Fifteen Million Years in the Making: The Geologist’s Tour of Oregon City

A conversation about deep time, catastrophic floods, ancient fish, and why this particular drop in the river changed everything. This is the origin story for a self-guided geologists tour of Oregon City.

Many people who visit Oregon City think of the more recent, pioneer history. They want to talk about the Oregon Trail, the oldest incorporated city west of the Rockies, the municipal elevator. ~1843 onward.

Dr. AK Kotash wants to talk about 15 million years ago.

Dr. Kotash joined Clackamas Community College‘s geology faculty two years ago. They arrived from a university post in Michigan’s flatlands with a particular relief: Oregon’s topography changes everything about how geology can be taught. Here, you can deliver a lecture and then walk outside to point to an ancient volcano vent or identify Boring Lava basalt being uncovered during a campus renovation.

Dr. Kotash’s academic specialty is undersea volcanoes, but at CCC, they teach introductory geology. They are planning field trips around the extraordinary examples sitting outside the classroom door.

They agreed to meet for lunch at The Highland Stillhouse on South Second Street, a Scottish pub with an open-air deck. Next door is the Museum of the Oregon Territory. It features top-floor ballroom venue views and a curated collection beginning at the moment human written history arrives in this valley. They also have plans for a natural history exhibit that will explore the valley’s formation.

Between us: seasonal mocktails. Below us: Willamette Falls, roaring away as it has for thousands of years, mostly unbothered by the fact that a highway, a museum, and a pub has been built near it.

The few people who stop at the overlook, across Highway 99E from us, think they’re simply looking at a powerful waterfall surrounded by industrial history. What they’re actually looking at is 15 million years of geology, an eroded fault line, the remnants of a catastrophic inland flood, and one of the oldest migratory fish runs on the continent. All at once. In one glance.

That is the story Dr. Kotash agreed to help tell, starting not with what’s most visible, but with the oldest known event and working up from there.

Let’s start at the bottom layer.

Layer One: What Came Before the Flood Basalts

Before the great lava flows arrived, this landscape had a deep history already in progress. The oldest rocks in the Oregon City area are ancient volcanic remnants from the Eocene epoch, now buried under everything that came after. This first layer is essential to understanding the evolution of the Cascade Range and the foundational geology of the Willamette Valley.

Layer Two: The Flood Basalt Lava

Around 16 to 17 million years ago, a series of flood basalt eruptions sent lava flowing across a vast area of the Pacific Northwest from vent systems in eastern Oregon and Washington. Individual flows covered thousands to tens of thousands of square miles. Stacked on top of each other over millions of years, they built up to hundreds of feet in collective thickness. It’s the layered geological record Oregon City sits on today.

From the Stillhouse patio, Dr. Kotash points toward the basalt shelf at the lip of Willamette Falls and identifies what’s visible. The falls go over the Sentinel Bluffs Member of the Grand Ronde Basalt. The columns rising in the bluffs above the river on either side are the Ginkgo flow within the Frenchman Springs Member of the Wanapum Basalt; a younger set of flows sitting above the Grand Ronde.

Between those two formations is a thin but significant layer: the Vantage Member “horizon,” a deposit of sandstone and weathered material that formed during a long pause in eruptions when the exposed surface began to erode. That interval of quiet is visible in roadway cuts in the hills around Oregon City and along Highway 205, though vegetation often obscures it.

“That layer matters more than people realize,” our friend, Tima joins in on the conversation, “It’s a weak zone. It’s part of why this area has significant landslide history.”

Tima Carlson is a registered and licensed geologist with over 20 years of geotechnical experience, specializing in environmental site investigations throughout Oregon and Washington. She also happens to be a co-founder and volunteer at Soulflags Art Community Center on Main Street just across the 99E intersection from the future Tumwata Village site.

“That rock under the water right there,” Kotash continues, gesturing toward the falls shelf, “traveled here almost as a liquid from eastern Oregon. It cooled here. And then it sat for millions of years, resistant to erosion.” 

It was waiting for a fault to find it. More on that shortly.

Layer Three: The Troutdale Formation

After the basalt flows cooled, the landscape entered a quieter chapter. Beginning at the end of the Miocene and continuing into the Pliocene, the ancestral Columbia River spread thick layers of sand, gravel, and rounded cobbles across the basin, burying the basalt under a different kind of record entirely. Geologists call this the Troutdale Formation, and it tells a story less about fire than about water and time: a river system patiently reworking material as it shifted course across the landscape for roughly two million years. As the Cascade Range continued rising to the east, the Columbia’s course changed and re-cut its way through the accumulating gravel more than once, eventually settling into something closer to the channel it follows today. The Troutdale Formation is the buried record of all that wandering, quietly stacking up before the next chapter of volcanic activity arrived to bury it in turn.

Layer Four: The Volcanic Hills You Drive Past Every Day (and they’re not so Boring)

Before the ice age floods arrived, there was one more round of volcanic activity worth knowing about and it happened in Oregon City’s backyard.

The Boring Volcanic Field comprises more than 80 small volcanic vents and lava flows dispersed across the greater Portland basin. Individual cones can reach up to 650 feet above their surroundings. Each vent was the product of a single eruptive event linked to the Cascadia subduction zone and active from roughly 2.7 million years ago. The hills are now threaded through the Portland metro. Mount Tabor, Rocky Butte, and Powell Butte are some of its remnants.

Oregon City sits near the origin point of that entire field. The basalt of Canemah, one of the oldest Boring Volcanic Field units, was dated to approximately 2.5 million years ago from samples at Water Board Park in Oregon City. The hills visible across the plateau east of the city are part of this same system, each the product of a single brief eruption.

