Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Pe Ell Landslide, Pe Ell, Lewis County

The Pe Ell Landslide failed during the December 3rd Storm of 2007, closing State Route 6 just west of Pe Ell.

Pe Ell Landslide - Photo by WSDOT

Pe Ell Landslide - Photo by WSDOT

The debris avalanche/slide flowed across the highway and pushed a truck into the living room of the house across the way. Remarkably, most of this was caught on tape by the residences of the house.

Pe Ell landslide impact of house - WSDOT Photo

Pe Ell landslide impact of house - WSDOT Photo

On December 11, Kelsay and I arrived at the landslide. The drive through the Chehalis valley was spooky to me, a lingering stench filled the air and misery could be seen all around. Home after home, farm after farm all showed damage from the floods. By time we arrived, WSDOT had already arranged for an emergency contract with Scarsella (on December 9th) to begin work on clearing State Route 6. Unfortunately, with all of the heavy equipment working on the site, we decided to stay on the periphery of the landslide and investigate the damage to the structures.

The damage was localized to the western lobe of the landslide. It impacted the houses at a low speed, warping and pushing them.

Pe Ell landslide impact to a house - DNR/DGER Photo

Pe Ell landslide impact to a house - DNR/DGER Photo

Pe Ell Landslide impact to second house - DNR/DGER Photo

Pe Ell Landslide impact to second house - DNR/DGER Photo

Meanwhile, WSDOT was working hard on figuring out the landslide. The WSDOT Geotechnical Division has access to many really neat tools to help with their investigations. Here is a 3-D representation of the landslide mass created by their division:

Pe Ell Landslide 3D Model - WSDOT Geotechnical Division

Pe Ell Landslide 3D Model - WSDOT Geotechnical Division

They also compiled a small scale geologic map of the landslide mass (with an amazing aerial photo of the landslide):

Pe Ell Landslide Geologic Map - WSDOT Geotechnical Division

Pe Ell Landslide Geologic Map - WSDOT Geotechnical Division

In the end, WSDOT removed over 47,000 cubic yards of material to stabilize the landslide mass at a cost of around $4 million dollars. The project was completed on March 13th, 2008, over three months after the storm.

The landslide prompted a debate on logging, landslides, and highway safety. The landslide itself was logged weeks before the storm. The interesting part, this landslide wasn’t caused by root strength loss, it was probably too deep anyway to have much impact. The lack of canopy, however, might have played a roll in the landslide initiation. Canopy plays a role in reducing the rate rainfall from reaching the ground (to a certain point) or slow melting of snow on the ground by reducing rain rates and buffering changing temperature. It is difficult to say in an intense storm how much it might have slowed the rainfall, or reduced snow melt (by reducing the warm rain and temperature from reaching the snow), but the lack of trees, even with this intense rainfall, probably did increase the likelihood for its initiation.

Cause aside, the cost of repairing these landslides is expensive. This is just one of probably hundreds of landslides to fall on our highway systems each year. Figuring out why these landslides fail and if we can either mitigate or possibly find better management practices to help reduce landslides would help save millions of dollars and reduce injury and death.

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Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Racehorse Creek Landslide, Whatcom County

On January 6th, 2009, a Pineapple Express (actually to have formed off of Hawai’i) flowed into Washington State, hitting the northern counties first and moving into the southern counties. Whatcom and Skagit Counties were first to report landslides late on January 6th as warm rains melted away snow and thawed the ground. As the rain continued, a small rain on snow type event occurred, spawning over 1,500 landslides. Debris flows and debris avalanches were the most common landslide to have formed from the storm event and the majority of the landslides occurred on the flanks of the Cascade Mountain Range.

January 2009 storm map with an incomplete landslide inventory

January 2009 storm map with an incomplete landslide inventory

This map shows the storm intensity overlain with landslide initiation points, primarily from DNR/DGER aerial surveys after the storm and reported landslides from public and private folks.

One of the largest landslides during the January 7-9th storm occurred along Racehorse Creek in Whatcom County.

Racehorse Creek Landslide; DNR/DGER Photo

Racehorse Creek Landslide; DNR/DGER Photo

The landslide occurred in two major components, the main debris avalanche and near lateral debris flows. The main debris avalanche is over 160,000 square yards and moved a significant amount of trees into Racehorse Creek. The debris flows scoured into the ground, removing timber in its way, also reaching Racehorse Creek. Once in the swollen waters of Racehorse Creek, the moved debris moved downstream, forming a massive logjam.

