National Geographic News
Aerial photo of the mudslide.

This aerial photo of the mudslide near Oso, Washington, was taken Saturday, March 23, 2014. The debris flow was up to 15 feet (4.5 meters) deep in some areas.

PHOTOGRAPH BY MARCUS YAM, THE SEATTLE TIMES/AP

Brian Clark Howard

National Geographic

Published March 24, 2014

A fatal mudslide in rural northwestern Washington State over the weekend underscores the dangers of this fast-moving natural hazard.

On Saturday morning, a mudslide moved down the Stillaguamish River near the small former fishing village of Oso, Washington. Authorities have confirmed eight dead, eight injured, and as many as 108 people missing or unaccounted for as of Monday morning. The one-square-mile (2.6-square-kilometer) track of the mudslide also destroyed about 30 homes.

Jim O'Connor, a research hydrologist with the U.S. Geological Survey in Portland, Oregon, told National Geographic that the mudslide, which was up to 15 feet (4.5 meters) deep in some areas, was caused by ground made unstable by heavy rainfall.

"This area has had slides in small increments over the last several years, but this took a huge bite of the hillslope this time," says O'Connor.

Not only has there been a lot of precipitation in the area over the past few months, but the Stillaguamish River also has been eroding away the base of the hillside, or "undercutting it," making it less stable, says O'Connor.

"A whole section of a hillside, about 700 feet [213 meters] high above the river, collapsed all at once," says O'Connor. "It's amazing how much terrain it ended up covering."

Photo of a house with an American flag on a highway after the mudslide.
This house on Highway 530 near Arlington, Washington, was destroyed by the mudslide on Saturday.
Photograph by Lindsey Wasson, The Seattle Times-Pool/Getty

What Is a Mudslide?

A mudslide, also called a debris flow, is a type of fast-moving landslide that follows a channel, such as a river. A landslide, in turn, is simply when rock, earth, or other debris moves down a slope. (See photos of a mudslide and a video on landslides.)

Mudslides occur after water rapidly saturates the ground on a slope, such as during a heavy rainfall. According to O'Connor, it doesn't take high relief in the topography to create a slide. Rather, it just takes a pull of gravity strong enough to bring down material that is made fluid enough by water.

Mudslides tend to happen during wet seasons, says O'Connor. For the Pacific Northwest, that's generally during winter or spring.

Mudslides are also often triggered by earthquakes or by disturbances in hillsides caused by fires or human activity.

In the United States, mudslides and landslides result in an average of 25 to 50 deaths a year, according to the Centers for Disease Control and Prevention (CDC).

Photo of a house destroyed by the mudslide.
The fast-moving mudslide razed this nearby house.
Photograph by Lindsey Wasson, The Seattle Times-Pool/Getty

How Are Mudslides Prevented?

Strategies to decrease the risk of mudslides include draining water off hillsides, armoring the bases of hills so they are not undercut by rivers, and "loading the toe," says O'Connor. In the case of "loading the toe," engineers put heavy mass, such as large rocks, at the base of a hill to try to anchor the slope and prevent it from coming loose.

O'Connor says the piles of rock that are often seen at the base of roadcuts are the most visible example of that strategy.

O'Connor adds that when it comes to the Stillaguamish River area, the valley is scalloped with the evidence of many past slides.

"This isn't a situation where [the authorities] should have done something [to prevent it] because there is so much terrain there that this could have happened to," he says.

The CDC recommends that people exercise caution around steep slopes during rainfall. Immediate signs of a pending slide include tilting trees and sudden increases or decreases in rivers.

Follow Brian Clark Howard on Twitter and Google+.

39 comments
Kapil Roy
Kapil Roy

It is again proved from the method that not to change anything from the nature . Keep everything as the same as it was before.

Jess Richardson
Jess Richardson

Don't forget to point out that the big businesses kept on fracking for their own benefit.

