Debris Flow

Muddy Fork Debris Flow at 20 Years

/ Tom Kloster / 4 Comments

20 years later, it’s hard to imagine the massive debris flow that roared down the verdant Muddy Fork valley in 2002…

Earlier this year I posted a “then-and-now” look at the changes to the Cooper Spur area on Mount Hood’s east flank over the past 20 years, including dramatic changes to the Eliot Glacier. This article provides a similar look at forces of nature that have once again reshaped the Muddy Fork canyon, on Mount Hood’s steep western flank. 

The story begins in the winter of 2002-03 with a massive debris flow triggered by a landslide in the upper Muddy Fork canyon. The event occurred sometime during the winter season, when deep snow-covered hiking trails, and thus went unnoticed until the snowpack cleared that spring. The event immensely powerful, mowing down whole forests and raising the valley floor of the upper Muddy Fork by as much as 20 feet. Whole trees were snapped off and carried downstream with the debris, forming huge piles that still give mute testimony to the power of the event.

Stacks of downed trees still give mute testimony to the violence of the 2002 event

Though the exact origins of the initial landslide remain unknown, the event was probably not triggered by a collapse within the Sandy Glacier that looms above the Muddy Fork, as there were few signs of the debris flow near the glacier, above the section of the upper canyon where the landslide scars were obvious. Instead, the debris flow likely began as a major slope collapse within the steep confines of upper canyon, where the Muddy Fork tumbles between sheer rock cliffs and steep talus slopes. 

The scars from the 2002 collapse are still plainly visible today (below), but the debris fields it created downstream are rapidly being reclaimed by new forests. The landslide created new cliffs and steep walls within the upper canyon, including new waterfalls along the Muddy Fork where the stream suddenly plunged over the newly exposed bedrock. 

The massive 2002 cliff collapse in the upper reaches of the Muddy Fork canyon gave birth to a debris flow that spread for miles downstream

Below the cliff-lined section of the upper canyon, the debris flow fanned out, spreading the landslide debris across the broad floor of the Muddy Fork valley. Nobody witnessed the event, so it’s unknown exactly how it played out. However, the wide debris fields of rock and sand clearly resulted from a major event, as did the complete removal of a standing forest. 

Trees swept up in the flow were stacked in piles that suggested a lot of water content in the debris flow – as much mud as it was rock – due to saturated winter soils and possibly a sudden snowmelt, perhaps from an unusually mild winter storm.  Whole trees were rafted on their sides until they were beached in giant log jams against forest stands along the valley margins that somehow survived. Evidence of the flow only became apparent when hikers returned that spring to find the Timberline Trail completely erased where it had crossed the Muddy Fork.

The raw cliffs and talus slopes surrounding McNeil Falls are still recovering from the event after 20 years

Over the course of the two decades that have since passed, the Muddy Fork quickly cut through the new debris to reach the old valley floor, revealing splintered stumps from trees that were snapped off during the event, then buried in the debris (below). This has confined the stream to a deep channel in the upper valley that limits its once-meandering ways across the valley floor, at least for now. However, the event only affected the north branch of the Muddy Fork, leaving the south branch almost untouched. 

Large trees by the thousands were snapped off and upended by the debris flow, then buried on the floor of the Muddy Fork valley

The Muddy Fork quickly excavated a new channel in the debris flow, unearthing trees like these that had been buried on their sides under the debris

While the north branch is currently confined to its newly cut channel, this is a temporary condition. Debris flows along Mount Hood’s glacial streams are a nearly constant reality, and even major events like the one that occurred on the Muddy Fork in 2002 are not uncommon. As jarring as these events are to witness, they also give us a privileged glimpse into the very processes that have shaped the mountain we know today. In time, smaller debris flows will gradually choke the current channel with debris, and the Muddy Fork will once again meander across the valley floor – just 20 feet higher than it was in 2002.

The Muddy Fork debris flow then… and now

Though it wasn’t apparent to casual visitors from a distance, hikers who knew the mountain immediately spotted the debris flow from the open slopes of Bald Mountain, where the Timberline Trail provides a sweeping view of Mount Hood’s west face. Before the debris flow, the two main branches of the Muddy Fork had similar floodways at the head of the valley, below the Sandy Glacier. As the 2003 image (below, left) shows, the north branch was suddenly much wider. Today, forest recovery has nearly erased signs of the debris field (below, right) from this vantage point.

