TIMESCALES OF RESPONSE TO A CATASTROPHIC FLOOD: QUANTIFYING “EVENT” AND “POST-EVENT” EFFECTS TO ASSESS STREAM RECOVERY FOLLOWING TROPICAL STORM IRENE

The rarity of catastrophic floods has critically limited our understanding of these important events and their aftermath. In temperate environments, stream recovery following extreme flood events is thought to occur rapidly due to frequent competent flows and climate suitable for rapid revegetation. However, seven years after Tropical Storm Irene generated the flood of record on many streams in the New England, we observe that sediment yield and transport rates remain elevated above background averages. We present work investigating stream recovery following Tropical Storm Irene, as motivated by three research questions: 1) what are the likely timescales of recovery in the stream system, 2) how do years of post-flood processes extend the impact of the event. To answer these questions, we analyze multi-temporal lidar from 83 landslide sites, as well as measurements of suspended sediment and channel geometry made throughout the 14,000 km² study area. We find that landslides continue to erode at rates of ~10 cm/yr, and that suspended sediment concentrations remain accordingly elevated an order of magnitude above pre-Irene rates. Though some landslide revegetation and slope stabilization occurs during summer and fall, this recovery is reliably interrupted by spring snowmelt runoff and large precipitation events. We show that this elevated input is likely to continue until landslide toeslopes are isolated from adjacent river channels, and thus are able to recover without continued bottom-up destabilization; for some landslides, this process may take decades. As a result, we predict that the elevated post-flood suspended sediment regime will persist at least a decade following the Tropical Storm Irene, possibly doubling the initial impact of the event.