The role of chronic and episodic disturbances on channel-hillslope coupling: the persistence and legacy of extreme floods

Landscape form represents the cumulative effects of de‐stabilizing events relative to recovery processes. Most geomorphic research has focused on the role of episodic rare events on landscape form with less attention paid to the role and persistence of chronic inputs. To better establish the interplay between chronic and episodic extreme events at regional scales, we used aerial photography and post‐flood sediment sampling to assess stream and hillslope response and recovery to a 100–300 yr. flood caused by Tropical Storm Irene in New England. Within a 14 000 km2 study area, analysis of aerial photographs indicated that the storm initiated (n = 534) and reactivated (n = 460) a large number of landslides. These landslides dramatically increased overall estimates of regional erosion rates (from 0.0023 mm/yr. without Irene to 0.0072 mm/yr. with Irene). Similarly, Irene‐generated LWD inputs of 0.25–0.5 trees/km exceeded annual background rates in a single event, and these concentrated inputs (101–102 of trees/landslide) are likely to result in large jams and snags that are particularly persistent and geomorphically effective. Finally, we found that landslide scars continue to provide elevated sediment inputs years after the event, as evidenced by sustained higher suspended sediment concentrations in streams with Irene‐generated landslides. Overall, our results indicate that infrequent, high‐magnitude events have a more important geomorphic role in tectonically stable, more moderate‐relief systems than has been previously recognized. Understanding the role of these events has particular relevance in regions such as New England, where the frequency and magnitude of extreme storms is expected to increase. Further, these effects may force reconsideration of conservation and restoration targets (for example in channel form and large wood loading and distribution) in fluvial systems. Copyright © 2016 John Wiley & Sons, Ltd.