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Sediment yield, spatial characteristics, and the long-term evolution of active earthflows determined from airborne LiDAR and historical aerial photographs, Eel River, California

Mackey, Benjamin H. and Roering, Joshua J. (2011) Sediment yield, spatial characteristics, and the long-term evolution of active earthflows determined from airborne LiDAR and historical aerial photographs, Eel River, California. Geological Society of America Bulletin, 123 (7-8). pp. 1560-1576. ISSN 0016-7606. doi:10.1130/B30306.1.

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In mountainous landscapes with weak, fine-grained rocks, earthflows can dominate erosion and landscape evolution by supplying sediment to channels and controlling hillslope morphology. To estimate the contribution of earthflows to regional sediment budgets and identify patterns of landslide activity, earthflow movement needs to be quantified over significant spatial and temporal scales. Presently, there is a paucity of data that can be used to predict earthflow behavior beyond the seasonal scale or over spatially extensive study areas. Across 226 km^2 of rapidly eroding Franciscan Complex rocks of the Eel River catchment, northern California, we used a combination of LiDAR (light detection and ranging) and orthorectified historical aerial photographs to objectively map earthflow movement between 1944 and 2006. By tracking the displacement of trees growing on earthflow surfaces, we find that 7.3% of the study area experienced movement over this 62 yr interval, preferentially in sheared argillaceous lithology. This movement is distributed across 122 earthflow features that have intricate, elongate planform shapes, a preferred south-southwesterly aspect, and a mean longitudinal slope of 31%. The distribution of mapped earthflow areas is well-approximated by a lognormal distribution with a median size of 36,500 m^2. Approximately 6% of the study area is composed of earthflows that connect to major channels; these flows generated an average sediment yield of 19,000 t km^(−2) yr^(−1) (rock erosion rate of ∼7.6 mm/yr) over the 62 yr study period, equating to a regional yield of 1100 t km^(−2) yr^(−1) (∼0.45 mm/yr) if distributed across the study area. As such, a small fraction of the landscape can account for half of the regional denudation rate estimated from suspended sediment records (2200 t km^(−2) yr^(−1) or ∼0.9 mm/yr). We propose a conceptual model for long-term earthflow evolution wherein earthflows experience intermittent activity and long periods of dormancy when limited by the availability of readily mobilized sediment on upper slopes. Ultimately, high-order river channels and ephemeral gully networks may serve to destabilize hillslopes, controlling the evolution of earthflow-prone terrain.

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Additional Information:© 2011 Geological Society of America. Received 26 March 2010; Revision received 10 October 2010; Accepted 12 October 2010. This research was funded by a National Science Foundation (NSF) grant (Geomorphology and Land Use Dynamics, EAR-0447190) to J. Roering. B. Mackey was partially supported by the “Fulbright EQC Award in Natural Disaster Research” from New Zealand. The LiDAR data were acquired by the National Center for Airborne Laser Mapping (NCALM) in 2006. We thank Calvin and Wendy Stewart for field access and generous hospitality, and the Lone Pine and Island Mountain ranches for fi eld access. We benefited from fruitful discussions with Harvey Kelsey and Jim McKean over the course of this research. We thank Harvey Kelsey, Bill Haneberg, Grant Meyer, and Associate Editor Jon Major for insightful reviews that greatly improved this manuscript.
Group:Keck Institute for Space Studies
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New Zealand Fulbright EQC Award in Natural Disaster ResearchUNSPECIFIED
Issue or Number:7-8
Record Number:CaltechAUTHORS:20110620-135047046
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:24067
Deposited On:20 Jun 2011 23:02
Last Modified:09 Nov 2021 16:20

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