Emily Axt
Emily Axt

"A fatal mudslide in Washington State points to the dangers of heavy rainfall" Can you have a more inaccurate byline? The mudslide points to the dangers of CLEAR-CUTTING OUR FORESTS.


Kathy B.
Kathy B.

My thoughts and Prayers are with all the lives lost and their families! Just wanted to bring the attention back where it should be. Not interested in "who's making money", or in offending anyone here!

Lynn Goldfarb
Lynn Goldfarb

Like the floods in Colorado last year, this is another manifestation of climate change. If it is exacerbated by clear-cutting, that is just another part of the corporate greed and irresponsibility that drives climate change disasters. The fossil fuel industry is spending over a billion dollars year on a campaign to deny climate change, modeled directly on big tobacco's denial that smoking causes lung cancer. They even use on of the crooked PR firm, Heartland, that used to be in the denial business for the tobacco industry. The cynicism and. The moral bankruptcy and cynicism of thee corporations is beyond belief. And the media does. Othing to call them out, although Scientific Americnan and others have documented their activities.

Jane Feldman
Jane Feldman

Decision makers could have zoned this dangerous area as unsuitable for homes, schools,  and businesses. It seems that there was ample evidence, and this evidence was well known information, that indeed this an area that has historically has had a lot of landslide activity. City and county planners shouldn't let development occur is places ripe for danger. Neither should insurance companies.

Stephen Funk
Stephen Funk

Looking on Google Maps, I can't see any logging scars or roads in the main slide area.  I'm a forester, but I have seen a lot of these situations and read what I could in the literature.  I'd call this about 95% naturally caused.  The houses and roads, ironically, were on the opposite side of the river from where the slide started.  In a little less massive slide they would have been safe.

David Blomstrom
David Blomstrom

Yes, it is Nature - with a healthy dose of logging. Washington's environment simply can't stand up to the Seattle Mafia, which will never stop trying to squeeze money out of everything from taxpayers to trees.

Sandy Reidelberger
Sandy Reidelberger

It's nature.. It has done this since the beginning of time.. I suggest you shouldn't build house in these areas..

Guy White
Guy White

The US lumber industry has destroyed every square inch of WA state and OR as well, I'm amazed there aren't slides like this much more often. As Tano Kram stated below, the upper area was clear cut bald over old glacial slur combined with ancient volcanic ash making it very unstable earth to begin with, just add water for the perfect recipe for disaster. And?! No trees or forest floor to stabilize and voilà! People die, lives ruined... Why does Weyerhaeuser clear cut, why do they move huge mills to Mexico so EPA standards don't affect them? Cause they don't give s***!! Look at this photo and you be the judge on how and where the slide originated....     https://www.facebook.com/photo.php?fbid=10203520335988228&set=a.3128990422906.162368.1209079691&type=1&theater

Tony Cooley
Tony Cooley

Mudflows or debris flows require a particular type of geology and are not the only kind of landslide.  To get a mudflow or debris flow, the strength of the soil after failure must plummet to a fraction of its initial value so the failed material moves away from the slide and does not provide continuing lateral support for the remaining slope.  This is called a "sensitive" material, one whose residual strength is much less than it peak strength.  It is metastable, like a house of cards.  The initial failure can be small, but it progresses backwards as the front part of the slide flows away, leaving the steep headwall at the back of the previous failure unsupported.  When this fails in turn, a progressive failure results and continues until the final headwall is stable.  When the material sliding is not "sensitive", the slide does not progress laterally very far, so most damage would occur to features at the top of the slope and those immediately at the toe.  Thus, it would be alarmist to suggest mudflows and debris flows could occur anywhere.  The best guide is what has happened in the area in the past.


