Long-term kinematics and sediment flux of an active earthflow, Eel River, California

Abstract

Although earthflows are the dominant erosion mechanism in many mountainous landscapes, estimates of long-term earthflow-driven sediment flux remain elusive because landslide displacement data are typically limited to contemporary time periods. Combining high-resolution topography from airborne LiDAR (light detection and ranging), total station surveying, orthorectified historical aerial photographs, and inventories of meteoric ¹⁰Be in soil pits, we quantified ~150 years of slope movement on a 1.5-km-long earthflow in the Eel River catchment, northern California, United States. Using LiDAR-derived topography, we mapped the upper half of the earthflow into three distinct kinematic zones: an upslope source area, a long narrow transport zone, and a mid-slope compressional zone. From our air photo analysis (1944–2006), average velocities are fastest in the transport zone (1.7 m/a), slowest in the source zone (<1 m/a), and decrease monotonically over the past 30 years in all three zones. Meteoric ¹⁰Be inventories systematically increase with distance downslope of the source area, consistent with the notion that the elongate transport zone acts like a relatively undeformed soil conveyor that can be used to quantify long-term displacement. Because our ¹⁰Be-derived transport zone velocity of 2.1 m/a averages over the past 150 years, pre-1944 velocities likely approached 2.5 m/a, suggesting that twentieth century land-use practices have not increased rates of sliding. Although our results reveal a progressive decline in velocity that may reflect exhaustion of readily mobilized source material, velocities temporarily increased in the mid-twentieth century due to major hydrologic events. Given an average velocity of 2 m/a, the Kekawaka earthflow is deflating its source area over 20 times faster than the regional erosion rate, emphasizing the localized and vigorous role of active earthflows in landscape evolution.