[click here for a large version of this photo pair]

A closer look at this photo comparison reveals the scale of the debris flow in the summer of 2003, shortly after the event (below, left). The debris field was up to 1/4 mile wide and left up to 20 feet of debris in the channel of the north branch of the Muddy Fork. After 20 years (below, right), a carpet of green, recovering forest has already reclaimed much of the new debris field.

[click here for a large version of this photo pair]

Down at ground zero, the scene at the head of the Muddy Fork valley in 2003 (below, left) was of astonishing destruction. Whole forests were toppled and piled like matchsticks along the margins of the debris flow, pushing into standing forests just high enough on the valley walls to have escaped the waves of debris. 

Twenty years later (below, right), most of the forest debris remains, though new logs were added to the jumble in the September 2020 wind event (described in this WyEast Blog article) that swept over Mount Hood. The new, mostly decidious forest rapidly emerging on the debris flow can also be seen in the 2023 image as the bright green band in the mid-background.

[click here for a large version of this photo pair]

The debris flow was still raw and unstable in the summer of 2003 (below, left), but after 20 years, a dense young forest (below, right) of Red Alder, Cottonwood and scattered Douglas Fir is quickly stabilizing the debris field. The health and vigor of this young forest growing on a 20-foot layer of boulders, gravel and sand is testament to the remarkable fertility of volcanic soils. While this new deposit contains almost new organic matter or true soil, the mineral content is rich in iron, potassium, phosphorus and other mineral nutrients essential to plant growth.  

[click here for a large version of this photo pair]

Red Alder and Cottonwood are no surprise, here. Both are pioneer species known for their ability to colonize disturbed areas – but the presence of Douglas Fir is a surprise (two can be seen in the foreground of the 2023 image). If the young firs growing within this largely deciduous new growth can keep pace with their broadleaf neighbors, the new forest could begin to be dominated with evergreen conifers within a few decades, speeding up the succession process that typically unfolds in a recovering forest.

Looking downstream (west) from the center of the debris field in 2003 (below, left) provided a true perspective on the scale of the event, with large debris deposits mounded against heaps of stacked, toppled trees. After 20 years, the recovery (below, right) is rapidly obscuring the view, though Bald Mountain can still be seen over the young tree tops. The tallest of the young trees in the 2023 view are Cottonwood and most of the smaller tree are Red Alder. The conifer in the foreground is a young Douglas Fir – roughly 15 tears old and about eight feet tall.

[click here for a large version of this photo pair]

Turning back (east) toward the mountain from roughly the same spot, the view in 2003 (below, left) revealed a new channel through the debris that the Muddy Fork had almost immediately begun excavating. By 2023 (below, right) the channel has been widened over the years, though its depth has since stabilized at the old valley floor level. The second photo also shows the new forest quickly hemming the channel in from both sides.

[click here for a large version of this photo pair]

Still, the Muddy Fork is a glacial stream, and therefore volatile. It continues to expand, then refill its new channel with debris from smaller flood events that occur almost every year.

On one of my first visits to the debris field, I spotted a row of tree stumps (below) that marked the original valley floor – or perhaps an ancient valley floor? In this spot, the Muddy Fork had cut down through the loose flow material, exposing these sure markers of a former level of the valley floor. 

One surprise is how quickly the Muddy Fork settled in to its new landscape after the debris flow. As these photos show, the stream quickly cut its way to the former valley floor then mostly stopped cutting any deeper in the many years that followed, despite its famously volatile flow. These stumps – and even the two large boulders to the left – remain today much as they were 20 years ago, despite being directly adjacent to the stream and exposed to the many flood events that occur here.

[click here for a large version of this photo pair]

Looking downstream (below) from just above the spot where the stumps were revealed, you can see how little change to the new channel has occurred since it was initially carved in the year after the event. The large boulder on the lip of the channel (left side of these images) is still perched there – and the three Noble fir growing it that survived the original event are still thriving today, nearly twice as tall. 

[click here for a large version of this photo pair]

The most notable difference in the above photo pair is how debris has begun to refill the channel, as evidenced in the 2023 photo on the right. This process will continue over time until the channel has filled and the Muddy Fork is once again meandering across the main valley floor.

I don’t have good photo records of the narrow, upper canyon of the north branch Muddy Fork from prior to the 2002 debris flow. However, I have seen both photos (and even paintings) of this idyllic scene from the 1980s and 90s that show a waterfall here. Based on those earlier images, I do think that a single waterfall existed before the debris flow, roughly where the new falls is located today. Waterfall hunters have dubbed this “McNeil Falls”, referencing nearby McNeil Point – just off to the right in the photo pair, below.

