In mountainous landscapes with weak, fi ne-grained rocks, earthfl ows can dominate erosion and landscape evolution by supplying sediment to channels and controlling hillslope morphology. To estimate the contribution of earthfl ows to regional sediment budgets and identify patterns of landslide activity, earthfl ow movement needs to be quantifi ed over signifi cant spatial and temporal scales. Presently, there is a paucity of data that can be used to predict earthfl ow 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 orthorectifi ed historical aerial photographs to objectively map earthfl ow movement between 1944 and 2006. By tracking the displacement of trees growing on earthfl ow surfaces, we fi nd 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 earthfl ow features that have intricate, elongate planform shapes, a preferred south-southwesterly aspect, and a mean longitudinal slope of 31%. The distribution of mapped earthfl ow areas is wellapproximated by a lognormal distribution with a median size of 36,500 m 2. Approximately 6% of the study area is composed of earthfl ows that connect to major channels; these fl ows 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 earthfl ow evolution wherein earthfl ows 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 earthfl ow-prone terrain.

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We demonstrate the ability of coupled remote sensing tools to characterize large, slow‐moving landslides in the Eel River catchment, northern California. From a stack of ALOS interferograms, we identified 5 large (>1 km long) landslides that exhibited significant activity from February 2007 to February 2008. For the Boulder Creek earthflow, we used orthorectified air photos taken in 1964 and unfiltered airborne LiDAR flown in 2006 to map the displacement of trees growing on the landslide surface. Combining those displacement orientations with stacked DInSAR data, we observed average downslope velocities of 0.65 m yr−1 through the central transport zone of the landslide. Given landslide depth estimates, minimum sediment transport and denudation rates are estimated to be 4100 m3 yr−1 and 1.6 mm yr−1, respectively. Our results demonstrate the highly erosive role of large, slow‐moving landslides in landscape evolution and suggest that the superposition of dense, ephemeral gully networks and rapidly moving zones within the landslide may facilitate delivery of slide‐mobilized sediment into adjacent fluvial channels.

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