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The Big Floods — Tualatin, OR
Posted by: Groundspeak Regular Member Dunbar Loop
N 45° 21.663 W 122° 47.060
10T E 516890 N 5023081
Quick Description: Located near the Ibach Park playground, this historic sign helps explain the enormous floods that took place when prehistoric Lake Missoula let loose torrents of water during the Ice Age.
Location: Oregon, United States
Date Posted: 1/20/2012 11:10:10 PM
Waymark Code: WMDJJF
Published By: Groundspeak Charter Member Moun10Bike
Views: 5

Long Description:
The Big Floods

A Fiery Start

Look off to the east toward the basketball courts. On a clear day you may see Mt. Hood on the horizon, a snowy reminder of what was once a fiery volcano.

Volcanoes shaped much of the Northwest landscape. The recent eruption of Mt. St. Helens bears testimony to the power that still lurks beneath the Cascades, power that once covered much of Oregon with layers of basalt lava.

The rocks you see before you were probably placed here by a bulldozer, not a volcano, but other rocks, of a type not usually found in Oregon, were once scattered across the broad Willamette Valley. Where did they come from, these huge, house-size boulders of granite and rhyolite, quartzite and pegmatite? How did they get there? Geologist wondered the same thing.

The Bretz Floods

A geologist from the University of Chicago came up with an extraordinary theory. In 1923, J. Harlan Bretz proposed a flood, an enormous ice-age flood, that carried these huge boulders, called erratics, locked inside icebergs all the way from the glaciers in Montana, here to the Willamette Valley.

If you think Dr. Bretz's theory of a massive flood sounds a little far fetched, you are not alone. Bretz's own colleagues scoffed at his ideas. Many could not fathom the idea of such an enormous flood, a flood that would carry huge boulders thousands of miles, and scour the face of four states before rushing down the Columbia River to the sea. It took the advent of aerial photography to put together the pieces of this ancient geologic puzzle.

Dr. Bretz's flood theory is now widely accepted, though no less extraordinary. The Bretz or Spokane Floods, as they are called, were a series of cataclysmic floods that originated during the last ice age, about 15,000 years ago.

Lake Missoula

The process that would so drastically shape the Northwest landscape started in the Idaho panhandle when a narrow finger of the Continental Ice Sheet called the Purcell Lake dammed off the Clark Fork River. A huge lake, called Lake Missoula formed behind this half-mile high wall of ice.

Lake Missoula was hundreds of miles long, covering 3,000 square miles of western Montana. It was 2,000 feet deep at the ice dam, twice as deep as Lake Superior. Lake Missoula held 500,000 cubic miles of water, pressed against a wall of ice.

Imagine how loudly the ice dam groaned. How quickly the ancient ice shattered. When the ice dam could no longer contain Lake Missoula, it took on 48 hours -- two days for the lake to empty.

A Wall of Rushing Water

The wall of water that rushed west from Lake Missoula was 400 feet high. It moved at speeds of 65 miles per hour. This massive flood of water, ice and debris blasted through formations of columnar basalt formed during even older floods of molten lave, leaving deep canyons called coulees.

Huge rivers flowed over the broad plateau of eastern Washington, carving a streambed 100 miles across. Massive waterfalls formed in a landscape forever changed. the largest torrent, at the lower end of the Grand Coulee was 3.5 miles wide over 300 feet high.

Floodwaters backed the Snake River for 100 miles, reversing the river's flow. They literally roared down the Columbia River Gorge until they hit the narrows near present day Kalama, Washington. here, the flow slowed, A backwater formed, forcing water south into the Willamette Valley.

Lake Allison Covers Portland

Have you ever heard of Lake Allison? You won't find on any modern maps of Oregon. Lake Allison is the name given the arm of the Bretz floods that reached south into Oregon, filling the Willamette Valley all the way to Eugene.

Lake Allison was named for Ira Allison, the geologist who mapped the location of hundreds of boulders scattered across the Willamette valley. These glacial erratics mark the high waterline of Lake Allison at 400 feet. Imagine Portland submerged under 400 feet of water. The West Hills would be little more than islands in a vast sea. Would your home have been spared?

It Came From Outer Space

The Willamette Meteorite is perhaps Oregon's most famous relic from the Bretz floods. This huge hunk of nickel-iron fell to earth millions of years ago, probably somewhere on the ice sheet over Montana or southern Canada. It was one of the many glacial erratics carried west by the surging waters of the Bretz floods.

In 1902, a local farmer named Ellis Hughes found the Willamette meteorite in the woods about two miles of the Tualatin River from West Linn. He hitched his horse to a crude wooden wagon, enlisted the help of this wife and son, and spent the next three months, inched the 15 ton meteorite nearly a mile to his own property. There he built a shed and charged the public 25 cents to gaze at his unusual find. Would you have paid to see the meteorite?

The journey of the meteorite did not stop there. Oregon Iron and Steel, who owned the land where the huge stone was found, sued Hughes for the meteorite's return. They won. In 1905, the meteorite was rafted down the Willamette river for display at the Lewis and Clark World's Fair. It was then shipped east to the New York Observatory, where it remains to this day, despite efforts to return it to Oregon soil.

The Tonquin Scablands

The Tualatin River Valley hears its share of scars from the Bretz floods. Oregon geologist, John Allen, suggest that the floods change the course of the Tualatin River, moving its mouth north from Lake Oswego, to where it now joins the Willamette at West Linn.

Water rushing up the Tualatin River breached a low divide near the town of Tonquin, between Sherwood and Tualatin, just a few miles southwest of Ibach Park. The floods carved a series of 14 channels in the balsalt divide -- the Tonquin Scablands.

Scablands is an unfortunate term geologists use to describe areas where soil has been scraped away, exposing irregular areas of the underlying basalt. Can you think of a better name for these scars on the landscape?

The Tonquin Scablands are western Oregon's best example of this unique landform. yet, they are dwarfed by the channeled scablands of eastern Washington which bore the brunt of the flood's enormous power.

An Ancient Mystery

It may not be Jurassic Park, but Tualatin had its own ancient animals. A massive jaw bone was found when early farmers drained the wetlands behind what is now the Tualatin Fred Meyer store. The fields were planted to onions which grew particularly well in the thick, black soil. And the bone? It remained a mystery.

The remains of this strange creature were not fully unearthed until 1962, when a student named John George followed up on rumors of strange teeth and a rib bone found in Charlie Robert's tomato field back in 1947. George excavated the old tomato field for a science project. Under buckets full of thick, black mud he found the left side of the animal well preserved. The bones were boxed and taken to Washington Park Zoo for identification. What do you think they found?

The Tualatin Mastodon

The bones were those of a female mastodon, 8 feet tall and 12 feet long, now on display at the Tualatin City Offices. She probably met her death in a pocket of quicksand that rimmed the many swamps of the Tualatin Valley 11,000 to 12,000 years ago.

What is a mastodon? It most closely resembles a present-day elephant with a thick, shaggy, reddish-brown coat. Mastodons were slightly smaller than wooly mammoths and had narrower tusks which allowed them to move more easily through groves of alder, cottonwood and oak. The Tualatin mastodon probably shared her ancient Oregon landscape with sabre tooth tigers and giant sloths. What do you think happened to these strange animal species?

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