[click here for a large version of this photo pair]

While the landslide and debris flow that it triggered in 2002 did seem to move McNeil Falls somewhat, the most notable change was to produce a twin waterfall – something waterfall hunters (the author included!) prize. However, as the reshaped falls has continued to evolve over the two decades since the event, the two segments have gradually begun to merge into one – or so I thought until I took a closer look for this article. 

In the following photo pair, you can see that the Muddy Fork has actually been carving away debris from the sloped bedrock, and simply moved the falls northward and down the cliff scarp. You can see the shift by matching the rocks marked “A” on the right, the dark notch to the left of the letter “B” and the protruding rock marked “C” that – surprisingly – used to divide the two tiers of the falls! Today, the south (right) tier of the semi-twin drop is really the original north tier, and a completely new tier has formed to the left of this original tier where landslide debris was cleared from the rock ledge. 

[click here for a large version of this photo pair]

Look closely at the above photo pair and you can also see some very large boulders perched to the left of the falls as they existed in 2003. These have since been eroded away, and contributed to the pile of rock debris that has accumulated at the base of the falls in the 2023 photo –shortening the falls a bit.

What will the future bring for McNeil Falls? My guess is that it will continue to shift north a bit more, likely becoming a single tier – twin waterfalls are rare! But even as it find its way to a lower brink along that cliff scarp, the stream will also gradually move loose rock away from the base of the falls as the canyon walls stabilize, so it might become taller over time. There are other examples of exactly this phenomenon on other big waterfalls around Mount Hood – most notably, Stranahan Falls, on the Eliot Branch, which has gained at least 30 feet in height from an eroding canyon floor at its base. Of course, McNeil Falls will also continue to suffer the brunt of the Muddy Fork’s volatile nature and keep changing and reinventing for centuries (and millennia) to come.

Downstream from the falls, the bed of the Muddy Fork continues to gradually collect new debris as the channel carved in the years immediately after the debris flow continues to fill. This can be seen in the photo pair (below), where large boulders now fill the floor of the new channel – and even support young alder and willow pioneers on small midstream islands.

[click here for a large version of this photo pair]

The downstream effects of the 2002 debris flow on the Muddy Fork were less dramatic, yet still reshaped the way the river flows through its valley. This view of the valley (below) from Bald Mountain is roughly two miles below the source of the debris flow. The large rock and gravel deposits along the stream are still plainly visible twenty years later, though the bright green alder and willow colonies have begun to reforest the flooded area. In this view, you can also see the bleached ghost trees along both sides of the stream that were killed by the debris flow. 

Though riparian trees like Sitka alder and willows are making inroads, the Muddy Fork continues to meander across the debris flow where the valley is less steep and the stream less channeled

These disturbed areas now serve as important habitat for raptors and cavity-nesting birds alike, along with many other species that require standing skeleton trees to survive. The dense new riparian growth (below) is equally important to many species that require streamside habitat. While major events like the 2002 debris flow might be shocking for us, it’s also a reminder that violent processes like this are built into the cycle of the ecosystem – they are required for the species who have adapted over millennia to forests that continually cycle through disruption and recovery. 

Large areas of the debris flow have recovered further downstream from the event, marked by the bright green stands of alder, willow and cottonwood along the Muddy Fork in this view from Bald Mountain

Debris flow pioneers

Whether from fire, debris flow or even human-caused events like logging, watching our forests recover and rebuild provides invaluable insight into the role individual species play in the health of forests. Where we used to value our forests mostly for the lumber that could be harvested (and therefore, mostly for big conifers) we now know that non-commercial species like alder, willow and cottonwood are as essential to forest health as the conifers.

Twenty years into the recovery, I expected to find Sitka alder (below) dominating the young forests returning to the debris flow, as these tough, adaptable trees among the first to reclaim disturbed ground wherever it might occur. They are often called “slide alder” for their ability to survive in avalanche chutes, as they freely bend and give under the heaviest of winter snow loads, often popping new leads from horizontal, snowpack-flattened limbs. But alders have a super-power that is especially important in forest recovery: they are nitrogen fixers that enrich the soil as they grow, making them uniquely suited as pioneers in disturbed areas.

