A 62 YEAR RECORD OF REGIONAL EARTHFLOW MOVEMENT DETERMINED FROM LIDAR AND AERIAL PHOTOGRAPHS, EEL RIVER, CA

In deep-seated landslide and earthflow prone terrain, such as the Franciscan mélange of the northern California coast ranges, active earthflows are recognized to contribute significantly to both landscape morphology and the transfer of sediment from hillslopes to the channel network. Despite the importance of earthflows to landscape evolution, we struggle to reliably quantify the extent, distribution, and activity of earthflows across a drainage basin, which requires detailed, long-term observations of earthflow movement across a broad area. In turn, this has hindered the development of a robust conceptual framework describing how earthflows operate as a geomorphic process.

Here, we accurately constrain the spatial and temporal distribution of earthflow activity along a section of the main stem Eel River, northern California. In this catchment renowned for widespread earthflow activity, states of activity range from long-dormant slumps, to sustained earthflow movement presently approaching 1 m/a. A high resolution 230 km^2 LiDAR dataset provides a meter-scale template of the modern earthflow morphology and vegetation, and we use LiDAR-derived maps and elevation models to accurately rectify a series of historical aerial photos. By comparing the location of trees growing on the earthflow surface as observed on the historical photos to the LiDAR, we can map the location, timing and magnitude of earthflow displacement since 1944.

Despite much of the landscape displaying the morphological signature of mass movement (>50% of the study area has headscarps, levees, pressure ridges, tension cracks and hummocky terrain that appears qualitatively active on the LiDAR imagery), we find only 7% of the terrain has experienced quantifiable slope displacement since 1944, distributed across 80 individual earthflow features.

For eight of the larger, continually active earthflows, sequential aerial photos reveal the history of earthflow velocity over the 62 year record. Typically, the earthflows show increased rates of displacement over the 1960s and 1970’s, with some earthflow velocities temporarily exceeding 4 m/a. This is followed by a marked deceleration over the past 3 decades to ~1 m/a. The uniformity of temporal pattern across different earthflows suggests a climatic driver dominated earthflow behavior since 1944.