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Hazards

Landslides, Erosion and Land Use

In urban areas the human and infrastructure costs of landslides are often mediate and devastating. Their ecological impact is complex. A landslide that causes a local disturbance to a riparian ecosystem may also be a factor in providing a complex stream habitat.

Portions of the Willamette River Basin provide favorable conditions for landslides including steep hillsides and abundant rainfall. The region also experiences common landslide triggering events - freeze/thaw cycles, rapid snowmelt, periods of intense rainfall, and earthquakes. Most triggering events in Oregon are seasonal and occur during the winter and spring. The conditions that favor landslides may develop over long periods of time and be intensified by human activities (such as road building or clearcutting) that acts on the natural slope and concentration of water. Common landslide types are slides, flows, spreads and topples/falls (PDF).

The more process of erosion is the wearing away of the land surface by running water, wind, ice, gravity, and other geological activities. Rates of erosion are increased by unstable soils and steep slopes. Erosion contributes sediments to the streams of the Willamette River Basin and can cover spawning beds with fine sediment as well as carry nutrients from fertilizers into the water. Dams in the basin serve to trap sediment upstream but at the same time erosion rates are increased downstream. This "downstream" sediment is made up of smaller sized particles which may serve to transport nutrients and toxic constituents more easily.

On a per acre basis, urban areas contribute the greatest amount of this suspended sediment to the Willamette River. Sediment transport varies seasonally. The greatest levels are reported after major storms and the spring snow melt. This rapid water runoff carries pollutants directly into streams without the natural filtration that occurs with slower passage through soil.

Sources

Landslide Hazards in Oregon. Portland, OR: Oregon Department of Geology and Mineral Industries

Allan, S., A.R. Buckley, and J. E. Meacham. 2001. Landslides. Atlas of Oregon, 2nd Ed., pp. 140-141. Eugene, OR: University of Oregon Press

Allen, J., A. Salamack, and P. Schoonmaker. 1999. Natural Environment: Erosion. Restoring the Willamette Basin: Issues and Challenges. pp.17-19. Salem, OR: Willamette Restoration Initiative

Compiled by Bonnie Avery, Natural Resources Librarian (2007)

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