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

Abstract

Although earthfl ows are the dominant erosion mechanism in many mountainous landscapes, estimates of long-term earthfl ow-driven sediment fl ux 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, orthorectifi ed historical aerial photographs, and inventories of meteoric 10 Be in soil pits, we quantifi ed ~150 years of slope movement on a 1.5-km-long earthfl ow in the Eel River catchment, northern California, United States. Using LiDAR-derived topography, we mapped the upper half of the earthfl ow 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 10 Be 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 10 Be-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 refl ect exhaustion of readily mobilized source material, velocities temporarily increased in the midtwentieth century due to major hydrologic events. Given an average velocity of 2 m/a, the Kekawaka earthfl ow is defl ating its source area over 20 times faster than the regional erosion rate, emphasizing the localized and vigorous role of active earthfl ows in landscape evolution.