DYNAMICS OF THE SWIFT CREEK LANDSLIDE, NORTHWEST WASHINGTON

Deep-seated landslides significantly influence the relief of mountain landscapes in Washington State, yet relatively few of these landslides have been studied in detail. We selected the Swift Creek landslide, a large (approximately 136 acres), deep-seated landslide located on Sumas Mountain in northwest Washington, to be the site of a detailed study. Our study, the first phase in a planned long-term study to be conducted by WWU, consisted of geomorphic mapping, repeated GPS surveying of monitoring points (consisting of six surveys from July 2002 to June 2003), tree-core analysis (dendrogeomorphology), and historic aerial photograph analysis.

The landslide appears to be rooted in altered ultramafites, primarily consisting of serpentinite at the surface, and closely resembles an idealized earthflow (even though the underlying material appears to be bedrock). The serpentinite most likely weathers to weak smectite clay. The landslide consists of a series of large scarps at the head, presumably failing by rotational movement, and transitions to a flow toward the toe of the landslide. The toe of the landslide is almost completely devoid of vegetation (most likely from the high Mg and low Ca content of serpentinitic soils and the high activity of the landslide), and is the site of frequent failures during the winter.

Horizontal movement rates of monitoring points on the landslide ranged from 37 m (a point on the toe) to less than 2 meters during the course of this study (July 2002 to June 2003). The greatest amount of movement generally occurred on the toe. Rapid movement of points on the toe (i.e. >10m annually for many of the points) is related to shallow mass wasting events caused by high precipitation. Points upslope from the toe generally moved <4-5 m during the course of this study.

Based upon historic aerial photographs and dendrogeomorphic analysis, the Swift Creek landslide became active in the 1930s. Geomorphic activity on the landslide increased to a maximum in the 1970s and has slightly diminished since that time. The toe of the landslide has enlarged steadily in both thickness and horizontal extent since the initiation of the landslide and continues to enlarge (trees near the toe are being engulfed by landslide debris). The landslide should remain active for the foreseeable future.