St. Peters Dome rising above the January 13 Bucher Creek debris flow that swept across I-84, killing one person (ODOT)

It seems a world away as we enter yet another summer drought, with record-breaking heat waves and an early wildfire season in WyEast country. Yet, just a few months ago, on January 13th, the tragic story of a Warrendale Resident being swept away in her car by a winter debris flow in the Columbia Gorge filled our local news. The event closed a 10-mile section of I-84 from Ainsworth State Park to Tanner Creek and the area was evacuated after the National Weather Service issued a flash flood warning. 

Some of the local media coverage also connected the dots, reporting on the long history of dangerous debris flows in this part of the Gorge. This was not a freak tragedy, but rather, a completely predictable event. The well-known hazard zone stretches from Ainsworth State Park on the west to Yeon State Park, five miles to the east, encompassing the hamlets of Dodson and Warrendale in its path. While the steep walls throughout the Gorge are infamous for producing rockfall and landslides, this stretch is notoriously active. Why?

Slip-sliding away…

Geoscientists don’t have a particular name for this geologically active area, but the unifying feature is a near-vertical wall that I will call the Nesmith Escarpment for the purpose of this article. The name that comes from Nesmith Point, which has the distinction of being the tallest feature on the Gorge rim, rising nearly 4,000 feet from the banks of Columbia River. The Nesmith Escarpment was largely created by the ancient, catastrophic Missoula Floods that shaped much of what we know as the Columbia River Gorge during the last ice, more than 13,000 years ago. These floods repeatedly scoured the Gorge with torrents hundreds of feet deep, often enough to overtop today’s Crown Point and Rowena Plateau.

Tumalt Creek is the largest of the volatile streams that flow from the towering, over-steepened Gorge walls of the Nesmith Escarpment(ODOT)

As the massive Missoula Floods cut into the slopes below Nesmith Point, the over-steepened terrain began to collapse into the river. It’s a process that continues to this day, gradually expanding the escarpment and leaving behind sheer basalt towers of resistant bedrock along the lower slopes. Of these, St. Peters Dome is the most prominent, along with Rock of Ages and Katanai Rock (the informal name for the impressive monolith that rises just east of St. Peters Dome).

The headwaters of Tumalt Creek flow from the highest walls of the Nesmith Escarpment, where the red, volcanic layers of the Nesmith Volcano that rests on the Gorge rim have been exposed by erosion  (ODOT)

Adding to the geologic uniqueness of the Nesmith Escarpment is Nesmith Point, itself. Located at the top of the escarpment, the familiar layer-cake stack of basalt flows that make up so much of the Gorge geology gives way at Nesmith Point to bright red and yellow layers of clay and cinders that reveal the uppermost part of the escarpment to be the remains of a volcano. The northern half of the volcano has been torn away over the millennia by the growing escarpment, leaving a visible cross-section of the volcanic dome. The surviving, southern half of the Nesmith volcano is gently sloping, like other dome volcanoes that line the Oregon side of the Gorge (the familiar peaks of Larch Mountain and Mount Defiance among them).

[Click here for a large version of the schematic]

The result of all this erosion is a 3-mile-long amphitheater of collapsing layers of volcanic debris and basalt walls resting uncomfortably and over-steepened upon ancient sediments at the base of the cliffs that make for a slippery, unstable foundation. Rain, winter freezes and gravity will therefore continue to chip away at the escarpment for millennia.

Over the many centuries since the Missoula Floods, this relentless erosion has built a huge apron of what geoscientists call an “alluvial fan” at the base of the Nesmith Escarpment. This name describes the flood debris that accumulates where canyon streams prone to flash-flooding suddenly reach a valley floor, slowing and depositing debris over time. The resulting layers typically form a broad, gently sloped wedge shaped like a fan. For the purpose of this article, the fan at the base of the Nesmith Escarpment will be referred to as the Nesmith Fan. 

(Source: State of Wyoming)

(Source: City of Scottsdale)

One of the defining features of an alluvial fan is the erratic, constantly shifting course of the streams that create them. Because of their shallow slope and the accumulation of debris, these streams continually change course as they spread their loads of rock and gravel on the fan.

