0:00

city of Portland, light rail, overhead views, animation of waves covering the city

Imagine a city, any modern city. It offers convenience, security, a lot of protection against the unpredictable power of nature.

But sometimes nature is unimaginably forceful. And this is the story of nature out of control. Of a flood that nature unleashed very long ago. Portland, Oregon is a city that has grown in the old flood path. The flood today would swamp Portland's skyscrapers. The buildings would be under 400 feet of water.

Pause tape and ask:

- Does anyone recognize this city?

- How does it look like our city?

- How is it different from our city?

0:50

glaciers calving, sliding down hill

For many thousands of years now, Earth has enjoyed a moderate climate -- one comfortable for human beings.

But just 13,000 years ago, the world was not so friendly. Scientists say some slight variation in the Earth's angle toward the sun made summer weather cooler in the northern hemisphere. The change was enough to tumble the Earth into a deep freeze -- an ice age -- that lasted maybe 15,000 years.

Ask:

- What is happening to the ice?

- Why is it falling?

- Do you know what that is called? (calving)

Pause tape and discuss:

For the last few seconds, what have we been looking at? Rewind the tape and play the scenes of glaciers. Watch and think about what you are seeing.

1:30

animations of the flood

The Ice Age had buried much of the upper half of North America under a very thick sheet of ice.

Near what is now Missoula, Montana, an ancient glacier had blocked mountain rivers and streams, creating an inland sea. By 17,000 years ago, the vast reservoir may have contained 520 cubic miles of water. Its surface covered 3,000 miles.

Pause tape and ask:

Which states are represented on this map?

2:12

animation of lake undercutting, dam failures, Lake Missoula breaking free

The lake was 2,000 feet deep at the point where the ice was holding the water back.

Slowly, pressure forced water under the ice, undercutting the glacier. Finally, in a series of spectacular dam failures, the inland sea -- known to geologists as Glacial Lake Missoula -- broke free.

Pause tape and suggest:

Watch as the ice dam is undercut by water and Lake Missoula breaks free!

2:35

floods across eastern Washington, animation, and then images of the scarred landscape

Lake Missoula probably drained and reformed perhaps a hundred times over 2,000 or 3,000 years. And the resulting inundations were perhaps the greatest floods ever on Earth. Each of the huge roaring torrents bulldozed across much of eastern Washington. They scoured away whole landscapes of eastern Washington's rich soil and pulled up the underlying lava rock. They left behind a scarred landscape that has not healed in more than 10,000 years.

3:15

computer generation of water through the Gorge to the Willamette Valley

And the floodwaters carved a pathway that can still be tracked through the Columbia River Gorge. They deposited much of eastern Washington's topsoil 600 miles away -- in the Willamette Valley south of Portland.

3:25

scientist Richard Waitt, USGS, scraping soil

"A figure that's often used for this is a full bore flood was flowing here at a rate of ten times the discharge of all of the world's rivers. You know that includes things like the Amazon, of course, in that figure. So it's an enormous amount of water. There's nothing like this recorded anywhere else on Earth that we know of." Meet United States Geological Survey Geologist Richard Waitt, "A lot of this stuff is quite rounded. It's traveled some distance ..." Dr. Waitt's careful study of flood sediments reveals details of the floods.

His research -- and that of a pioneer in Missoula Flood study, J Harlen Bretz -- have been essential.

Pause tape and ask:

What is he looking for? (clues in the sediments to the history of the area.)

3:55

J Harlen Bretz

In the 1920's, Bretz's description of the floodwaters -- and his interpretations of the scabbed landscape that they created -- generated scorn among geologists. The floods occurred on a scale so huge, after all, their reality proved almost incomprehensible to Bretz's fellow scientists.

4:35

computer animation of flood over falls

"It's all of Lake Erie plus all of Lake Ontario together. If you can imagine this huge amount of water within two days, maybe, all of it going out and traveling across this landscape. And there's no way the landscape can contain it." Maybe to understand the scale of the floods -- and their impact on the land -- requires aerial study of the path the floods followed. Such a perspective was not available to scientists in the early part of the twentieth century.

Pause tape and ask:

How might having an aerial view of an area help us understand its geology?

Time Marks: 0:00-5:00 | 5:00-10:00 | 10:00-15:00 | 15:00-23:20