A large reduction in strength with failure leading to mud or debris flows commonly involves a loose soil whose grains have been cemented together with mineral or clay deposits.  This is common in arid areas where torrential flows deposit sediments in alluvial fans and then these soils dry out until the next storm adds more soil on top of the last deposit.  This natural cementation provides enough strength to allow a slope to form.  When such a material becomes saturated with water, as during these rains, it increases in weight as the pores between the grains are now filled with water instead of mostly air.  More important, once the peak strength is exceeded, the soil along the failure plane will try to collapse to a denser state.  This is the house-of-cards aspect.  If the material did not have the natural cementation that forms early in the deposition, it would have already densified under the weight of additional soils as these are deposited on top.  The cementation, commonly of dried clay films or gypsum in arid areas, provides enough strength to keep the original loose structure.  


If the landslide occurred during a dry period, the failure of the cementation would cause the soil to densify as it fails, but this would merely compress the air in the pores and the failure is unlikely to propagate.  The soil grains adjust into a denser framework of grain-to-grain contacts that provides the soil with its shear strength due to friction between the grains.  However, if the pores are filled with water, once the cementation fails, the soil grains can only densify by expelling the water in the pores.  During the interim while this water is being expelled, the excess load not borne by the soil strength is transferred to the water like in a hydraulic jack and the pressure of this pore water increases. Because the frictional strength of the soil grains depends on the contact pressure between grains and this is directly decreased by the pore water pressure, this increase in pore water pressure decreases the soil strength of the materials adjacent to the failure plane.  Also, as the soil slides, the soil mass breaks up and distorts because the failure plane forms following the local weaknesses and higher stresses at the time of failure, but the sliding block has to take the shape necessary to stay in contact with the underlying irregular surface as it moves.  This distortion of the sliding mass destroys the cementation of an increasing fraction of the soil mass, so progressively more of it drops to residual strength with high pore water pressures.  If enough free water is available, this distortion can even locally dilate local portions of the failed soil mass as it moves, working additional water into the sliding mass. This is how the mud part is generated.   It then flows as a viscous fluid.  From looking at the videos connected to the article, it is also possible that there may also be an erosion and fluid transport component if enough water is coming out of the slope where material is also moved by viscous traction of the flowing water like in flood.


I have given only a thumbnail explanation of sensitive soil slides to match what I am speculating applies to this slide.  There are also sensitive clay slides in Quebec and Scandinavia where the sensitivity is due to cation exchange of calcium for sodium in glacial marine montmorillonitic clays.  There, the clays were deposited in salt water which allowed a certain strength when sodium ions affected the clay structure, but when fresh groundwater replaces this with calcium, the clay becomes metastable and behaves similar to these mudflows out west, though for a different reason.


As far as engineers and geologists predicting a failure, the susceptibility of a deposit to a particular kind of failure can be determined by observing the evidences of past failures and mapping the extent of the vulnerable materials.  Where a particular development is considered, additional, though expensive site characterization can be done to determine site-specific risk and protective measures or avoidance can be done.  However, the exact timing of a slide cannot generally be done as this depends on weather and other factors including the simplifications and generalization necessary to analyze slope stability.  The calculation of slope stability is imprecise both for theoretical reasons and limitations on our ability to fully characterize a natural, variable deposit.  The approach is generally to make things stable or avoid a hazardous area rather than to predict when something will fail.  


I am sorry I did not have time to edit this more rigorously to explain this more clearly.  If you want to look into this more, you can get general discussions in geological literature under the topics of "mudflows" obviously, "mass wasting", and "landslides".  The engineering evaluation of these is in the field called "geotechnical engineering", a sub-field of civil engineering, and "soil mechanics".  Within that you should look for "sensitive soils" and "progressive failure".

James Marshall
James Marshall

@Bruce Shand,


Bruce, 


Crap does happen, dirt does hit the fan, but  laziness coupled with greed is usually to blame. Today's capitalism keeps people sick and system dependent. Aspirin causes permanent brain damage,  yet they tell you it's safe. (read the small print) Aspirin is deadly. Hospitals keep you functionally sick, do they heal you with carrots and veggies like many famous doctors have proven in the past? No, they like the blood letting Catholic Priests of times past, destroy the more healthy parts of you, so that you become more and more dependent upon pharmaceuticals. After all, the doctor has to be paid his commission for selling you the drug which he's not sure will work anyway. But you the patient, and he the doctor are going to take a flying guess at trying to suppress your symptoms so that you can feel minimally better, go back to work, so you can enjoy your new enslavement to a grossly over inflated doctor bill. 


