Inventory and Analysis of Landslides In Redwood Creek, California, Following the 1997 Storm

Mass movement processes are a dominant means of supplying sediment to north coastal California rivers, but the timing of sediment input is highly episodic. In 1997, an 8-year return interval storm event with high antecedent precipitation triggered 258 landslides larger than 400 m2 in the 720 km2 Redwood Creek basin in northwest California. Using 1:6000 aerial photographs and limited field reconnaissance, we mapped shallow debris slides and debris torrents and collected geologic, topographic and land-use data. The volume of landslide material delivered to Redwood Creek from this storm was about 674,000 Mg, representing about half of the annual suspended sediment load measured near the mouth of Redwood Creek in WY 1997. The highest density of landslides (4.7/km2) occurred on a specific bedrock type (metamorphosed sandstones and mudstones of the Grogan Fault Zone), and the volume per unit area from this lithology was 100 times greater than the basin average of 930 m3/km2. Areas of recent commercial timber harvest within a steep inner gorge adjacent to Redwood Creek produced 2300 m3 of material per kilometer of channel length, about three times the rate from the rest of the inner gorge. Sixty-three percent of the landslides occurred within 30 m of a road, and the average volume for these landslides was almost 50 percent larger than the average for those located farther from roads.