Volcano Hazards including Lahars

Key Points

• Each of Washington's volcanoes is still active. In fact, all of them except for Mount Adams have erupted in the last 250 years. Volcanoes do not erupt at regular intervals, so it is difficult to know exactly when or where the next eruption will happen.

• Washington State is home to five active volcanoes located in the Cascade Range, east of Seattle: Mt. Baker, Glacier Peak, Mt. Rainier, Mt. Adams and Mt. St. Helens (see figure [Cascades volcanoes]). Washington and California are the only states in the lower 48 to experience a major volcanic eruption in the past 150 years.

• Major hazards caused by eruptions are blast, pyroclastic flows, lahars, post-lahar sedimentation, and ashfall. Seattle is too far from any volcanoes to receive damage from blast and pyroclastic flows.

  • Ash falls could reach Seattle from any of the Cascades volcanoes, but prevailing weather patterns would typically blow ash away from Seattle, to the east side of the state. However, to underscore this uncertainty, ash deposits from multiple pre-historic eruptions have been found in Seattle, including Glacier Peak (less than 1 inch) and Mt. Mazama/Crater Lake (amount unknown) ash.
  • The City of Seattle depends on power, water, and transportation resources located in the Cascades and Eastern Washington where ash is more likely to fall. Seattle City Light operates dams directly east of Mt. Baker and in Pend Oreille County in eastern Washington. Seattle's water comes from two reservoirs located on the western slopes of the Central Cascades, so they are outside the probable path of ashfall.
  • If heavy ash were to fall over Seattle it would create health problems, paralyze the transportation system, destroy many mechanical objects, endanger the utility networks and cost millions of dollars to clean up. Ash can be very dangerous to aviation.

• Lahars are mudflows and debris flows that originate from the slopes of a volcano and travel down river systems. Mt. Rainier is the only volcano connected to Seattle via a river system.

• Lahars from Mt. Rainier have buried the Kent Valley in the past, but there is no evidence a lahar has reached Seattle in the past 10,000 years. A Washington Department of Natural Resources analysis states that it is possible for a lahar to reach Seattle but would be extremely unlikely.

• Seattle faces vulnerabilities from a lahar reaching the Kent Valley. Interstate 405, as well as oil and natural gas pipelines, water lines, power lines, and sewer mains that serve Seattle all cross the potential lahar area in the Kent Valley. This area also hosts many of Seattle's major food distributors.

• Lahars can cause floods that transport massive amounts of sedimentation farther downstream. In a Mt. Rainier eruption, if lahars reach as far as the Kent Valley, Seattle's Duwamish Valley could experience post-lahar sedimentation.

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