If the Nesmith Escarpment and debris fan were located in a desert environment, these defining features would be exposed and easy to see. But in the forested western Gorge, the dense rainforest vegetation quickly covers debris flows with new growth, often within five or ten years, making it hard to recognize how active the geology really is. It’s therefore easy to understand why settlements like Dodson and Warrendale were built upon on the Nesmith Fan, where the fertile ground and gentle terrain were friendly to farming and home sites. The spectacular cliffs of the Nesmith Escarpment simply provided a beautiful backdrop for these communities. Yet, it’s also an increasingly hazardous place for anyone to live.

The image below shows the Nesmith Escarpment and debris fan in a way that wasn’t possible until LIDAR technology was developed. LIDAR allows highly detailed images of topography even in areas like the Gorge, where dense forests cover the terrain. The LIDAR view shows the steep walls of the escarpment in stark relief, including the hundreds of steep ravines that have formed along the escarpment.

Lidar view of the Nesmith Escarpment and debris fan

The LIDAR view also reveals the alluvial deposits that make up the Nesmith Fan to be a series of hundreds (or even thousands) of overlapping debris flows from the roughly dozen streams that flow from the Nesmith Escarpment, each helping to gradually build the enormous alluvial fan. The wrinkled surface of the fan reveals the hundreds of flood channels that have developed over the millennia as countless debris flows have swept down from the cliffs above.

This view (looking east toward Dodson from Ainsworth State Park) shows the vulnerability of I-84 and the Union Pacific Railroad where they cross the 3-mile-wide expanse of the Nesmith Fan. The 2021 debris flows and flooding damage to the Ainsworth interchange can be seen at the center of the photo, where the interstate was temporary closed by the event (ODOT)

[Click here for a large version of this image]

During the very wet winter of 1996, a series of major debris flow roared down from the Nesmith Escarpment, sweeping cars off I-84 and closing the freeway for several days. A train on the Union Pacific line was knocked off its tracks and many home were damaged.

During the event, debris from Leavens Creek, near St. Peters Dome, swept toward the Dodson area, eventually engulfing the Royse house, which was located near the Ainsworth interchange. The scene was shocking, burying the home in debris that rose to the second floor and destroying outbuildings on the Royse farm. You can read Carol Royse’s riveting account of the event on Portland State researcher Kenneth Cruikshank’s excellent web page describing the 1996 debris flows here.

The Royse House in Dodson (with St. Peters Dome beyond) after a series of debris flows on Leavens Creek engulfed the structure in 1996 (The Oregonian)

The Royse home stood half-buried and visible from the freeway for many years, becoming a prominent reminder of the power of the Gorge. By the mid-2000s, a new forest of Red alder and Cottonwood had already enveloped the debris path and the Royse home, eventually obscuring it from view until the Eagle Creek Fire destroyed both the structure and newly established forest in 2017. 

The more recent debris flows in January of this year struck some of the same spots that were impacted in the 1996 and 2001 events. The Tumalt Creek drainage was once again very active, sending debris onto I-84 and closing the freeway. To the west, the Leavens and Bucher creek drainages also sent debris onto the highway and the site of the former Royse home.

As jarring as these changes are to us, this cycle of destruction, rebirth and more destruction has unfolded hundreds of times on the Nesmith Fan. It’s simply part of the ongoing evolution of the landscape.

How do they start?

Debris flows are a mud and rock version of an snow avalanche. They typically begin with oversaturated soils on steep terrain that suddenly liquifies from its own weight. Once it begins to move, the flow can incorporate still more oversaturated soil as it gathers speed, just as a snow avalanche triggers downslope snow to move. The steepness of the terrain is a key factor in how fast a debris flow can move, and on very steep slopes they can reach as much as 100 miles per hour, though they typically slow as the debris reaches the base of the slope and spreads out to form alluvial fans.

These towering twin cascades where Bucher Creek originates along the Nesmith Escarpment rival Multnomah Falls in height. The impossibly steep terrain here is the source of both the debris and sudden flash floods that have helped build the Nesmith Fan, far below (ODOT)

A heavy rain event can also trigger a debris flow by creating stream flooding that erodes and undermines stream banks, causing debris to slide from canyon walls. This form of debris flow is common in the larger canyons in the Columbia Gorge, but less so on the Nesmith Escarpment, where most of the streams are small and only flow seasonally. Here, it’s the steepness of the slopes and the unstable geology that makes the area so prone to debris flows.

