Temporal and Spatial Distribution of Landslides in the Redwood Creek Basin, Northern California

Mass movement processes are a dominant means of supplying sediment to mountainous rivers of north coastal California, but the episodic nature of landslides represents a challenge to interpreting patterns of slope instability. This study compares two major landslide events occurring in 1964 to 1975 and in 1997 in the Redwood Creek basin in north coastal California. In 1997, a moderate-intensity, long-duration storm with high antecedent precipitation triggered 317 landslides with areas greater than 400 m 2 in the 720 km 2 Redwood Creek basin. The intensity-duration threshold for landslide initiation in 1997 was consistent with previously published values. Aerial photographs (1:6000 scale) taken a few months after the 1997 storm facilitated the mapping of shallow debris slides, debris flows, and bank failures. The magnitude and location of the 1997 landslides were compared to the distributions of landslides generated by larger floods in 1964, 1972 and 1975. The volume of landslide material produced by the 1997 storm was an order of magnitude less than that generated in the earlier period. During both periods, inner gorge hillslopes produced many landslides, but the relative contribution of tributary basins to overall landslide production differed. Slope stability models can help identify areas susceptible to failure. The 22% of the watershed area classified as moderately to highly unstable by the SHALSTAB slope stability model included locations that generated almost 90% of the landslide volume during the 1997 storm.