Landslides triggered by Pacific Northwest Storms, November and December 1998

By Rex L. Baum and Alan F. Chleborad

Two separate storms, November 25 and December 27-29, 1998, resulted in flooding and landslides in parts of the Pacific Northwest. The November 25 storm triggered several small landslides in south King County, Washington. A few blocked roads, including northbound Interstate 5 near SeaTac airport, but no serious damage resulted from the slides.

The December 27-29 storm followed a cold spell. Runoff from snowmelt and rainfall caused flooding and triggered landslides in western Washington and Oregon. A few of the landslides caused significant damage and many temporarily blocked roads and highways. Landslides were locally numerous in western Oregon. Types of landslides included earth slides, rockslides, rock falls, rapid earth flows and debris flows. Geologic materials involved in the slides consisted mainly of colluvium and weathered residuum derived from sedimentary, volcanic, and plutonic rocks.

We conducted reconnaissance surveys of landslides in King and Pierce counties, Washington and Benton, Lincoln, and Tillamook Counties, Oregon January 5-9, 1999. According to news reports landslides also occurred in Pacific, Wahkiakum, Lewis, Cowlitz, and Skamania Counties, Washington, and Multnoma and Clackamas Counties, Oregon as a result of the December 27-29 storm.

Location Map

Location Map Counties of Washington and Oregon where landslides have been reported are highlighted. Red dots show locations of landslides described in table 1.

Several damaging landslides of note are depicted in accompanying photographs and characterized in the accompanying table.