Does Timber Harvesting Cause Landslides?

Landslides are a relatively common, naturally occurring hazard in some parts of Oregon. Landslides in Oregon are typically "triggered" by periods of heavy rainfall and/or rapid snowmelt. Certain geologic formations and locations with very steep slopes are most susceptible to landslides. Although landslides are a natural geologic process, their incidence can be accelerated by human activities, including forest practices.

A recent study by the Oregon Department of Forestry confirmed that landslides occur in forests of all ages, whether or not they have been harvested. This study, the largest ground-based investigation ever conducted of landslides, found that timber harvesting on very steep slopes will likely increase landslide occurrence for a period of roughly 10 years. In addition, forests between 10 and 100 years of age typically had lower landslide densities and erosion than found in mature forest stands, a finding new to this study. Landslide occurrence on less steep slopes (typical of most forestland) was low.

Roads associated with forestry activities and other land uses have been a significant source of landslides in steep terrain. Roads built under newer Forest Practices Act rules have resulted in smaller and fewer slides than roads built to older standards.

Landslides have historically contributed to fish habitat by providing sediment, logs, and other material to streams. Forest practices may change the size and composition of what a slide deposits in a stream. This is why the Oregon Plan For Salmon and Watersheds encourages placement of logs and other habitat features in streams and strategic locations for fish.

The Oregon Legislature recently granted authority to the State Forester to regulate harvesting where there is a significant risk of a rapidly moving landslide.

*Nearly four-fifths of the landslides that entered streams during the 1996 storms occurred on slopes of greater than 70 percent, and none entered streams from slopes under 40 percent.

SOURCE: Oregon Forest Resources Institute, Forest Fact Book.