(From the Eugene Weekly, Earth Day 1998 edition)

Upstream
The violent, drenching storms of 1996 brought the immediate and local
consequences of human alterations in water's movement into sharp
focus. The Oregon Department of Forestry estimates the number of
landslides triggered by those storms in the "tens of thousands." The
steeper the slope, the more likely it is to slide when it receives a large
dose of rainwater and loses the stabilizing effect of tree roots. Debris
flows from slides can travel fast and far, scour streambeds to bedrock,
and kill people and animals in their path.

Landslides are a hot topic partly because of increased awareness of
downstream consequences such as fish habitat destruction and estuary
silting, but largely because significantly more people have been exposed
to them as Oregon's population grows and develops more types of land.
Logging, building and road construction can trigger landslides by altering
the way water reaches and travels through the soil, often creating
impervious surfaces so that the soil can't deeply soak up water.

Clearcutting in particular has been implicated in increasing landslides.
Historically, foresters have relied on aerial surveys to reveal landslides in
forested and logged areas. New ground-based research is proving the
method unreliable, but there's no consensus on whether more landslides
occur on clearcut slopes than forested ones or whether aerial surveys
have simply missed many slides in both types of terrain.

Hoping to clarify the question, a team including University of Washington
geomorphologist David Montgomery spent 10 years watching "the
steepest, most slide-prone terrain we could find" in the Oregon Coast
Range, Montgomery said. "There were 35 landslides in the decade after
clearcutting in this small area. It's a significant acceleration." The team
also reviewed data from many other sites in the Northwest. Although his
team is still analyzing, Montgomery believes the landsliding rate in
clearcuts is at least twice and perhaps as much as 10 times the natural
background rate.

 

In the long term, Montgomery added, another issue may trump our
immediate concerns. "The rate at which we're bleeding soil off the
landscape is higher than it's being produced. We don't tend to worry
about that, but somebody 600 years from now might be mad. Trees
falling over is the primary soil-producing agent in the Northwest, and
trees don't fall over anymore." The soil we're losing washes into coastal
estuaries that don't especially benefit from it.

Geotechnical engineer Keith Mills of the Oregon Department of Forestry
says the department is conducting its own landslide research to develop
management policies for logging and other uses in slide-prone areas. The
state also has a task force on landslides and public safety to cope with
the problem in more populated areas, Mills said. After the 1996 storms,
the state Legislature prohibited forest operations on high-risk sites above
homes and high-use public areas until January 2000. Mills agrees with
Montgomery that scientific evidence indicates "there's an increased
frequency for a period of time after harvest" in steeply sloped areas with
shallow soils, but cautioned that "It's far more complex than saying you
don't have slides unless there's logging."

It's ironic and appropriate that 1998, the International Year of the
Ocean, coincides with the strongest El Niņo on record and comes
shortly after some of Oregon's rainiest years in recent memory. The
mythical True Oregonian can cheerfully cope with any type of water
event-our belief that we can truly harness water-is probably an
industrial-era fantasy. As the global climate shifts and human activities
change the landscape, we are reminded that the combination of gravity
and water can overwhelm our most grandiose assumptions and our most
ambitious projects.