Willamette Falls Geology: A Story of Two Floods

Thu 20 Apr 17

Hundreds of people visited the Willamette Falls Legacy Project booth at downtown Oregon City’s First City Celebration on July 23. Project staff, designers and volunteers from the friends group were on hand to talk with visitors, answer questions and seek community input on seasonal activities.

The creation of Willamette Falls is the story of two floods. First came a flood of lava creating the foundational basalt bedrock that shaped the falls we know today. Millions of years later came a series of floods so high it covered the entire Willamette Valley with four hundred feet of water at its height.

We sat down with Dr. Scott Burns, Professor of Geology at Portland State University—and outstanding Oregon scientist— to learn more about what historic and geologic events came together to create the falls we see today.


Volcanoes Create Basalt Formations

Sixteen million years ago volcanos three hundred miles away starting erupting. In a land where the Cascade Mountains had yet to appear, miles of liquid magma flowed overland, displacing the Willamette and Columbia rivers as it found its way to the Pacific Ocean along their riverbeds.

“These volcanoes were located where Oregon, Washington and Idaho meet today,” says Dr. Burns “they’re the kind of volcanos we often see in Hawaii now—where magma pours out, creating land as it cools.”

Multiple flows ensued for primarily the next two million years creating countless layers of rock that we call today Columbia River Basalt.


Ice Age Floods Further Carve the Falls

Fifteen-to-eighteen thousand years ago a series of cataclysmic floods, called the Missoula Floods, swept periodically through the region further carving out the Willamette River, Willamette Falls and creating the space West Linn and Oregon City occupy today.

“The Missoula Floods were the result of periodic, sudden breaks of the ice dam on the Clark Fork River that created Glacial Lake Missoula in Montana,” says Dr. Burns “after each ice dam broke, thousands of square miles of water would rush down the Clark Fork and the Columbia River, flooding much of eastern Washington and the Willamette Valley. After the rupture, the ice would reform, creating the dam again until the pressure mounted and it broke the next time. We think there were about forty of these floods over the next three thousand years that reached Portland—though the first was the largest.”

“We’re not sure where on the river the Willamette Falls were originally,” says Dr. Burns, “but we know the Missoula Floods brought the falls back to where they are now by eroding the basalt each time they came through.”

The Missoula Flood flooded eastern Washington and the Willamette Valley up to 400 feet. Image by Wine Folly.


More About the Falls

“A lot of people don’t know this,” says Dr. Burns, “but the Willamette River is tidal all the way until the falls.” Meaning you can see the tide rise and fall along the Willamette all the way from where it meets the Columbia, through downtown Portland to the base of Willamette Falls. Turns out the tide can’t jump the falls like spring Chinook!

The Willamette River is also the longest river in the United States that flows primarily north and has been called “The River that Flows Uphill”. It’s the 13th largest river in the United States in volume of water and begins its nearly three-hundred-mile journey in the high Cascades southeast of Eugene. Nearly 70 percent of the state’s population lives in cities along its banks.

“One more thing,” Dr. Burns says on our way out, “remind people Willamette Falls is the second-largest waterfall in North America by volume—second only to Niagara Falls!”

You got it Dr. Burns, thanks for the interview!

Dr. Scott Burns, Professor of Geology at Portland State University and a specialist in our region’s geology.

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