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How the 2014 Oso landslide dammed a river and cut a Washington highway

The Oso landslide killed 43 people on 22 March 2014 and blocked the North Fork Stillaguamish River and State Route 530. Water overtopped the debris dam within about 25 hours. The river cut a new channel while road crews worked to restore access.

Sources checked

Disaster Register Editorial Team

Landslide and recoveryOso river change and highway recovery in 2014
  1. 22 March 2014
    Landslide crossed the valley

    The slide near Oso travelled almost two kilometres. Its deposit crossed State Route 530 and blocked the North Fork Stillaguamish River. · Landslide Source 1

  2. 23 March 2014
    River overtopped the deposit

    Water began flowing over a low part of the deposit within about 25 hours of the landslide. · River Source 2

  3. 24 March 2014
    Aerial surveys measured the site

    The USGS assessment lists lidar at one-metre horizontal resolution and aerial imagery at 0.3-metre resolution for this date. · Survey Source 1

  4. 26 March 2014
    Crews opened an alternate access route

    State and county crews cleared the Mountain Loop Highway as work also connected State Route 530 with a utility road for responders. · Road and access Source 3

  5. Early April 2014
    Pilot channel excavated

    County and cooperating agencies cut a pilot channel in the eastern part of the deposit to speed drainage during the search effort. · River and response Source 1

  6. 25 April 2014
    Piloted public access contracted

    Washington State Department of Transportation contracted work to guide public traffic on a separate utility access road. · Road and access Source 3

Show 7 more dates
  1. May 2014
    River channel re-established through the deposit

    A USGS study reports a deeply inset channel 20 to 40 metres wide through the deposit by May. · River Source 2

  2. 30 May 2014
    Highway clearing completed

    The county project account records about 127,000 tons of debris moved during the State Route 530 clearing project. · Road and access Source 3

  3. 31 May 2014
    Highway reopened to piloted traffic

    State Route 530 reopened through the area to one-way piloted traffic. · Road and access Source 3

  4. 1 June 2014
    Standing pool receded at lower flows

    USGS reports that by this date no standing pool remained at river flows below about 57 cubic metres per second. · River Source 1

  5. July 2014
    Water surface approached its earlier profile

    USGS found the channel water surface about one metre above its pre-landslide profile. · River Source 2

  6. 22 September 2014
    Highway reconstruction substantially complete

    The state recorded substantial completion of the emergency roadway reconstruction project. · Road and access Source 3

  7. First 18 months
    Sediment moved downstream

    USGS estimated 820,000 metric tonnes of sediment eroded from the deposit into the river over the first 18 months. · River Source 2

Choose a milestone to trace the first day, the river’s changing channel or the return of road access.

Source 1 Source 2 Source 3

The deposit created a temporary lake

The preliminary USGS assessment estimated an impoundment about eight metres deep, three kilometres long and holding about 3.3 million cubic metres of water. Once it overtopped, the river began cutting into the deposit through a new channel. Source 1

The water surface fell as the new channel deepened

USGS reports that average water-surface elevation through the deposit fell about seven metres over roughly two months. By July 2014 it was about one metre above the pre-landslide profile. The profile and the persistence of a standing pool were separate indicators of the river's change. Source 2 Source 1

The highway reopened in stages

The May reopening allowed one-way piloted traffic after road-clearing work. The project account separately dates substantial completion of emergency reconstruction to 22 September. A date for limited access describes a different stage from completing the reconstruction contract. Source 3

Sediment continued moving after the initial response

The 18-month study estimates that erosion from the deposit delivered about 820,000 metric tonnes of sediment to the river. It estimates that silt or clay made up 77% by mass. Road-clearing debris figures describe material moved by crews under a separate project. Source 2 Source 3

Sources

Open the data and source notes