Hazardous Slopes & Bluffs

  • Many Puget Sound Bluffs are Naturally Unstable.

    Before buying shoreline property, investigate slope hazards. Consider hiring a geologist to evaluate the site for stability and erosion potential.

    Information on the stability of Puget Sound slopes is available in the Coastal Zone Atlas of Washington. This atlas can be found in the public library or at your local county planning or engineering department. Not all slides may be mapped, however. Many slides may be hidden under trees, shrubs, or eroded materials.

    Bluff Hazards to Consider

  • Ground and Surface Water Problems

    Along Puget Sound, most slides occur in winter and spring, when groundwater levels peak. Construction and the diversion of storm runoff can cause erosion and slide problems.

  • Layers Can Slide

    Where till overlies sand or gravel, the lower unit erodes faster undercutting the till, which can collapse in large blocks. Where sand overlies till, silt, or some other impermeable material, groundwater can soak the sand layer. This wet layer of sand can weaken a slope and cause slumping and mud flows.

  • What Grows There?

    Crooked or bent tree trunks can signal creeping and sliding soil. If there is no vegetation on a bluff or slope, it may be too steep or unstable to support growth. Plants with shallow roots, such as salmonberry can mask slide activity.

  • Keep Septic Drainfields Away From Bluffs

    Septic tank effluent can saturate soils and make slopes unstable. Carefully plan on-site septic system locations. Keep drainfields well away from bluffs and slopes. Make the sale contingent upon passing a percolation test.


    A Massive Slide, Winter 1999

    A number of homes in the Carlyon Beach / Hunter Point area of Thurston County were damaged by a large slide...

    Related Topics

    Slides, What causes slides along Puget Sound?
    Geology, How glaciers left a legacy of unstable bluffs.
    Understanding Erosion, What to consider on land loss.

    Related Link

    Geologic Hazards: Landslides, USGS. National landslide hazards program and national landslide information center. 

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