Dr. Kotash has a personal anecdote that puts the scale of all this into perspective. While teaching in Iceland and having discussions with Icelandic geologist colleagues, Kotash mentioned that some local formations reminded them of Oregon. The Icelandic geologists stopped them and said Iceland’s volcanic landscape cannot compare to the amazing scale and volume of the flows in the Pacific Northwest. That feels like high praise coming from people who live on one of the most volcanically active places on Earth! 

“We are sitting in the middle of something geologically extraordinary,” Dr. Kotash said, “and most locals and visitors have no idea.”

Layer Five: The Missoula Floods

A massive glacial lake in what is now Montana was held back by an ice dam. That dam failed repeatedly, sending floods racing across eastern Washington and down the Columbia River. As much as 60 cubic miles of water flowed past the site of Willamette Falls during the largest events, filling the Willamette Valley and submerging Oregon City at the confluence of the Willamette and Clackamas Rivers, reaching roughly 500 feet above the current river level.

The floods scoured the river channel and the basalt bedrock directly at Oregon City. They are also responsible for much of the landslide instability in the area, saturating slopes and triggering failures that shaped the terrain we see today.

“We’re sitting on the floor of that lake right now,” Carlson says. “Everything below a certain elevation (500 feet) in Oregon City was underwater.” The soil that made this valley worth settling (the reason people walked 2,000 miles to get here) is flood deposit. 

Layer Six: The Waterfall

The Bolton Fault line, running north of the current falls, created the original step in the basalt. The place where resistant rock was uplifted just enough to produce a drop in the riverbed. The falls began there, at the fault, just under a mile downstream from where they sit today. The Missoula Floods then drove them south (upstream) through repeated scouring of the basalt shelf, to their current position between Oregon City and West Linn.

The Willamette River now plunges 42 feet over the basalt across a crest 1,500 feet wide. It is the largest waterfall in the Pacific Northwest by volume, second only to Niagara in the United States. Dr. Kotash told their geology students this fact and asked whether anyone already knew it. None of them did. 

Many Portland area residents and visitors don’t know it either. Power plants, mills, fences, private property. Three or four generations of people just couldn’t get to it. The waterfall didn’t go anywhere. It just got hidden by industry.

Two projects are working to restore public access to this site. The Willamette Falls Legacy Project is a multi-agency initiative focused on the West Linn side. On the Oregon City side, the Confederated Tribes of the Grand Ronde are developing Tumwata Village at the former Blue Heron mill site. It will be a cultural and community space honoring the deep Indigenous history of Tumwata, a Tribal name for Willamette Falls. 

No fault, no falls. No falls, no fish concentration. No fish concentration, no indigenous gathering place. No gathering place, no city. 

Speaking of fish. 

Living Through the Layers: The Fish That Predate It All

Here is where geology and biology braid together into something genuinely humbling, very cool, and a little weird.

Pacific lamprey have lived on Earth for 450 million years. They were here before the trees.

To them, the Columbia River Basalt flows are recent news. They have been returning to Willamette Falls to spawn for longer than the falls have existed in their current location, adapting through cooling lava, catastrophic floods, and a century of industrial pollution without disappearing.

The tradition of harvesting lamprey at the falls has sustained Indigenous communities for thousands of years. Lamprey are a First Food, alongside salmon and sturgeon, central to the cultures of tribal nations who have fished this site since long before American settlement. Willamette Falls holds profound cultural significance for the Yakama Nation, the Confederated Tribes of Warm Springs, the Confederated Tribes of the Umatilla, and the Nez Perce Tribe, all of which maintain treaty-reserved access to harvest here.

The Yakama Nation Fisheries Pacific Lamprey Project has made Willamette Falls a centerpiece of its scientific and cultural revitalization work. Each summer, the Yakama Nation hosts the Willamette Falls Lamprey Celebration at Clackamette Park in Oregon City — open to the public, free to attend, and unlike any other event in the region. 

Before the 1970s, annual lamprey counts at Bonneville Dam exceeded 250,000 fish. Thirty years later that number had dropped to 52,000. Conservation work is expanding but remains underfunded relative to the scale of the problem. Lamprey and salmon are part of the same ecosystem, which has the potential to fail without this important work to honor, restore and protect it.

What Oregon City Is Built On

While wrapping up our lunch and nerding out over rocks and dinosaur fish, Dr. Kotash mentioned how great it would be to have a self-guided geological tour of Oregon City, perhaps starting from the museum next door. Carlson mentions another once-popular geological site featuring an eroded basalt plug called Coalca’s Pillar that she’d like more people to know about.  We imagined a mapped out path with stop locations, photographs, explanatory graphics, and on-site descriptions that connect each location to the geological story told here. Visitors interested in seeing the basalt layers, fault exposures, flood deposits, and Boring Lava vents in person will be able to follow the route at their own pace and read the landscape the way a geologist would. 

Listen, we’re proud nerds. 

“Oregon City gets credit for being the first city in the state of Oregon,” Carlson says. “But that’s really just the most recent chapter. The place itself is so much older than the city. The city just happened to be built on top of something extraordinary.”

You don’t have to tell me twice! Stay tuned for upcoming information regarding this self-guided Geologists’ Tour of Oregon City; a collaborative project we’re excited to dig into.  

Geologic information in this article is also sourced from the Oregon Department of Geology and Mineral Industries Geologic Map of the Oregon City 7.5′ Quadrangle (GMS-119, Ian P. Madin, 2009) and the U.S. Geological Survey.