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Abstract 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 km²study 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 (10¹–10²of 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. 

Considerable research over the past several decades shows that dams, especially large, flow regulating structures, fragment watersheds and serve to disconnect the normative downstream flux of sediment and nutrients. Less attention has addressed smaller, channel-spanning Run-of-River (RoR) dams that are more commonly distributed throughout watersheds. Taking advantage of a suite of RoR dams in New England (USA), we quantify bedload flux into, through, and beyond the reservoir of five RoR dams and calculate the residence time of gravel clasts within the reservoir. To accomplish this goal, we embedded Radio Frequency Identification (RFID) PIT tags in 791 gravel clasts ranging in size from 15 mm to 81 mm which were subsequently deployed within and upstream of the impounded reservoirs. Among the 503 tracers that were transported from their deployment location, the median cumulative distance traveled was 30 m and the maximum cumulative displacement during the study period 758 m. Of the total tagged rocks placed at all five sites, 276 rocks were displaced over the dam, 204 of which spent time in the reservoir between high discharge events; the rest were transmitted downstream in a single high discharge event. Among those tracers that spent time in the reservoir prior to transmission over the dam, the average reservoir residence times at the different sites ranged from 19 - 203 days. The median grain size of tracers that were transported over the dam were identical to those that moved during the study period and similar to the median grain size of the channel bed. The distribution of virtual velocities of those tracers that moved was approximately log-normal and very broadly distributed over more than six orders of magnitude. An analysis of variance revealed that the distribution of velocities was partitioned into two statistically similar groups; with slower velocities in the two smaller watersheds (13 km2 – 21 km2) compared to the larger watersheds (89 km2 – 438 km2). We conclude that RoR dams transmit and trap the upstream sediment supply within the same range of physical conditions that produce mobility and trapping in the river’s natural reach-scale morphological units. Since RoR dams are likely not trapping more sediment than is typically sequestered in natural river reaches, these dams do not disconnect the normative downstream flux of sediment nor result in channel morphological disequilibrium downstream of the dam. However, the minimal effect that small, channel spanning RoR dams have on the morphological equilibrium state of a channel does not suggest that RoR dams have no ecological footprint. 

Abstract The time scale of channel recovery from disturbances indicates fluvial resiliency. Quantitative predictions of channel recovery are hampered by multiple possible recovery pathways and stable states and limited long-term observations that provide benchmarks for testing proposed metrics. We take advantage of annual channel-change measurements following Tropical Storm Irene’s 2011 landfall in New England (eastern USA) to document geomorphic recovery processes and pathways toward equilibrium. A covariate metric demonstrates that channels can adjust rapidly to ongoing boundary condition shifts, but that they adjust along a continuum of possible stable states. Moreover, the covariate equilibrium metric indicates sensitivity to warm-season high discharges that, in this region, are increasing in frequency. These data also show that the channels are resilient in that they are able to recover an equilibrium form within 1–2 yr of disturbances.