Logjam formed by the Racehorse Landslide, looking towards Kendall Creek; DNR/DGER Photo

Logjam formed by the Racehorse Landslide, looking towards Kendall Creek; DNR/DGER Photo

Logjam formed by the Racehorse Landslide, looking towards Racehorse Creek; DNR/DGER Photo

Logjam formed by the Racehorse Landslide, looking towards Racehorse Creek; DNR/DGER Photo

The size of the landslide has caused many to scratch their heads as to what possibly might have triggered this landslide. Some point to an earthquake as a possible trigger (one did occur on January 6th, 2009), others, natural factors of erosion and saturation. Or, as is common, a combination of saturated ground, erosion of the toe and a bit of shaking from an earthquake.

Whatcom County Earthquake Map; DNR/DGER

Whatcom County Earthquake Map; DNR/DGER

Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Sultan River Debris Avalanche, Snohomish County

When I first started at DNR, I lived in Seattle (commuting to Olympia), but most of my field work was in or near the Sultan Basin. The Sultan River watershed was the first watershed I worked on for the LHZ Project. My first year working was almost comical, every day I was in the field, it rained. In the summer, we would have long stretches of dry weather and one day of rain every couple of week and those were the days I ended up in the field.

On December 10th, Pat Pringle and I were in the field investigating the Sultan River basin. We didn’t hike or really get out of the car much due to a torrent of rain coming down. We never quite got over to the area of the Sultan River debris avalanche, but we were just about across the way from it. I did end up heading out there after it failed.

Sultan River Hydrology

Sultan River Hydrology

On December 11th, a group of Kayakers decided to ride the Sultan, mostly because of the elevated water levels from the storm. During their ride down, they ran into more than they bargained for.

Sultan River Debris Avalanche Location Map

Sultan River Debris Avalanche Location Map

Sultan River Debris Avalanche

Sultan River Debris Avalanche

A large debris avalanche/fall came down right after the kayakers passed by. As you can see in the video, the landslide dammed the Sultan River for a short time, but eventually over topped/breached the dam. The kayakers, a bit shaken up by the landslide decided to leave the river and hike out and head back to civilization. Unfortunately, they didn’t know the area and headed east of the the Sultan Basin, eventually reaching a small nudist community. They all make it out safely.

To make matters worst, the City of Sultan received a report of a large landslide damming the Sultan River. Their main concern was a dam burst flood that would damage the city. The Snohomish County sheriffs office had a helicopter in the air trying to find the landslide and see how bad the blockage was. This was a real thing to worry about. The Sultan River is entrenched and a dam could potentially block a large amount of water. Strangely enough, many of the rapids (that the kayakers seem to enjoy so much) came from old landslides that probably partially or fully dammed the Sultan River and slowly eroded out, just like this landslide.

Landslides are rarely caught on video and when caught, they are a valuable source of scientific information. In this video, you can see how the landslide overcomes what almost looks like a small rock that is holding back the mass of material break apart shortly before movement. You can almost calculate the acceleration as it heads towards the river and what happens as a mass of material impacts into the water, the size of the waves and so forth. We can also see how the river responds to the debris dam and how it returns to normal flow without breaching the dam.

This video also caught the attention of others and it was part of a Discovery Channel series called Raging Nature. I was interviewed by the show, but I didn’t appear on camera for the show (probably the background river sound, which was really loud). The narrator during the show did say much of the exact words that I said (which was kinda creepy for me).


Geology

Bedrock Units

Regional bedrock that includes the Sultan River watershed belongs to the Western Mélange Belt, part of the Western and Eastern Mélange Belts (WEMB) terrain. The WEMB includes Mesozoic (late Jurassic to early Cretaceous) marine sedimentary rocks, along with lenses of Paleozoic limestones, Mesozoic intrusives, and other rich types in fault-bounded bodies that were tectonically juxtaposed (Tabor et al, 1993). The WEMB rocks underwent high pressure, low temperature metamorphism in the late Cretaceous orogeny at about the time they were juxtaposed against the Northwest Cascade System terrain to the North.

