Landslides are a natural geologic process closely associated with periods of heavy rainfall and/or rapid snowmelt in this region. In areas of high natural risk, the incidence of shallow, rapidly moving landslides can be accelerated in the near term by human activities, including road construction and timber harvesting on steep slopes.

There are special forest practices rules for road construction and harvesting operations on high landslide hazard locations. Where public safety is at substantial risk, road construction and timber harvesting are prohibited. If a road is needed, a steep grade to the ridge top might be required to avoid cutting the road across a slope that might be slide prone. When roads are built on steep slopes, soil and rock that is removed must be loaded in trucks and hauled to a stable location. It cannot be dumped over the side onto steep slopes where it might trigger a landslide.

Timely reforestation is required after timber harvest. This reduces slide risk over time by shortening the period that trees are not occupying the land.

Landslides can affect fish habitat both positively and negatively. Fine sediment entering a stream can cover fish eggs and suffocate them or block young fry as they emerge. On the other hand, logs and coarse gravels delivered to streams in landslides can create spawning beds as well as pools for rearing habitat.

The Storm of 1996

Two major storms and resultant floods in 1996 prompted the Pacific Northwest’s largest-ever ground-based landslide study on the relationships between forest practices and shallow, rapidly moving landslides. These are some of the conclusions:

• For the first 10 years after a timber harvest, the risk of this type of landslide is increased 1.5 to 2 times.
• Younger forests (age 30-100) typically have lower landslide densities and erosion volumes than are found in mature forest stands.
• There is significant landslide risk on very steep slopes regardless of forest age.
• Landslides from recently harvested forests and older forests have similar dimensions, including depth and volume.

Road-related landslides have been more of a problem than those associated with forest operations. Significant research and changes in road location, design and maintenance have been implemented to reduce this problem.