Landslide of the Week – Sultan River Debris Avalanche

June 8, 2009

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).


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.


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.


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