REACTIVATION OF ANCIENT LANDSLIDES NEAR PORTLAND, OREGON, WINTER OF 2006-2007

The winter of 2006-2007 in the Portland area started out with a very high rainfall month over 35 cm. Although the rest of the winter had normal precipitation, the antecedent rainfall eventually helped reactivate some very large landslides months later. First, a very large landslide over one million cubic meters moved downslope near Stevenson, Washington (to the east of Portland in the Columbia Gorge). The old slide had been undercut by the stream at the bottom of the cliff, and the migration upstream in the last 20 years of the 6 meter high waterfall had removed the buttress at the bottom of the slope. The scarp moved upslope to the edge of two homes. At the beginning of January another ancient landslide in Astoria, Oregon (to the west of Portland at the mouth of the Columbia River) started moving. It has first moved in 1954 destroying 18 homes. This slide near 1st and Commercial Streets covered 4 city blocks and continued to move for 3 months. In mid February 2 homes of the 2006 Street of Dreams began to move. This large development of $2-3 million homes was constructed on an ancient landslide in the fine-grained Troutdale Fm. near Oregon City which is SE of Portland. Abernethy Creek at the toe of the slide eroded the toe so one house, called Moxie, lost its side yard, but the house remained intact because it was built on over 50 pilings. The second house, called Romeo and Juliet, started moving downhill and caused considerable damage. On March 29th a rockfall of only 300 cubic meters completely stopped traffic on Highway 26 when it covered the west bound lanes with debris. It had moved the last time in 1993.