Bedrock in the Sultan River watershed is mainly composed of the Western Mélange Belt (Phipps et al., 2003; Dragovich et al., 2002). These rocks were deposited during the late Jurassic to early Cretaceous (170 to 100 million years ago) periods (Carithers and Guard, 1945). Sediment was thickly deposited in a marine setting, comprising mostly of silt and mud. Hydrothermal systems and submarine eruptions (similar to black smokers) formed from intruding magma, creating large pyritic deposits (such as the Lockwood Pyrite deposit) and overlaid the marine sediment with volcaniclastic and mafic flows (for example, basalt) material (Olson, 1995; Snohomish County, 1979). This magma chamber underwent differentiation, where the heavier mafic material (rich in iron and other metallic minerals) filtered to the bottom of the chamber and lighter felsic material (rich in silica, such as quartz and feldspar) rose to the top (Stewart, 2005). These rocks were then metamorphosed (exposed to heat and pressure), folded, uplifted and eroded. The metamorphism changed the marine sedimentary and volcaniclastic rocks into argillite (metamorphosed siltstone) and phyllite (metamorphosed mudstone). The granitic magma chamber also experienced metamorphism, altering the granitic rocks into meta-tonalites (light colored granitic rock), meta-gabbros and meta-peridotites (dark colored granitic rock).

As the rocks experienced pressure from the west (most likely from the oceanic plate colliding with the North American continental plate), they tilted the stratigraphic section to the northeast. This tilting, along with erosion of the overlying rock, exposed the relict magma chamber (gabbro and peridotite in the west, grading east to tonalite) in the western part of the Sultan River watershed. The metamorphic marine rock, which overlies the relict magma chamber, can be found primarily in the southern and eastern parts of the watershed. The metamorphic volcaniclastic rock, which overlies the marine rock, is located primarily on Blue Mountain, in the northeast part of the watershed.

The meta-tonalite rocks, where not overlain by glacial drift, is very stable, even with slopes steeper than 60% (A prime example of this is the large hill, located in T. 28N R. 8E, section 2 and 11). The meta-marine rocks can be unstable, especially when the beds are tilted to near vertical. The north flank of Blue Mountain is an excellent example, where the meta-sedimentary rocks are tilted to near vertical and failures are frequent within the section. The meta-volcanic rocks can be very unstable and appear to be very susceptible to slope failures when the rock is exposed to water. A prime example of this is the water run-off from the radio tower located at the highest peak on Blue Mountain; many debris flows initiated from this deposit, independent from harvest or road construction.

Poorly-Consolidated Surficial Units

Surficial units in the study area consist of continental glacial drift. Other surficial deposits are composed of alpine glacial drift, colluvium, and alluvium. About 14,000 years ago, the Puget Lobe of the Cordilleran ice sheet, which represents the most recent advance of continental ice sheet, flowed into surrounding valleys. This advanced was named the ‘Vashon Glaciation’ locally. Tongues of the Vashon glacier dammed valleys that were tributaries to the Puget Lowlands, creating large ice dammed lakes. Glaciers advanced up the Pilchuck River system and the Sultan valley, covering the northwestern portion of the watershed (Tabor et al., 1993). This blocked the paleo-Pilchuck River, creating a large ice-dammed lake and depositing deltas and lake deposits on the north flanks of Blue Mountain to Bald Mountain. This rising lake eventually overflowed, washed over Olney Pass, and deposited fluvial outwash across the plains in the west and south parts of the Sultan River watershed.

Ice margins near Lake Chaplain and Echo Lake also produced significant outwash towards the town of Startup (Booth, 1990). As the glaciers retreated, the terminal moraine (called the Pilchuck plug) blocked the upper drainage of the Pilchuck River, creating the new Sultan River watershed (Coombs, 1969; Bliton, 1989). The Sultan River established a channel, rapidly incised into the glacial material, cut into the bedrock, and became entrenched. This incision is probably due to easily eroding glacial material and isostatic rebound of the bedrock in the area. Old meander bends and channels can be seen near the main channel of the present Sultan River.

Near the confluence of the Sultan and Skykomish River, glacial lakes formed by the advancement of the Cordilleran ice sheet, creating thick lake deposits in the southern extent of the watershed (Booth, 1990). These lake deposits formed low-permeability clay and silt layers that perch water and spawn large landslides during high precipitation. The silt and clay layers are commonly overlain by permeable glacial outwash from the paleo-Spada Lake and ice-margin flows. This combination of silt, clay and sand makes much of the hillsides in the southern part of the watershed susceptible to shallow and deep-seated landslides.

