over time, fatally weaken the structure of an ice dam, causing it to fail. The current plot twist to the scablands story is that a deluge happened not once, but repeatedly, as ice dams reformed and the glacial lake refilled, only to empty again and again onto the scarred terrain of what is now eastern Washington.