You defend capitalism likes it's God's gift to man. 


Let me tell you what I think about capitalists. They're good people when they care more about people,  and about the moral of the story, rather than how much money they will put in the bank. 


Take the New Madrid Fault, lot's of earthquake swarms! Hah! What do you know, they're fracking for oil and gas in the New Madrid Seismic Zone. Who was the idiot capitalist who thought it was OK to take a horizontal drilling rig into an already unstable piece of land, and begin through hydraulic fracturing to crush the supporting shale underneath the already unstable sandy soil New Madrid Super Fault Line?


Don't the "Capitalists" know that the whole city of Memphis Tennessee will sink into the sand it's built upon if a (big one) is triggered by some innocent oil and gas rig drilling and fracturing the wrong shale plate at the wrong time??? 


The moral of the story is important, more than the dollars earned. 


Here is the moral of this story, when capitalists have rigged the game so they make money no matter what happens good or bad, something is wrong in America. 


If Memphis sinks beneath the dirt because a capitalist was betting on the stock market Memphis stock would drop like a rock, there is a problem.


When "capitalist develope horizontal drilling, so one state can steal another states oil, that's flat wrong. It's thievery. When your doctor experiments upon your body, not sure if he's giving you the right treatment, he's a quack! Yet to this day, your personal doctor has about a 20% chance of healing you from any form of disease. 


And Advil, called safe by doctors, when you read the small pint, warns you it can cause your liver to shut down. 


And the President of the United States, and your representative conservative/liberal  congressman never lie. 


What kind of narcissistic nervauna do you live in?   



Point being a capitalist is OK, if you fear God. But if you live by the law of finance where you make money at the expense of your fellow man, it's the same as stealing. 

Such as war... Yes capitalist start wars also, and kill hundreds of thousands per decade... Are you saying because capitalists are in the war business, state sanctioned murder is OK? 


Seth Ward
Seth Ward

People, your concern for the environment is admirable but it does not qualify you to comment on geology. Any infraction here has less to do with loggers than it does with the engineers, insurance underwriters and county commissioners who should have listened to their instincts and blocked development in such a place. (Read: money talks.)


There is a similar issue going on near Big Sky in Montana, now, where owners of multi-million dollar homes have discovered the developer went on selling lots knowing full well the whole area had slid at some time in the last 100 years and would continue to do so. A dozen years ago, the local university had maps that showed this in graphic detail, but was not allowed to publish because of it being "too political." (See also: money talks.)

bruce shand
bruce shand

Man! The hyperbolists really come out here don't they?

Greedy capitalists, criminal logging companies.

Shit never happens does it? It's always someone's fault.

I'm sure mudslides never happened until capitalists (greedy ones, of course) and loggers came along.

Philip Vieira
Philip Vieira

We are harboring environmental criminals. Why do we claim to want to do things about this. And one thing for you deniers out there: What climate change? If you ask yourself this, look up the recent weather. It is out of control. Stop greenhouse gas pollution, unless you want to be drowned in a storm surge gone wrong. 

Tano Kram
Tano Kram

I know this area, this slope, and the range above it. It has all been clearcut at different points. To act like this is a slide-prone are is ludicrous. THe entire state wherever there is glacial till, is a slide-prone area, but what keeps it together is trees. The roundtop mountain top above this area has been clearcut bald. Areas directly to the west and above where this land gave way were clearcut within the last 20 years. The area along the river, which slid before was bare before 2003? Why is that? Because it was probably clearcut after years of selective cutting by loggers who used to not clearcut because they knew it destroyed the viability of the area.


Logging criminals are out of control in the USA, and for Nat Geo to not call it as it is, is a sure sign that they were bought out by moneyed interests and not what they used to be, a source of truth.

