Produced by the Pacific Rivers Council

Fact Sheet: Clearcuts, Landslides, and the 1996 Storms

Scientific studies for more than twenty five years have proven over and over that clearcutting causes very large increases in landslides. The American Fisheries Society is the independent professional society for fisheries scientists. The Society's 1991 publication "Influences of Forest and Rangeland Management on Salmonid Fishes and their Habitats" [William R Meehan ed, AFS 1991] reviewed all the scientific literature, and concluded:

"The frequency of mass erosion [landslides, debris flows, earthflows, etc] is strongly linked to the type and intensity of land treatment in the basin. Although most mass movements are associated with roads and their drainage systems, many originate on open slopes after logging has raised soil water tables and decreased root strength." [Meehan, 1991 page 194]

"The increase in mass movement due to clear cutting varies widely, ranging from 2-4 times in Oregon and Washington...to 31 times in the Queen Charlotte Islands... An increase of 6.6 times...is probably closer to the norm." [Meehan, 1991, page 194]

In other words, scientific studies show that clearcutting, at a minimum, doubles or quadruples the number of landslides, and the average effect is much greater yet.

The 1996 Storms

Intense winter storms in February and November/December of 1996 have triggered thousands of landslides in the Pacific Northwest, concentrating in the Coast Range of Oregon, the Western Cascades of Oregon, in SW Washington, and most recently in Umpqua and South Coast Regions of Oregon. Similar patterns of landslides occurred in the Skagit Basin in Washington in the early 1990s, and throughout the region in the large storms of 1964 and 1955.

The 1996 storms affected areas that have been subject to very high levels of logging and roadbuilding in recent decades, particularly the 1980s. The absence of really severe storms over the last fifteen years or so means that many logged hillsides and roads are only now being hit for the first time. And the resulting picture is devastating. Aerial reconnaissance surveys documented more than 650 landslides in the February storms alone. More than 70% of these landslides were associated with recent clearcuts.

The report titled Aerial Reconnaissance Evaluation of 1996 Storm Effects on Upland Mountainous Watersheds Oregon and Southwest Washington, [Weaver and Haggans, 1996] is attached. Weaver and Haggans found that: "Within any particular area there was an obvious and visible association between roads and landsliding, and between recent harvesting (clearcutting) and landsliding." [Weaver and Haggans, 1996, page 7]

Prepared by the Pacific Rivers Council. Direct questions to David Bayles, 541 345 0119.