Chronic and episodic disturbances: The persistence and legacy of extreme floods

The response and recovery of fluvial systems to disturbance can operate on a range of time scales depending upon the pre-event initial conditions and the magnitude and sequencing of post-event flows. Although most previous research on magnitude-frequency relationships focuses on extreme events and their overall effectiveness on landscape form, the relationship between the long-term co-occurrence of chronic and episodic events in fluvial systems remains poorly understood. Impacts of steady events are often well chronicled, but estimating the effect of extreme events is more difficult due to their rarity and spatially variable impacts. To better establish the interplay between chronic and episodic extreme events at regional scales, I examine stream response and recovery to a 100--300 yr flood caused by Tropical Storm Irene in New England.;Of 994 mapped landslides in a 14,000 km2 study area, 534 were newly initiated by Irene. The remainder were reactivated scars initially triggered before Irene. These landslides were point sources for sediment and large woody debris (LWD) inputs to streams. Including Irene inputs in a 30-yr regional sediment budget elevates the inferred background erosion rate from 2.3 mm/kyr to 7.2 mm/kyr. Episodic events like Irene are thus necessary to account for regional erosion rates of ~4--6 mm/kyr. Irene-generated LWD inputs of 0.25--0.5 trees/km equal annual background rates in a single event, but generate more snags and jams because of their concentrated inputs (10s--100s/landslide).;Landslide scars continue to input sediments to streams 3--4 yrs after Irene. Suspended sediment fluxes are an order of magnitude higher than pre-Irene fluxes across a wide range of discharges. The eleven greatest suspended sediment concentrations ever recorded in the region have been collected during non-flood post-Irene flows. The geomorphic impact of LWD input is similarly persistent. LWD regulates stream flow, sediment transport, and ecology for many years until the woody debris undergoes downstream transport or decay. The initial and persistent impacts due to Irene and similar extreme events are an essential component of regional fluvial dynamics, and may govern long-term recovery from disturbance.