Reference

Booth, Derek B., 1990, Surficial geologic map of the Skykomish and Snoqualmie Rivers area, Snohomish and King Counties, Washington: U.S. Geological Survey Miscellaneous Investigations Series Map I-1745, 2 sheets, scale 1:50,000, with 22 p. text.

Bliton, William S., 1989, Sultan River project. IN Galster, R. W., chairman, Engineering geology in Washington: Washington Division of Geology and Earth Resources Bulletin 78, v. I, p. 209-216.

Carithers, Ward; Guard, A. K., 1945, Geology and ore deposits of the Sultan Basin, Snohomish County, Washington: Washington Division of Mines and Geology Bulletin 36, 90 p., 1 plate.

Coombs, H. A., 1969, Leakage through buried channels: Association of Engineering Geologists Bulletin, v. 6, no. 1, p. 45-52.

Olson, Duane F., 1995, Geology and Geochemistry of the Lockwood Volcanogenic Massive Sulfide Deposit, Snohomish County, Washington: Western Washington University Master of Science thesis, 118 p., 8 plates.

Phipps, Richard W.; McKay, Donald T., Jr.; Norman, David K.; Wolff, Fritz E., 2003, Inactive and abandoned mine lands–Spada Lake and Cecile Creek watershed analysis units, Snohomish and Okanogan Counties, Washington: Washington Division of Geology and Earth Resources Open File Report 2003-3, 36 p.

Snohomish County Public Utility District No. 1; Washington Department of Ecology, 1979, Sultan River project, Stage II; Application for amended license, FERC project no. 2157–State of Washington final SEPA EIS and FERC environmental report (exhibit W): Snohomish County Public Utility District No. 1, 2 v.

Stewart, Richard, May 27th, 2005, Personal Communication

Tabor, R. W.; Frizzell, V. A., Jr.; Booth, D. B.; Waitt, R. B.; Whetten, J. T.; Zartman, R. E., 1993, Geologic map of the Skykomish River 30- by 60-minute quadrangle, Washington: U.S. Geological Survey Miscellaneous Investigations Series Map I-1963, 1 sheet, scale 1:100,000, with 42 p. text.

Hyak Landslide

June 3, 2009

One of the more talked about landslides from the January 7-8th storm was the landslide that occurred at Hyak. The landslide started at the Summit at Snoqualmie ski area and moved into the Hyak community.
This landslide got a lot of press and originally, it was thought that is might be an avalanche. I remember having a discussion about this at a NOAA/NWS video conference meeting. Although, looking at the photos, it seemed that instead of the snow scraping up the soil beneath it, the slope gave way and moved the snow along. Something like a debris/snow avalanche.

Hyak Landslide Location Map

Hyak Landslide Location Map

The landslide occurred at approximately 11:40 a.m. Wednesday, January 7, 2009. Heavy rains (probably at about its elevation limit before turning into snow) from the storm had reached the summit earlier, warming the hillside and inundating it with rain.

Here at DNR – Division of Geology and Earth Resources (Washington Geological Survey), we were in emergency mode. We mobilized all of our geologist and sent them into the field to document landslides, but more importantly, sent them to check on residences that were impacted from landslides and to make sure they were safe from future landslide movement. Unfortunately, I was in the office, directing geologists to specific areas and creating updated maps of where we had located landslides or had damaged houses or blocked roads. I sent one of our geologists on the east side to get to Hyak and investigate what had happened and determine if it was an landslide or a snow avalanche. Plus, it did damage a handful of houses and the hillside had the potential to continue moving.

Aerial photo of the Hyak Landslide - DGER Photo

Aerial photo fo the Hyak Landslide - DGER Photo

Oblique aerial view of the Hyak Landslide - DGER Photo

Oblique aerial view of the Hyak Landslide - DGER Photo

Our geologist arrived I think late on January 7th and found numerous other crews investigating the landslide. Talking to them and doing some investigation herself, it was determined that it was most certainly a landslide and further, the slope was completely saturated. The scarp and material had woody debris within it and within the scarp, casts of old logs could clearly be seen, most with water gushing out the casts. It turns out that the slope had been modified about 40 or so years earlier and it appears they incorporated woody debris into the material. Over 40 years, the wood deteriorated and probably allowed water to more easily infiltrate into the subsurface, probably to the contact between the fill and rock/soil.

