June 15, 2009
Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.
Hazel Landslide, Snohomish County
The Hazel Landslide is a persistent deep-seated landslide that is probably driven by groundwater and erosion by the North Fork of the Stillaguamish River.
The landslide has caused some headaches for DNR, as a catastrophic failure and partial blockage of the Stillaguamish River around 1988 spurred many to consider logging as the culprit.
Logging in the northwest area above the landslide came into question as a area that was a groundwater recharge area. The addition of water from the removal of trees was considered to be at least a partial catalyst for the failure of the Hazel Landslide and it is seen as a poster child of what groundwater recharge in sensitive areas can do.
Since 1988 the forest land has recovered and groundwater recharge should have been diminished, but in January of 2006, following a period of prolonged precipitation, the Hazel Landslide once again moved, diverting the river into a small community of houses. This landslide was another large landslide and has been called the Steelhead Haven Landslide. During that month, about 8 inches of rain fell, well above the average 4.5 inches that typically falls during the month of January (in 93 months of record). This would overwhelm any recovery that may have occurred with the maturing forest.
The landslide produced a lot of sediment, which I hear from some of my sport fisherman up there, caused quite a poor year of fishing. The long term effects might not have been devastating, but with a weakening population of fish, it certainly hasn’t helped any.
In the landslide world, the Hazel Landslide is certainly one of the more well known landslides. The unstable nature of the glacial lakebed lithology it sits in has caused countless landslides throughout Western Washington and is a legacy of our glacial history. However, it has taught us some valuable lessons in how groundwater affects lacustrine beds and its potential sensitivity to water.