Stillaguamish is flowing again.

Dan McShane has been keeping an eye on the river gage on the Stilliguamish below the landslide. His latest post shows that the river is no longer backing up behind the landslide dam. Dan includes a graph from the river gage. I’ve copied his screen capture below- click to enlarge. You can see that discharge [= flow measured in cubic feet per second] dropped instantaneously just after noon on 3/22, from 2000 cfs to 900, and continued to decline down to 700 cfs until Sunday at around 3 PM.  Then the level of the impounded river had risen enough to begin overtopping the low point on the slide surface and began to flow downstream again. The last data point on the graph is from Monday at about 6 PM, and flow seems to have stabilized. You can track the discharge yourself at the gage website.

Stilliguamish gage data. Click to enlarge.

Stilliguamish gage data. Click to enlarge.

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16 Responses

  1. I’ve been watching this gauge since the slide…kinda like whitewater porn for us river rats.  The pool that has accumulated is the result of about 1200 cfs (2000 – 800) over 24 hours.  Figuring about 8 gallons per cu. ft. = about 64 pounds per cu. ft.  The pool contains about 104 million cu. ft. = about 830 million gallons = about 3.3 million tons of water.  Now, if I lived downstream of that I’d get to higher ground until it drains (hopefully slowly).  I live for high water and big waves, but the added mud & debris is a bit much for even this adrenaline junkie.


    Kelly
    aka, Water Dragon, Woody D’Bree, Lahars, Rowan N. Circles

  2. Is that tiny peak immediately before the drastic reduction a ‘tsunami-like’ rush of water flooding away from the incident? Or is it just a contrast artifact?

    • Mike,
      Interesting observation, and a good hypothesis. I don’t know. Wiould be interesting to make a study of past gage behaviors during landslide, see if this appears in other times and places.
      Dave

    • Hmmm, note that the river gauge is about six miles upstream of Arlington, about 8-10 miles downstream of the slide, and there was a delay of approx. four hours from slide to gauge drop. Two miles per hour is typical for low gradient and low flow like this. Could that blip be caused by a ‘push’ of water caused by the slide? Like a ripple from a rock dropped in a pond? Maybe, but that’s quite a distance with many bends, pools, and shallow gravel bars in the river that would tend to dissipate such a ripple.
      All that speculation in mind, I check many river gauges daily (whitewater boaters are like Pavlov’s dog) and have seen oddities like this little bump that have no obvious explanation.
      ?electronic gremlins?

    • This is from ‘Discovery News’ update:

      In fact, there’s evidence that the landslide released a great deal of water at its base as it occurred. The National Weather Service tweeted an image of data from a nearby river gauge on the Stilliguamish River showing a spike in the water level at the time of the landslide, followed by a dramatic drop in the river. The latter was caused by the landslide damming the river.

  3. I too have been carefully watching the hydrograph at Arlington. The important thing is that the slide debris did not block the river very deeply, since it is the depth more than the volume of water that determines hydrostatic head and thus how forcefully the overtopping waters will flood the area down valley. Consider past history…. for example, 1929 in the Gros Ventre River east of Jackson Hole, Wyoming. A spring rain-triggered landslide blocked the river deeply, and when the impounded lake overtopped the debris dam, a flash flood of tremendous volume devastated the small down of Kelly, Wyoming. It is tragic that so many have been killed in this recent landslide. since in the long run, slides were inevitable in this historically and prehistorically slide prone area. (which contrary to many news reports, was NOT due to the heavy March rainfall…that was merely the trigger for underlying causes of weak materials and steep topography). There are policy issues involved when housing is permitted directly below geologically recognized high-risk areas. Recognized risk may not have been the case when Steelhead Drive was originally subdivided, but it surely is an important issue in light of this event for future development or …Heaven forbid!…redevelopment on this same site?
    Science and policy aside, I grieve for those who lost love ones in this landslide. Our extended family has owned river-front property just downstream from Oso for about 70 years, so I have a special personal interest in this valley.

  4. Hi Dave — how does the Oso slide compare to the Slide Mountain slide (1200 c.e.?) along (under) Mt. Baker Highway?

    • I can give a partial answer to Eric’s question. The Slide Mountain slide into the North Fork of the Nooksack River valley is larger than the recent landslide into the N. Fork of the Stillaguamish River. I don’t have the reference handy that estimates the volume of the prehistoric Slide Mountain slide (not to be confused with the January 2009 slide above and into Racehorse Creek that has been thoroughly described in this blog by Dave), but it was large enough and fell down far enough to behave as a long-runout landslide, where horizontal travel distance is several times greater than vertical fall distance. The material is mostly sandstone bedrock blocks of the Chuckanut Formation, and it probably behaved as a rock avalanche. It too would have blocked the river for a time. Blocks of sandstone are clearly recognizable along the Mt. Baker Highway, but the most impressive display of its size is when one takes the North Fork road, which turns north off the Mosquito Lake Road just past the bridge near Welcome. You reach the edge of the slide just about where the pavement ends, and from there until the bridge across Racehorse Creek, a distance of about one mile, the entire area is littered with jumbled sandstone blocks, many of them the size of a one-car garage.