Sitka alder growing on the Muddy Fork debris flow

What I didn’t expect to find in the recovering forest atop the debris flow were Black cottonwood (below). Yet, many are thriving throughout the alder thickets, outpacing the alders since they can easily grow to 50-60 feet in mountain environments compared to just 30-40 feet for alders. Cottonwood are fast growers, too, especially where they can tap into a steady supply of groundwater – something the shallow water table in the Muddy Fork valley provides in abundance. They are important wildlife trees, too, including the browse they provide as young trees and the nesting cavities that form in the trunks of older trees as they mature.

Cottonwood growing on the Muddy Fork debris flow

An even more startling pioneer in the recovering forest on the debris flow are Douglas fir (below). These trees don’t require much of an introduction — as our state tree and a species found all over Oregon. I didn’t expect to find so many here, in part because we’ve been conditioned by the timber industry to believe these native conifers must be hand-planted after logging, and then only after all other vegetation that might compete has been killed with herbicides.  

Douglas fir growing on the Muddy Fork debris flow

Douglas fir are fast growers, and with their early arrival as pioneers in the new forests along the Muddy Fork, there’s a good chance they will keep pace with both the alders and cottonwood and quickly begin to restore a conifer overstory – though “quick” is measured in decades and centuries when describing forest recovery! 

Douglas maple growing on the Muddy Fork debris flow

Another surprise in the new forest growing on the debris flow is Douglas maple (above), a close cousin to our Vine maple, and sometimes called Rocky Mountain maple. These attractive trees are sprinkled throughout the young trees returning to the Muddy Fork and they are positioned to become part of the future understory of a mature conifer stand. Where alder and cottonwood are eventually shaded out by taller conifers, Douglas maple are more tolerant of shade, and can coexist with the big trees. They are also more drought-tolerant than their Vine maple cousins, and thus well-suited to the sandy upper layers of the debris flow that can become quite dry during summer droughts.

The cycle continues… except faster, now

Just as the recent series of wildfires in WyEast country have given us the gift of insight into how a forest regenerates after a burn, the debris flow on the Muddy Fork is providing a glimpse into the resilience of Mount Hood’s forests in the face of growing disturbances from climate-driven floods, landslides and debris flows. There is no way to know if the 2002 landslide in the Muddy Fork canyon was triggered by climate change, yet scientists do know that extreme rain events and unusually saturated soils are increasingly triggering such events. And while these events have always occurred, the extreme and often erratic nature of our storms in recent years has accelerated the pace and scale of flooding and debris-flow events on the mountain. The good news from the Muddy Fork is that our forests are – so far – coping well with these changes, especially in riparian areas where the restored habitat is most critical. 

Early 1900s scene along the upper Sandy River

Historic images and geologic evidence show these events to be part of a timeless cycle of destruction and rebirth. This image (above) show the upper Sandy River valley in the early 1900s, with mix of debris and young streamside vegetation (the willows and alders toward the background) that look much like today’s conditions. Clearly, periodic floods and debris flow events had always played a role, here.

This image (below) is a wider, hand-tinted view from about 1900 that shows the Muddy Fork branch of the Sandy River hugging the left side of the valley, with obvious signs of flooding and debris flows. There’s a story in the young forests covering the balance of the flat Sandy River valley floor, too (known as Old Maid Flat) in this view: at the time the photo was taken, just over a century had elapsed since the Old Maid eruptions on Mount Hood covered this entire valley with a debris flow that extended 50 miles downstream to the Columbia River, creating today’s Sandy River Delta. This very early view also shows burn scars (colorized as white snow) around the western foot of the mountain that have long-since recovered, and are now covered with dense forests of Noble fir.

The Muddy Fork is the open channel on the left in this colorized view of the Sandy River Valley from about 1900

While these events may seem random and jarring in from the perspective of a human lifetime, when you connect the dots between events over geologic time, the continuum is that of a mountain in a perpetual state of both eroding and occasionally rebuilding itself, one catastrophic event after another.

It is this long view that helps us understand and appreciate how our forests have evolved not to a specific end-state (the view from a logging perspective), but instead, have evolved to continually adapt to their conditions in a perpetual state of renewal and rebirth. The fact that our forests are rebounding so readily in places like the Muddy Fork or the scorched slopes of the Gorge fire — even now, in the midst of climate change – is both inspiring and reassuring in a time of unprecedented change in the world around us.

In the Realm of St. Peter

/ Tom Kloster / 8 Comments

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.