During a large storm in November, 1996, four people were killed near Roseburg when a landslide destroyed their home. The landslide originated in a clearcut, and many people have blamed logging for these deaths. Does clearcut logging cause landslides?

(click on thumbnail at left for larger view; click "back" button to return)

Landslides occur naturally. Landslides even occur in pristine forests where no trees have been cut and there are no roads. They are usually caused by the buildup of groundwater during intense rainstorms. High risk areas, or those areas where landslides are most likely to occur, are steep slopes just below ridgetops. High risk areas may have thin soil, and hollows or depressions that collect water during storms.

Most research indicates that clearcut logging can increase the risk of landslides, especially in high risk areas during the first ten years or so after logging. How much does the risk increase? That's hard to say, because the results from different studies don't agree ,and there are many variables, such as geology, weather, and logging practices. Some estimates of the effect of logging may be exaggerated, because, when viewed from the air, it is easier to see landslides in clearcuts than in older forests. Some studies have actually shown a decrease in landslides after logging.

Landslides flow downhill into streambeds at the bottom of canyons, so the debris from many landslides will be deposited in the same place. This makes it easier to predict areas that are in danger from landslides, no matter what the cause.

What about roads? Researchers have known for a long timethat roads can cause landslides. Roadcuts are steeper than natural hill slopes, and roads change the flow of surface water and groundwater.

However, road design and construction has improved in the last 20 years, and roads are more often built along ridges instead of on steep slopes. The Oregon Forest Practice Rules strengthened the requirements for forest roads in 1983. During the 1996 storms, roads built since 1983 appeared to cause fewer and smaller landslides than older roads.

The Oregon Department of Forestry can now restrict clearcut logging and road building on steep slopes above highways and homes if it is determined that it could present a hazard to public safety or property. Research is underway to learn more about the effect of logging on landslides. It is important to remember, however, that even if no trees are cut in high-risk areas, there is still a risk of landslides during intense rainstorms.

References:

• Public safety requires curbs on clear-cutting, construction. Statesman Journal, Salem, September 17, 1998.
• Pyles, M., P.W. Adams, R.L. Beschta, and A.E. Skaugset. 1998. Forest practices and landslides: a report prepared for Governor John A. Kitzhaber. Forest Engineering Department, Oregon State University, Corvallis.