According to Matt Cowan, Fire Chief of the Snoqualmie Pass Fire and Rescue, the landslide impacted eight houses, one which was pushed off its foundation, the other lightly damaged. Two people were injured.

(Photo from Hyak Flickr site)

May 27 photo of Hyak Landslide

May 27 photo of Hyak Landslide

The hillside might still pose a threat to future failures. If woody debris exists in the subsurface then continued weakness still exists. I am not sure if the ski resort is planning on regrading the hillside to make the slope usable to skiing, although I cannot imagine that they will abandon the ski area. Unfortunately, we have inherited a lot of legacy problems from early land-use modifications (from the early 1900’s to 1970’s) that still plague us today. They are rarely recognized as a hazard, but their results can be deadly.

Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Hornby Landslide, Glenoma Area, Lewis County

This is one of the more fascinating landslides that occurred during the January 2009 storm. Numerous debris flows and avalanches dotted the slopes above Mark and Jon Hornby’s farm.

Map of landslide in Glenoma Area, Lewis County

Map of landslide in Glenoma Area, Lewis County

Hornby Landslides

Hornby Landslides

Series of Events

At about 9:00 am on January 8, a large debris flow moved into Mark Hornby’s farm pasture, plugging a culvert and covering it with mud and debris. About a half an hour later, another landslide came off the slope, nearly striking Mark and his brother, Jon. I think this landslide also struck a bull and carried it a ways across the pasture. There full story can be found here.

View of debris flow deposit near house; DGER/DNR Photo

View of debris flow deposit near house; DGER/DNR Photo

The debris avalanche/flow in the middle of the clear cut slope is very shallow, less than 2 feet of incision in many places. The landslide incised down to bedrock in most places, which was probably why the landslide was so shallow. When it reached the pasture, the landslide turned into a short debris flow and then transformed into a hyperconcentrated flow and made its way across the Hornby’s Farm pasture. The landslide ponded against Highway 12, flowing to the Hornby’s driveway and then onto Highway 12. One of the oddities discovered in Glenoma was that many of the hyperconcentrated flows that reached the valley floor were entrapped into roads by snow berms from plowed snow.

Hornby Landslides; DGER/DNR Photo

Hornby Landslides; DGER/DNR Photo

January 7-8 Storm Summary

In December of 2008 and into January of 2009, cold air from British Columbia created an ideal condition for snowfall across Washington State. Snow accumulations preceding the storm were low in the Puget Lowland, with at only inches on the ground in most places. On January 7, a stream of moisture originating from around Kauai (Hawaiian Islands) flowed into Washington State, bringing warm temperatures and high amounts of rain, rapidly melting what snow remained in the lowlands and eating away at the snowpacks in the mountains. By January 8, the largest evacuation in the state’s history was under way, forcing more than 30,000 people living in the Puyallup River area to flee. The town of Orting, with a population of more than 26,000, was almost completely flooded. For the second year in a row, flood waters closed Interstate 5 in Centralia/Chehalis. In the rest of the state, rivers were also flooding—the Stillaguamish, Snohomish, Chehalis, Naselle, Hoko, Cedar, and Cowlitz were the most significant, peaking above the 100-year flood level.

King and Snohomish County were least affected, as a rain shadow from the Olympic Mountains shielded their low-lying areas. In the Puget Lowland, rainfall totals ranged from 1.5 inches in Seattle to 5 to 7 inches in southwest Washington and 3 to 6 inches in the northwestern counties. As the storm progressed into the Cascades, the higher elevation forced the clouds to release water as they moved over the mountains, leaving more than 20 inches of rain in two days. The rainfall saturated slopes, many already wet from melting snow, triggering debris flows and debris avalanches throughout most of western Washington. Areas sensitive to high-intensity storms, such as Glenoma, Concrete, and Van Zandt, were the site of numerous large debris flows, blocking roads, limiting emergency response, and destroying homes. In the end, more than 1,500 landslides were reported or recorded from Washington Division of Geology and Earth Resources (DGER) field and aerial surveys.