Susan Gawarecki
Susan Gawarecki

If you look at the big picture--at the top of the page--there is an escarpment to the left from a much older landslide.  So we know that this ridge is already prone to slide.  There also were several smaller slides from this zone in the past decades.  The river continued to undercut this particular section of hillside, which would have removed support from the mass of earth above it.  Ultimately, the weight of mud from heavy rain combined with its lubricating effect made another landslide inevitable.  Geologists could easily have predicted a disaster of this magnitude, but no one wants to believe it could happen to them.  We see this all the time when people choose to live in zones where they are at risk from flooding, rockfalls, mudslides, earthquakes, sinkholes and other natural hazards.


I'm so sorry for everyone who lost family and friends in this disaster.  I really hope that the lessons that the earth is sharing with us will not be forgotten as communities rebuild or expand.  Don't let the developers and real estate agents make those decisions--they don't have your best interests at heart.

Karen Johnson
Karen Johnson

From the looks of the laid-bare geology of that hillside overlooking the obliterated valley, there isn't a bit of bedrock.  Rather, the hillside seems to be comprised of that glacial till that formed the foothills of the Cascades during the Ice Age.  


Events like this mudslide could happen anywhere along the Cascade range, wherever the moraines have been deposited.  It's scary to think that all it took for this event to happen was a slight increase in seasonal rainfall.

Comarca Fontes
Comarca Fontes

MUITO TRISTA ESPERAMOS QUE ENCONTREM TODOS OS DESAPÁRECIDOS COM VIDA

Alec Sevins
Alec Sevins

Irony can be seen in an aerial view of Mt. St. Helens, which disrupted much less of the total landscape than the clearcuts that surround it for hundreds of miles. We tend to define something as "destruction" when it occurs in a short moment, but people have chipped away huge swaths of wilderness in the past few centuries, increasing in scale as the population grew.

Bill Yake
Bill Yake

While I have no love for clear-cutting, a look at the area on Google Earth (photo -- July 2013) and comparing it to the aerial photo/illustration in today's NYT, appears to show mature forest at the fracture line with no indication of recent clear-cutting in the area. However, it does look like there was a previous slide that started in much the same area -- and that ground had barely started to revegetate. It would be instructive to see an aerial view prior to the earlier slide.

Babul Khan
Babul Khan

The greedy capitalist America has made US the world's biggest desert by deforestation  of Douglas Fir Forrest all over US, the only rainy Forrest enclave left is in Oregon and and Seattle, now they even messing with vital environment. For the greed of money from timber export, US will do anything sinister for corporate America.. 

Andy Grogan
Andy Grogan

Honestly, I have to agree with the logging statement - and the clear cutting of forests and in particular the fire resistant hardwood plays a big role in many western forest fires. I sent a letter to Obama about that.


Also, I read engineers had tried several times to shore up the mud the banks of that mountain and failed. The land should have be declared unfit for habitation and evacuated years ago.


Finally, what technology do they had to search a disaster such as that? Human sight is not enough - there are special archeological search and map technologies that are used on ancient ruins that EMS rescue technicians, particularly in areas like this, should be trained to use. Also, I don't think it would be difficult to develop technology searching for infrared waves variability and other radiations to quickly find areas to search.


Good luck. I hope all those involved in the disaster have already been recovered.

Ted Smith
Ted Smith

The inevitable repercussions of clear cutting forests and indiscriminate logging; these problems are environmental and, despite the inevitable accumulation of water in the ground, can be mitigated by more responsible forest practices.

David Cockrill
David Cockrill

@Philip Vieira   According to scientists, thousands, maybe millions, of years ago, a giant natural dam of ice rapidly thawed and released a tremendous reservoir of ice water that cut across the entire North American continent, scooping up huge boulders as it went out across the land, and gouging the landscape and changing the appearance of land itself.  All of this happened long, long before mankind even came to the Americas, including what is now known as the Native Americans.  So take your ideas of global warming and put them in your collectivist-thinking tobacco pipe and smoke it.  I am not buying your communist bull****; follow the money and you'll find the truth behind the Global Warming theory.  A few people are making themselves rich at the expense of a lot of suckers out there.