  5. Dave. Hello. Thanks for these great updates on the Oso landslide. You are priceless in info! I remember 7 yrs or so ago I drove up to a logging road( oppisite ) Bedal campground mt loop hwy. And at around 4.ooo ft happen to see a huge landslide into the Sauk River mile or so below the Bedal Campground. Are ancient lahar areas responcible for these issues? Darrington, Oso( ancient ) lahar areas?

    Please keep me informed and may God protect the lost souls.

    Glenn

    • Glenn,
      One or more lahars from Glacier Peak flowed down the Suiattle River into the Sauk and, probably, onwards into the Skagit at modern Rockport. While it is possible that a large GP lahar descended the Stilly several thousands of years ago, no deposit has been mapped in that valley. Sloppy, wet landslides like the Oso slide, the adjacent 2006 Hazel slide, and the recent Clay Banks slides along the Nooksack are in Pleistocene glacial deposits, or in sediments deposited at the bottom of lakes dammed by those glaciers.
      Dave

    • Glenn, I’ve rafted that stretch of the Sauk many times over the years and have seen obvious evidence of numerous slides/slumps into the river. The one you describe sounds like it’s in the ‘Forgotten Mt. Estates’ area…a private in holding cluster of homes, many of which were washed away in the floods of 2003 & 2005. There is a particularly spectacular slide on the right bank of the Sauk at the end of the Beaver Lake trail (the trailhead is near the confluence of theI White Chuck river). These slides are all comprised of similar grey unconsolidated silty clay (real gooey stuff) with interspersed layers of sand and gravel…evidence of repeated events over the past 13,000 years or so.
      Hey Magmatist, please correct my watery descriptions or timeline.

    • Water Dragon- no correction needed.
      Dave

    • Oh, THAT one. It is the big one in these parts, at least as far as prehistoric but still fairly recent mass movement events. Eric, the Church Mountain slide, which is evident by the impressive, nearly mile-high scarp on the mountain’s south side and by the hummocky, bouldery terrain around the town of Glacier and as far as the 542 bridge across the Nooksack west of Glacier, has an estimated pre-erosion volume of 368 million cubic yards. Lush vegetation largely obscures the slide material from view, but there are a few roadcuts that reveal its poorly sorted nature. 368 million cubic yards is about 37 million dump truck loads of rock!
      Although I have not yet seen any estimated volumes for the N. Fork Stilly landslide, my wild guess is that it is probably about one tenth the volume…. maybe one twentieth….. of the Church Mountain slide.
      Doug
      p.s. – I just read a published estimate of the size of the Oso slide. King 5 News, as quoted in Wikipedia today, gives a length of 460 meters, width of 1300 m, and thickness of 12 m. Taking these as maxima, the computed volume of this landslide is about 7,176,000 cubic meters (9,386,000 cubic yards which is about 2.5% of the volume of the Church Mountain slide. But so far as we know, as huge as it was, the latter didn’t kill 25 people, with that number sure to go up (however, with due respect, the ancesters of today’s Nooksacks may have had a village where the town of Glacier is today, which clearly would have been utterly obliterated. Will we ever know?)
      While comparing volumes, the 1980 debris avalanche in the beginning of the May 18th, 1980 eruption had an uncompacted, uneroded volume of 0.67 cubic miles (2.8 billion cubic meters) and covered 23 square miles (60 square kilometers), according to the USGS. It and the lateral blast that followed would have killed many, many people if the area hadn’t been made off-limits to nearly everyone.
      None of this is meant in any way to detract from the impact of the Oso landslide. It wasn’t huge, relatively speaking, but terribly devastating. 😦

      Doug

  6. From USGS:

    Monitoring Water Levels and Flood Potential
    The USGS operates a streamgage to measure water levels about 12 miles downstream from the landslide, on the North Fork Stillaguamish River at Arlington. The river level at the gage dropped suddenly at about 1:30 pm PDT on Saturday. The drop in water level was about 1.2 feet, which is equivalent to a drop in discharge of about 1,200 cubic feet per second. Go online and see near-real-time data (http://waterdata.usgs.gov/wa/nwis/uv/?site_no=12167000).
    USGS scientists continue to monitor the streamgage remotely. The streamgage is operating normally (measurements every 15 minutes, transmissions to the web every hour) but could possibly be damaged if large debris comes down the river. Discussions are underway on whether and how to increase the reporting frequency of the streamgage, but this modification of instrumentation requires reprogramming at the field site. Due to safety concerns, USGS staff are currently not scheduled to go to the field-monitoring site.
    Consideration also is currently being given to making frequent streamflow measurements downstream of the slide at auxiliary sites and installing rapid deployment gages and turbidity meters. We are exploring deploying a buoy to measure the elevation of the pool elevation behind the blockage.
    The USGS is working with the National Weather Service (NWS) to do preliminary modeling of possible scenarios considering what would happen if the blockage on the Stillaguamish River were to break. This will help estimate the range of potential discharge from the pool that has formed behind the landslide. The NWS will use these data to develop flood inundation maps to estimate where flooding could likely occur.

  7. The gauge near Oso, about two miles downstream of the slide, has been re-activated as of Tuesday 3-25-14:

    http://waterdata.usgs.gov/wa/nwis/uv/?
    site_no=12166300&PARAmeter_cd=00060,00065

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