Debris flows are different from landslides. A debris flow is typically quite liquid and fast moving, like cake batter being poured into pan. Landslides are typically slow, with a large mass sliding as a whole, like an omelet sliding from a skillet onto a plate. In the Gorge, landslides are common and mostly occur where the underlying geology is oversaturated and allows the overlying terrain to move. The upper walls of the Nesmith Escarpment are scared by hundreds of landslides, and in the right conditions, these slide can trigger debris flows that spread far beyond the landslide.

What about fires and logging?

A third trigger for debris flows is the sudden removal of the forest overstory. The big trees in our Pacific Northwest forests capture and hold a tremendous amount of rain on their surfaces that never reaches the ground, with some of the moisture directly absorbed by the trees and much of it simply evaporating. Clear cut logging removes this buffer, allowing much more precipitation to suddenly reach the soil, triggering erosion, landslides and debris flows. 

Logging roads are especially impactful by cutting into the soil profile on steep slopes and allowing runoff to infiltrate under the soil layer and destabilized soils. This is well-documented as a source of major landslides in heavily logged areas. Thankfully, most of the forested western end of the Gorge is protected from logging, including the Nesmith Escarpment (though early white settlers logged these areas of the Gorge extensively)

The 2017 Eagle Creek Fire has not only destabilized steep slopes throughout the burn by killing the protective forest cover, it also revealed the tortured landscape of braided flood channels on the Nesmith Fan once hidden under dense vegetation. This image from just after the fire shows a volunteer trail crew scouting Trail 400 where it crosses the fan. The route curves in and out of the dozens of channels and debris piles formed by past flood events

Fire can have a similar effect on runoff when the forest canopy is completely killed. This is why new research shows that attempting to log recently burned areas can have serious effects by disturbing newly exposed soils and worsening the increased erosion that would already result from fires.

In the Gorge, the 2017 Eagle Creek Fire burned most of the Nesmith Escarpment, raising serious concerns about the debris flow activity accelerating here in the coming decades. The debris flows earlier this year may have been the first major events to have been triggered as much by deforestation from the fire as by oversaturated soils. The following photo pair shows the extent of the burn on the Nesmith Escarpment, with the first photo taken just a few weeks before the fire in 2017 and the second photo taken in 2018, when the fire’s impact was clearly visible.

The January 2021 debris flow

The Oregon Department of Transportation (ODOT) has been making regular flights over the Eagle Creek Fire burn since late 2017 to monitor for potential flooding and landslides. While the main purpose of these surveys is to anticipate impacts on the highway, ODOT is also amassing an invaluable library of historic photos that document the fire and resulting geologic events in a way that has never been done before.

Their most recent flight includes photos from the January 2021 debris flows that tell the story in a way that words cannot match:

This view looking west toward the Ainsworth Interchange shows how Bucher Creek had completely covered the south half of the interchange and sent mud and debris flowing east on the freeway, itself. The 1996 and 2001 debris flows impacted much of the same area (ODOT)

A closer look at the 2021 debris flows where the Ainsworth interchange was overwhelmed with debris. A green highway sign marks what used to be a freeway on-ramp (ODOT)

Bucher creek briefly pushed the lobe of mud and debris in the lower right of this view directly toward the home in the first photo, before changing direction to the path the creek is following in this photo. This is a good example of how accumulated debris regularly forces the streams that carry the debris into new channels. (ODOT)

This view looking back at the Bucher Creek debris flow lobe shows just how close it came to the home and outbuildings shown in the previous photo (ODOT)

The view down Bucher Creek debris flow toward St. Peters Dome and the Columbia River from near the crest of the Nesmith Escarpment (ODOT)

Landslide in burned timber near the crest of the Nesmith Escarpment. This landslide fed debris directly into the Bucher Creek debris flow, and onto the freeway more than 3,500 feet below (ODOT)

What to do?

It’s tempting to wish away future geologic hazards by taking comfort from what we perceive to be more predictable past. After all, the modern Gorge we know has been evolving for more than 13,000 years, and long periods of slope stabilization have marked recent centuries. But can we count on periods of stability in a future that will be shaped by global climate change? 

Almost surely not. All indications are for more volatility in both weather and flood events like those that have built the Nesmith Fan. Recent evidence increasingly supports the reality that our landscapes are changing along with the climate. In a 2016 report on landslide risks by Multnomah County, the number of events escalated over the past 25 years, including at the Nesmith Escarpment (see table, below).

