CREEPY HILLSLOPES: MULTIMETHOD ANALYSIS OF EARTHFLOWS IN HOLLISTER, CA

Earthflows are significant occurrences in tectonically-active mountainous regions with weak rock, clay-rich soils, and seasonally-varying precipitation. Despite comparatively slow movement, they can still seriously damage infrastructure and constitute large fractions of watershed sediment budgets. In California, where rainfall frequency and intensity can vary drastically year to year, earthflow movement is very intermittent. Structural influences, like differences in landslide size, hillslope position, and other morphological characteristics also affect behavior. Nonetheless, further longer-term studies are needed to understand effects of varying rainfall patterns and differing morphologic characteristics on slide motion. This research serves as a case study to elucidate these processes. Our research at the Hollister Hills SVRA examines 2 adjacent earthflow complexes with multiple landslides of various sizes and morphological features, all exposed to the same rainfall, allowing us to explore the influence of slide morphology on kinematics. To do this, we have quasi-annually tracked ground pegs hosted across 6 slides for about a decade. Additionally, we use coeval repeat lidar and drone photogrammetry to map volumetric surface changes from slide motion and gullying. Preliminary results indicate strong correlation of slide motion to extreme wet and dry seasons at the decadal scale but with strong variance across individual slides hosting different morphological characteristics. The recent 2022-2023 atmospheric river sequence resulted in a resurgence of earthflow activity, gullying, and shallow landslides. Notably, significant runoff acted to reactivate slide-associated gully systems that were dormant since at least 2010 and channel scour renewed motion of slide bodies adjacent to valley bottoms. These results lend evidence to the importance of atmospheric river events in driving earthflow evolution and episodic sediment delivery.