Karen Johnson
Karen Johnson

@Alec Sevins  The  Mt. St. Helens explosion itself "clearcut" the forests to the north of the mountain.  It took 20 years for nature to repair the damage. The ash debris totally obliterated Spirit Lake.  Neither the blast nor the ashflow amounted to "much less of the total landscape"; rather, much more.

Alec Sevins
Alec Sevins

@Bill Yake  In Google Earth, paste in 48.283621,-121.847355 and look at historical imagery going back to 1989 (when many of the homes weren't there). This location is far from stable. Building below it seems foolish in hindsight. It looks like they figured a slide would never cross the river.

Ste Schlappi
Ste Schlappi

@Babul Khan  Yes blame America. When in Europe take a look at the massive forests they never cut down. Perhaps you will have to look at old paintings of massive forests in Europe to see them. Grow up.

Eo Raptor
Eo Raptor

@Babul Khan  To the extent you're making a point about greed leading to environmental degradation, that's basic Economics 101. Your examples, however, don't quite make the point. Yes, the temperate rainforests are greatly reduced, but they never were much outside of Washington and Oregon. Douglas Fir never has, and still doesn't, grown natively west of the Rockies. 


If you want to be effective, understand your subject matter. 

Chris Webber
Chris Webber

@Babul Khan Dude - Douglas fir grow in the very rainy climate of the Pacific Northwest. They don't grow "all over US" as you say, not due to deforestation, but due to the wrong climate. East of the Cascades, less rainfall = no more Douglas fir, but long needle pines suited to drier climate. But I just lived in that area for 12 years, so maybe I don't know what I'm talking about...

Interesting you bash timber companies (relatively small by WalMart standards) while sporting the avatar of a murderer.


Eo Raptor
Eo Raptor

@Andy Grogan  Aargh! What hardwood forests? Sure, there are stands of hardwood, here and there, most of them human created. The western tall-tree forests are coniferous, and always have been, since the last ice-age. 


I don't like clear-cutting, and I think the Forest Service, Bureau of Lands, and other agencies charged with long-term forest management do a terrible job. For the most part, their clients seem to be the timber industry rather than we The People. But, damn! You'll never be a successful advocate if you don't get your facts straight.  


Oh, one last thing, yeah there's all sorts of new technology to find sites of ancient human habitation. But, dude, Oso isn't lost; everybody in the area knows where it is. The problem is finding the bodies under 4 - 5 meters of mud and debris from collapsed homes, then shoring up the sites sufficiently so that the rescuers don't become new victims. 

Eo Raptor
Eo Raptor

@Anonymous Author  Damn! I fell for it, and clicked the link. Hey, folks, save yourself the download time; the book is nothing but codswallop and woo! Really, where do these people come from? 

Robert Broska
Robert Broska

@Alec Sevins"a slide would never cross the river"

         That would be a fair assumption, for the myopic  monkey minded ones who don't swim. (Realtors and  land developers who would not live in the structures that they promoted to be built.

Tano Kram
Tano Kram

@Ste Schlappi @Babul Khan  Go To Switzerland, and see how they manage their forests there compared to here. You know nothing about the topic. Yeah, the europeans were the leaders in criminal enterprise until the center of that exploitation moved to the USA. THe US selectively cut for years with plenty of productivity and nowhere near the level of land-destruction going on since the Reagan 80s brought in the brilliant notion of clear-cutting to pay off junk bond debt. Wake up.

Philip Vieira
Philip Vieira

@Chris Webber  I disagree with anything that we do that hurts the environment, because of "progress". Doesn't chaos tear things down, just for the sake of "progress"? And capitalism has soured badly. We have twisted it beyond evil.

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