The best path for adapting to this reality and becoming more resilient in response to future events is to accept the ongoing risk from the Nesmith Escarpment. In the near-term, this means regularly repairing I-84 and the parallel Union Pacific railroad after flood events that will become increasingly common and disruptive. It also means installing early warning systems along these routes for the traveling public and commers, as well as the residents of the area who live in harm’s way. 

The 2021 debris flow along Tumalt Creek during this year’s series of flood events on the Nesmith Fan was a textbook example of why adapting in the near-term to protect existing infrastructure is a tall order. The following images show just how unpredictable and unmanageable this steam has become for ODOT.

Once Tumalt Creek reaches the foot of the Nesmith Escarpment and begins to flow across the fan, its course continually shifts and changes, making it very difficult to predict where each debris flow event might be headed (ODOT)

A single culvert (above) carries Tumalt Creek under the freeway and frontage road, but the Nesmith Fan is a maze of shifting streambeds by definition, making it nearly impossible to force streams to obey culvert locations (ODOT)

The channel carrying the debris flow on Tumalt Creek that overwhelmed the frontage road and I-84 in February later dried up, with the creek shifting to another channel after the flood (ODOT)

This screen was installed at another culvert that Tumalt Creek has swept through in past debris flow eventsl. While this device might keep small debris flows from overwhelming the culvert, it has no chance against the increasingly large debris flows that we can expect on the Nesmith Fan (ODOT)

This is the view from the frontage road looking upstream at the large, main culvert intended for Tumalt Creek – though it had shifted out of the channel when this photo was taken a few months after the February event. The flatness of the terrain on the Nesmith Fan is evident here, with no obvious stream chanel except for the grading and contouring by highway crews (ODOT)

Adapting to a new reality

In the long term, coping with debris flows also means facing some tough questions for those who live on the Nesmith Fan. For some, it’s a place where families have settled for generations. For others, it’s a dream home they’ve put their life savings into on the Columbia River in the heart of the Gorge. But for anyone who lives here, the risks are real and growing – as the death of a local resident in this year’s debris flows reminds us.

Across the country, climate change and rising sea levels are impacting millions of homes and businesses built in floodplains formerly classified as “100-year”, but now seeing regular flooding. In the past, the U.S. Government has provided public flood insurance for those living or operating a business in a flood zone, but the increasing frequency of catastrophic events in flood and hurricane-prone regions like the Mississippi Valley, Texas, Florida and Carolina coasts is pushing federal flood insurance premiums sharply up. This does not bode well for those living and working in hazard zones in the Pacific Northwest, including the rural communities scattered across the Nesmith Fan.

Notices like this will become a way of life for Nesmith Fan residents in coming years

In some places along the Mississippi Valley, the federal government has begun simply relocating homes, and even whole towns, rather than rebuilding them in harm’s way. Could this be a model for the Nesmith Fan? Possibly, though most of the private homes in the path of debris flows are not in the flood plain, and may not be eligible for any form of subsidized federal insurance or assistance, short of a disaster.

A more direct approach that could be taken at the state level is a simple buy-out, over time. Where flood-prone areas in other parts of the country might simply have value as farm or grazing land, the Gorge is a world class scenic area, and both public land agencies and non-profits are actively acquiring land for conservation and public use. As Gorge locations go, it’s hard to find a spot as spectacular as the Nesmith Fan and the escarpment that rises above it.

Already, the Forest Service and Oregon State Parks have acquired land on the Nesmith Fan for recreation and to provide habitat under the Columbia River Gorge National Scenic Area provisions, including at least two parcels with coveted river access. Permanent funding of the federal Land and Water Conservation Fund last year should also help jump-start public acquisitions in the Gorge that have stalled in recent years, and could help spur land owners considering their options.

Katanai Rock (left) and St. Peters Dome (right) rise above orchards at Dodson in this 1940s view from the old Columbia River Highway

Recent events are surely changing the dynamic for landowners in the Gorge, as well. Would some residents living on the Nesmith Fan be more open to a buy-out after witnessing the devastation of last year’s debris flows, knowing that more are likely to come in the wake of the Eagle Creek Fire? Probably. Others – especially the string of luxury homes along the Columbia River – might be more motivated by legacy, and for these folks, non-profit conservation trusts and easements could be a tool for transitioning private land into public ownership over time.

In the meantime, expect more flooding, debris flows and periodic closures of I-84 during the rainy months. And probably more fires in summer, too. This is the new normal in the Realm of St. Peter, after all, and it’s a cycle that will continue for all our lifetimes, and beyond.