Logging and Landslides

DGER and AEG hosted a field trip in the Glenoma Area (field trip guild). The purpose of this stop was to discuss logging and landslides. Unfortunately, the conversation never got going very well. It certainly caught my attention when I was looking over the photos coming in. One of the first things that caught my attention was the prominent deep-seated landslide on the west side of the clearcut. It is difficult to see if it is active from a photo, but when I first saw it from the aerial photo I thought it probably had some recent movement (within the last 100 years, maybe?). In the subsurface, there are places of thick, mostly unconsolidated pumice. So, is this logging related? It is possible. Was it illegal? Probably not. I didn’t do any detailed ground survey of this area, but just at a general glance, I cannot think of any forest practice rules that they might have broke. Maybe we need to look at if the FP rules are protecting our slopes, especially in Lewis County.

We tried getting into the Pilchuck headwaters yesterday to no avail. Roads were washed out and gates were locked. We did see a number of landslides on the Sultan Basin Road heading up to Olney Pass. These landslides bare the mark of a large rainstorm event and almost certainly moved during the January 7-8th, 2009 storm event

Sultan Basin Road Landslides

Sultan Basin Road Landslides

The picture has a backdrop of 2003(? I think it was later than that) Snohomish County LiDAR.

The most interesting of these landslides is a debris avalanche at the bridge crossing at Olney Creek. It was probably dealt a one-two punch, the swollen Olney Creek was probably eating away at the bank (and probably has been for years) and the saturated ground allowed enough driving forces to overcome the resistive forces. It also moved a good amount of timber into the creek, which might cause a problem down the road by creating a debris dam behind the bridge.

Sultan Basin Rd Debris Avalanche.  Photo by Carol Serdar

Sultan Basin Rd Debris Avalanche. Photo by Carol Serdar

Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Mt. St. Helens Debris Avalanche, Skamania County

Today is May 18th and it seems fitting that the Mt St Helens Debris Avalanche would be the landslide of the week. The Mt St. Helens Debris Avalanche is perhaps one of the most well known landslides in the world.

Area: 23 sq.mi.
Volume: .67 cu. mi.
Age: 29 years

Mt St Helens Debris Avalanche

Mt St Helens Debris Avalanche

An Excerpt from: REPORT: Eruptions of Mount St. Helens: Past, Present, and Future

“May 18, a Sunday, dawned bright and clear. At 7 a.m. Pacific Daylight Time (PDT), USGS volcanologist David A. Johnston, who had Saturday-night duty at an observation post about 6 miles north of the volcano, radioed in the results of some laser-beam measurements he had made moments earlier that morning. Even considering these measurements, the status of Mount St. Helens’ activity that day showed no change from the pattern of the preceding month. Volcano-monitoring data-seismic, rate of bulge movement, sulfur-dioxide gas emission, and ground temperature-revealed no unusual changes that could be taken as warning signals for the catastrophe that would strike about an hour and a half later. About 20 seconds after 8:32 a.m. PDT, apparently in response to a magnitude-5.1 earthquake about 1 mile beneath the volcano, the bulged, unstable north flank of Mount St. Helens suddenly began to collapse, triggering a rapid and tragic train of events that resulted in widespread devastation and the loss of 57 people, including volcanologist Johnston.

Debris Avalanche
Although the triggering earthquake was of slightly greater magnitude than any of the shocks recorded earlier at the volcano, it was not unusual in any other way. What happened within the next few seconds was described by geologists Keith and Dorothy Stoffel, who at the time were in a small plane over the volcano’s summit. Among the events they witnessed, they

“noticed landsliding of rock and ice debris
in-ward into the crater… the south-facing wall
of the north side of the main crater was especially
active. Within a matter of seconds, perhaps 15 seconds,
the whole north side of the summit crater began to
move instantaneously. … The nature of movement
was eerie…. The entire mass began to ripple and
churn up, without moving laterally. Then the entire
north side of the summit began sliding to the north
along a deep-seated slide plane. I (Keith Stoffel)
was amazed and excited with the realization that
we were watching this landslide of unbelievable
proportions. … We took pictures of this slide
sequence occurring, but before we could snap off
more than a few pictures, a huge explosion blasted
out of the detachment plane. We neither felt nor
heard a thing, even though we were just east of the
summit at this time.”

Realizing their dangerous situation, the pilot put the plane into a steep dive to gain speed, and thus was able to outrun the rapidly mushrooming eruption cloud that threatened to engulf them. The Stoffels were fortunate to escape, and other scientists were fortunate to have their eyewitness account to help unscramble the sequence and timing of the quick succession of events that initiated the May 18 eruption.

The collapse of the north flank produced the largest landslide-debris avalanche recorded in historic time. Detailed analysis of photographs and other data shows that an estimated 7-20 seconds (about 10 seconds seems most reasonable) elapsed between the triggering earthquake and the onset of the flank collapse. During the next 15 seconds, first one large block slid away, then another large block began to move, only to be followed by still another block. The series of slide blocks merged downslope into a gigantic debris avalanche, which moved northward at speeds of 110 to 155 miles an hour. Part of the avalanche surged into and across Spirit Lake, but most of it flowed westward into the upper reaches of the North Fork of the Toutle River. At one location, about 4 miles north of the summit, the advancing front of the avalanche still had sufficient momentum to flow over a ridge more than 1,150 feet high. The resulting hummocky avalanche deposit consisted of intermixed volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake. Covering an area of about 24 square miles, the debris avalanche advanced more than 13 miles down the North Fork of the Toutle River and filled the valley to an average depth of about 150 feet; the total volume of the deposit was about 0.7 cubic mile. The dumping of avalanche debris into Spirit Lake raised its bottom by about 295 feet and its water level by about 200 feet.

1983 Photo North Fork Toutle River

1983 Photo North Fork Toutle River


Mudflows and Floods

Volcanic debris flows-mobile mixtures of volcanic debris and water popularly called mudflows -often accompany pyroclastic eruptions, if water is available to erode and transport the loose pyroclastic deposits on the steep slopes of stratovolcanoes. Destructive mudflows and debris flows began within minutes of the onset of the May 18 eruption, as the hot pyroclastic materials in the debris avalanche, lateral blast, and ash falls melted snow and glacial ice on the upper slopes of Mount St. Helens. Such flows are also called lahars, a term borrowed from Indonesia, where volcanic eruptions have produced many such deposits.

Mudflows were observed as early as 8:50 a.m. PDT in the upper reaches of the South Fork of the Toutle River. The largest and most destructive mudflows, however, were those that developed several hours later in the North Fork of the Toutle River, when the water-saturated parts of the massive debris avalanche deposits began to slump and flow. The mudflow in the Toutle River drainage area ultimately dumped more than 65 million cubic yards of sediment along the lower Cowlitz and Columbia Rivers. The water-carrying capacity of the Cowlitz River was reduced by 85 percent, and the depth of the Columbia River navigational channel was decreased from 39 feet to less than 13 feet, disrupting river traffic and choking off ocean shipping.

Mudflows also swept down the southeast flank of the volcano-along the Swift Creek, Pine Creek, and Muddy River drainages and emptied nearly 18 million cubic yards of water, mud, and debris into the Swift Reservoir. The water level of the reservoir had been purposely kept low as a precaution to minimize the possibility that the reservoir could be overtopped by the additional water-mud-debris load to cause flooding of the valley downstream. Fortunately, the volume of the additional load was insufficient to cause overtopping even if the reservoir had been full.

On the upper steep slopes of the volcano, the mudflows traveled as fast as 90 miles an hour; the velocity then progressively slowed to about 3 miles an hour as the flows encountered the flatter and wider parts of the Toutle River drainage. Even after traveling many tens of miles from the volcano and mixing with cold waters, the mudflows maintained temperatures in the range of about 84 to 91 degrees (F); they undoubtedly had higher temperatures closer to the eruption source. Shortly before 3 p.m., the mud- and debris-choked Toutle River crested about 21 feet above normal at a point just south of the confluence of the North and South Forks. Another stream gage at Castle Rock, about 3 miles downstream from where the Toutle joins the Cowlitz, indicated a high-water (and mud) mark also about 20 feet above normal at midnight of May 18. Locally the mudflows surged up the valley walls as much as 360 feet and over hills as high as 250 feet. From the evidence left by the “bathtub-ring” mudlines, the larger mudflows at their peak averaged from 33 to 66 feet deep. The actual deposits left behind after the passage of the mudflow crests, however, were considerably thinner, commonly less than 10 percent of their depth during peak flow. For example, the mudflow deposits along much of the Toutle River averaged less than 3 feet thick.”